Asymmetric Mechanical Vulnerability of the Internal Carotid and Vertebral Arteries: The Investing Layer of the Deep Cervical Fascia–Carotid Sheath Coupling Hypothesis

1. Introduction

1.1. Problem Background and Operational Definition

1.1.1. Background

Activation of the trigeminovascular system, CGRP release, and central sensitization of the trigeminocervical complex are the current mainstream framework for understanding the initiation and maintenance of migraine and related syndromes. This framework primarily addresses the neurovascular event cascade after an attack has been initiated. It does not adequately answer a more upstream question: why the attack threshold in certain individuals is systematically lowered, such that routine head and neck movement or minor mechanical loading is sufficient to trigger symptoms.

The term "mechanical vulnerability" is used here as an operational term. It is defined as the property whereby the same upstream mechanical perturbation produces downstream events with different thresholds and different phenotypes across circulatory regions. This is an operational definition of the phenomenon to be explained, not an assertion that the differential response has already been demonstrated. The term specifically denotes differential responses among circulatory regions. When referring to individual-level susceptibility, the terms "individual susceptibility state" or "threshold lowering" are used.

1.1.2. Distinction from Mechanosensitivity and Mechanotransduction

Mechanical vulnerability is distinguished from the cellular-level concepts of "mechanosensitivity," the capacity to respond to mechanical stimuli, and "mechanotransduction," the process by which mechanical signals are converted into biochemical signals. The latter two are used here only to describe cellular and molecular steps. Although the title juxtaposes the internal carotid artery and the vertebral artery, the unit of analysis of the hypothesis is the anterior and posterior circulations, rather than the isolated properties of the two arteries themselves.

1.2. Deep Cervical Fascial Biomechanics and the Carotid Sheath as a Functional Convergence Point

1.2.1. Investing Fascia–Deep Cervical Fascia Continuity

Biomechanical research on the deep cervical fascial system has accumulated observations that have not previously been adequately integrated. The investing fascia, that is, the superficial layer of the deep cervical fascia, envelops the sternocleidomastoid and trapezius muscles. It forms an anatomically continuous mechanical closed loop with the galea aponeurotica and the pectoral fascia. Its chronic tension abnormality can be transmitted to deep structures through the fascial network.

1.2.2. Carotid Sheath as a Neurovascular Convergence Point

The carotid sheath is an anatomical convergence point for the cervical neurovascular structures. It encloses the internal carotid artery, the internal jugular vein, and the vagus nerve. It also lies tightly adjacent to the cervical sympathetic trunk. If the mechanical state of the investing fascia can substantially alter the mechanical environment of the carotid sheath and its contents, then a causal pathway from peripheral fascia to cerebrovascular–neural function would be anatomically feasible.

1.2.3. Biomechanical Feasibility and Unresolved Transmission Coefficient

Pires et al. [1] showed that the mechanical pressure required to reach the deep cervical fascia averages approximately 6 N/cm², with a value of 6.06 ± 0.186 N/cm². This threshold is consistent across the cervicothoracic region. Individual mechanosensitivity and subcutaneous fat thickness do not affect this pressure value. This provides a direct biomechanical feasibility basis for the proposition that a change in the mechanical state of the investing fascia is sufficient to affect deep structures. However, it does not measure the conduction efficiency from the investing fascia to the sheath wall.

1.3. Two-Level Hypothesis

1.3.1. Mechanical Coupling Chain

The investing fascia–carotid sheath coupling hypothesis comprises two levels. The first level is the mechanical coupling chain. Investing fascial fibrosis conducts abnormal stress through the deep fascial network to the carotid sheath. Within the sheath, this stress is amplified via PIEZO mechanotransduction and CGRP signaling. The result is superimposed effects along five pathways: arterial, venous, lymphatic/glymphatic, sympathetic, and vagal. Here, coupling refers to functional mechanical continuity, not necessarily anatomical fusion. The lymphatic and glymphatic pathways are combined into a single group. The glymphatic system is regarded as the parenchymal extension of the lymphatic pathway rather than an independent pathway.

1.3.2. Dual-Circulation Asymmetry Proposition

The second level is the dual-circulation asymmetry proposition. The anterior circulation refers to the territory supplied by the internal carotid artery system. The posterior circulation refers to the territory supplied by the vertebrobasilar system. The external carotid artery is not included in the anterior circulation perfusion analysis. The two circulations differ systematically in dynamic cerebral autoregulation and arterial wall mechanical responses. These differences cause the same upstream mechanical abnormality to produce downstream vascular events of different nature and different thresholds. In vivo evidence shows that the posterior circulation is more pressure-passive and tends toward stiffening under external loading.

1.3.3. Threshold-Dependent Five-Pathway Superposition (Conditional on the Sympathetic Hemodynamic Threshold)

The five-pathway superposition is threshold-dependent rather than simultaneous, with the threshold defined by the sympathetic hemodynamic threshold. The term "conditional" is used once here in its standard sense, namely dependent on a stated condition; thereafter the term "threshold-dependent" is used throughout to denote the same property. Below that threshold, the superposition mainly involves the arterial, venous, lymphatic/glymphatic, and vagal pathways. The sympathetic pathway joins only after its threshold is crossed. The direction of its contribution is modified by stimulation type, intensity, and anterior or posterior circulation site. Washio et al. [2] further indicated that site-specific differences exist between the vertebral artery and the posterior cerebral artery within the posterior circulation in dynamic cerebral autoregulation and CO₂ reactivity. Therefore, this article localizes posterior circulation vulnerability to the vertebral artery V2 segment as the peripheral input site. It does not treat the entire posterior circulation as a homogeneous entity.

1.4. Vertebral Artery Hypoplasia and Evidence Positioning

1.4.1. VAH as Structural Reference, Not Causal Evidence

Vertebral artery hypoplasia (VAH) is a congenital vessel-diameter variant. Systematic-review evidence indicates a stable association between the low-flow, low-reserve state of the posterior circulation and clinically vulnerable phenotypes. In this hypothesis, VAH evidence is not used for causal inference. It is not used to support the chain that acquired fascial constraint leads to low reserve. Its role is limited to two levels. First, it provides a logical corroboration for the low-reserve-to-vulnerability logic. Second, it provides a population-level parallel background. Neither level supports the chain from acquired fascial constraint to low reserve. This article therefore distinguishes logical corroboration from argumentative support. VAH provides argumentative support for the proposition that low reserve predisposes to vulnerability. It does not provide argumentative support for the proposition that acquired fascial constraint causes low reserve. In this hypothesis, VAH is therefore a reference framework for validation design, not causal evidence for the core chain.

1.4.2. Related Evidence and Estimable Sample-Size Basis

This subsection records evidence that provides epidemiological parameters, effect-modification variables, and sample-size basis for validation design. The meta-analysis by Triantafyllou et al. [3] showed that vertebral artery dominance, hypoplasia, and aplasia are not rare in the population. Pooled prevalence was 27.45%, 13.41%, and 5.39%, respectively. This provides an estimable sample-size basis for including individuals with congenital posterior circulation vessel-diameter variants in validation cohorts.

Neumann et al. [4] directly observed in hypertensive patients that those with VAH had lower resting posterior circulation blood flow. During simulated central hypovolemia, however, they maintained posterior circulation perfusion through a greater increase in total peripheral resistance. This suggests that the low-reserve state is accompanied by a systematic shift in compensatory strategy. It provides a prespecified effect-modification variable for the VAH subgroup: stress-state total peripheral resistance change.

Gaigalaite et al. [5] further showed that the effect of VAH on posterior circulation stroke or transient ischemic attack risk is significantly modified by the posterior configuration of the circle of Willis. This provides a prespecified stratification variable for the VAH subgroup. Sato et al. [6] showed that VAH and vertebral artery insufficiency can impair dynamic cerebral blood flow regulation, with reduced CO₂ reactivity on the hypoplastic vertebral artery side. This provides a supplementary functional endpoint for posterior circulation reserve.

Wang et al. [7], from the perspective of vertebral artery endothelial senescence in the context of cervical degeneration, proposed a candidate biological mechanism for posterior circulation vulnerability. They suggested that the low reserve of the posterior circulation may be not merely a passive attribute but an active process driven by chronic changes in the mechanical environment. This provides an exploratory biomarker direction, not a causal anchor.

1.5. Positioning of the Hypothesis

1.5.1. Complement to Trigeminovascular and Central Sensitization Theories

The hypothesis is positioned as a supplementary explanation at the peripheral mechanical upstream level. It is not a replacement for the trigeminovascular system or central sensitization theory. The mechanical state of the investing fascia–carotid sheath may act as an adjustable threshold variable. This variable may participate in determining the ease of attack onset and the direction of phenotypic differentiation.

1.5.2. Scope and Testability

The conditional claims of the hypothesis are presented in Section 6, Scope of Application. Anatomical adjacency is not equated with causal proof. All claims are conditional on the transmission efficiency, PIEZO activation, and sympathetic threshold conditions specified in Section 6.

2. Anatomical Basis

2.1. Mechanical Continuity of the Investing Fascia and the Deep Cervical Fascia

2.1.1. Anatomical Continuity and Biomechanical Feasibility

The investing fascia, that is, the superficial layer of the deep cervical fascia, envelops the sternocleidomastoid and trapezius muscles. It continues superiorly with the galea aponeurotica and transitions inferiorly into the superficial pectoral fascia. It thus forms a fibrous mechanical closed loop spanning the cranial vault, neck, and upper thorax. Pires et al. [1] experimentally quantified the mechanical pressure required to reach the deep cervical fascia from the body surface, obtaining 6.06 ± 0.186 N/cm². This threshold is consistent across the cervicothoracic region. This magnitude falls within the range of physiological head and neck movement. It provides a feasibility basis for cross-layer mechanical transmission through the deep fascial network, one of the structural premises of the hypothesis. However, the actual efficiency of cross-layer transmission has not yet been directly measured.

The review by Wilke et al. [8] further integrated anatomical and functional evidence for fascial force transmission. It suggested that fascia is not merely a passive protective packing but can participate in mechanical force transmission. Kalichman [9] likewise reviewed the anatomical and functional evidence for myofascial continuity. Aparici-Gil et al. [10] directly quantified the anisotropic constitutive behavior of deep fascia through ex vivo multiaxial mechanical testing. This provides a parameterizable experimental basis for the material properties of deep fascia.

Martínez-Sanz et al. [11] described three developmental stages of the middle deep cervical fascia in human embryonic and fetal specimens. They found that the mesenchymal primordium of the middle deep cervical fascia is continuous with the adventitial primordium of the infrahyoid muscles. The study further noted that the relationship between the middle deep cervical fascia and regional lymphatic and venous structures suggests that the middle deep cervical fascia may promote venous and lymphatic circulation. This provides a human developmental–histological microscopic starting point for the proposition that the deep fascial network is anatomically continuous, can transmit mechanical force functionally, and may be related to the lymphatic and venous drainage environment.

2.1.2. Magnitude References and the Unmeasured Transmission Coefficient

Querry et al. [12] showed, using balloon catheter measurements in humans, that the transmission efficiency from external cervical pressure to the carotid sinus can reach 82%–89%. This indicates that external cervical mechanical input is not substantially attenuated. However, this pathway is external pressure to the carotid sinus, not investing fascia to sheath wall. It therefore provides only a magnitude reference for the transmission coefficient. None of the above evidence can replace direct measurement of the transmission coefficient for the specific investing fascia-to-sheath wall pathway (Gap 1).

2.2. Compositional Controversy and Functional Positioning of the Carotid Sheath

2.2.1. Compositional Controversy and Layered Histological Evidence

The fascial composition of the carotid sheath is a long-unresolved point of controversy in cervical anatomy. The review by Bond et al. [13] explicitly noted that multiple aspects of carotid sheath anatomy are controversial, most notably its specific attachment sites and fascial composition. The traditional three-fascia model has been systematically challenged. Some authors deny the existence of a discrete fascial tube surrounding the internal carotid artery, internal jugular vein, and vagus nerve. Other histological studies report that the common sheath does not fuse with the investing fascia or the pretracheal fascia. Still other authors maintain that the carotid sheath is composed primarily of the alar fascia.

Zhao et al. [14], in a rabbit model, described a consistent three-layered fascial architecture within the carotid sheath. This architecture comprises an inner fascial layer, an intermediate zone, and an outer fascial layer. They found that the arterial inner fascial layer thickened earliest and most markedly. The inner fascial layers of the vein and nerve were relatively thin. The authors attributed this differential thickening to differences in the intrinsic pulsatile hemodynamic stress among the three structures. Potential fusion was observed between the outer fascial layers of adjacent structures. This suggests that the sheath wall itself is not a homogeneous structure. It is mechanically layered and heterogeneous in the rabbit model and at the developmental stage.

Hayashi [15], based on semi-serial sections of adult human cervical specimens, described the carotid sheath as a two-layer architecture of laminar adventitia plus common sheath. The laminar adventitia continuously and completely envelops each vessel. The common sheath does not fuse with the superficial fascia or the pretracheal fascia. It often fuses with the visceral fascia to form a thick plate. The common sheath and prevertebral fascia may be interrupted or indistinct. The study also reported significant inter-individual and site-dependent variation in the thickness of both layers. It explicitly concluded that the carotid sheath is a definite histological structure rather than an anatomical artifact. This provides a direct histological anchor in adult human cervical segments for the proposition that the sheath wall is layered, heterogeneous, and subject to inter-individual and site variation. However, this evidence is morphological and histological and does not provide mechanical property measurements.

2.2.2. The Non-Fusion Finding and Functional Continuity as a Premise to Be Verified

One finding reported by Hayashi that is unfavorable to the hypothesis must be addressed directly. Hayashi explicitly reported that the common sheath does not fuse with the superficial fascia or the pretracheal fascia. Therefore, direct anatomical fusion may not exist between the investing fascia and the sheath wall. Mechanical transmission from the investing fascia to the sheath wall through the deep fascial network cannot be presumed to be carried by anatomical continuity. The hypothesis is conditional on the functional mechanical continuity of the deep fascial network. It does not rely on anatomical fusion.

The non-fusion finding does not negate functional continuity. It does, however, require that functional continuity itself be treated as an unverified premise, not as a substitute for anatomical fusion. Functional continuity and anatomical fusion are two distinct premises. Denying the latter does not establish the former. Both require independent verification. This tension is the same in nature as the anatomical controversy reviewed by Bond et al. [13]. Both point to the fact that the anatomical premise of the investing fascia-to-sheath wall pathway has not yet been closed (see Section 9).

To avoid leaving functional continuity as an abstract assertion, four candidate implementation mechanisms can be listed for subsequent measurement and modeling. First, perivascular connective tissue radially constrains intrasheath vessels and transmits stress without anatomical fusion. Second, intrasheath pressure is transmitted through sheath-wall deformation to its contents. Third, shear coupling occurs between the investing fascia and the sheath wall through loose connective tissue layers. Fourth, an indirect route through the prevertebral fascia to the transverse foramen fascia may act mainly on the vertebral artery V2 segment rather than the internal carotid artery. None of these mechanisms requires direct anatomical fusion. All are therefore compatible with the non-fusion finding. Each nonetheless requires independent measurement or modeling verification and cannot be replaced by a reference to functional continuity.

2.2.3. Candidate Implementation Mechanisms and the Sheath as a Functional Convergence Point

Schachtel et al. [16], through human cadaveric and radiological analysis, further described the fascial composition of the anterior carotid sheath wall in the skull-base region. It is formed jointly by the stylohyoid fascia, stylopharyngeus fascia, pharyngobasilar fascia, and longus capitis fascia. It appears as a hypointense line on magnetic resonance imaging. The glossopharyngeal nerve penetrates the sheath wall approximately 9.0 mm below the skull base. This suggests that the fascial source of the sheath wall may differ across segments in the skull-base segment and normal adult anatomy. It further supports the view of the sheath wall as a functional convergence point rather than a single anatomical entity.

The hypothesis does not rely on the strong anatomical claim that the investing fascia directly constitutes the carotid sheath. It is conditional on the functional mechanical continuity of the deep fascial network. The carotid sheath is positioned as a functional convergence point at the cervical level. This functional convergence point is distinct from the nasopharyngeal lymphatic plexus, which Yoon et al. [17] described as an anatomical hub for cerebrospinal fluid drainage. Regardless of its precise fascial composition, the carotid sheath is the spatial convergence point where multiple cervical neurovascular structures coexist. A change in the mechanical state of the sheath wall can therefore affect multiple structures within and adjacent to it, either simultaneously or sequentially. This functional positioning accommodates the anatomical controversies rather than avoiding them.

Three lines of evidence support viewing the sheath wall as a mechanically heterogeneous interface rather than a single material entity: the layered histology of Zhao et al. [14], the layered description of Hayashi [15], and the regional heterogeneity reported by Schachtel et al. [16]. However, the non-fusion finding of Hayashi [15] requires that the functional continuity from the investing fascia to the sheath wall be treated as a premise to be verified rather than as established fact (see Section 2.2.2).

2.3. Anatomical Boundaries and Pathway Differences Between the Anterior and Posterior Circulations

2.3.1. Definition of Anterior and Posterior Circulations and the ICA–VA Contrast

In this article, the anterior circulation refers specifically to the territory supplied by the internal carotid artery (ICA) system. The posterior circulation refers specifically to the territory supplied by the vertebrobasilar system. The external carotid artery is not included in the anterior circulation perfusion analysis. Under this definition, the internal carotid artery is the representative artery of the anterior circulation in the neck. The vertebral artery (VA) V2 segment is the representative artery of the posterior circulation in the neck. The two travel through mechanically distinct environments.

Site-specific differences have been documented within the posterior circulation, between the vertebral artery and the posterior cerebral artery. The anterior circulation has not been similarly stratified in the existing evidence base, and the present hypothesis does not treat it as homogeneous. Two asymmetries in stratification resolution must be made explicit. First, the posterior circulation has already been resolved into a peripheral input node (VA V2) and an intracranial end node (PCA), following Washio et al. [2]. The anterior circulation has no comparable two-node resolution in the mechanical literature: Thomas et al. [18] measured the cervical ICA wall, whereas Friend et al. [19] indexed the ICA system without segmental resolution. Second, the anterior circulation differs internally in geometry, wall composition, and autoregulatory reserve across the cervical ICA, the MCA, and the ACA. Treating the ICA as the representative anterior circulation artery therefore simplifies anterior heterogeneity in a way that the posterior circulation is no longer permitted to do. To keep the anterior–posterior contrast from being confounded by unequal stratification resolution, a minimal three-node anterior stratification is adopted here: cervical ICA as the mechanically accessible peripheral node; MCA as the intracranially accessible regulatory node; and ACA as an exploratory node. This stratification is a design constraint, not an empirical finding. The justification is that the two in vivo paradigms on which the asymmetry rests — the dCA paradigm of Friend et al. [19] and the SWE paradigm of Thomas et al. [18] — are both anchored on the ICA system without segmental resolution. Any anterior–posterior ratio computed from them therefore inherits an unresolved anterior denominator. The minimal three-node stratification is the smallest design change that makes that denominator explicit.

2.3.2. Intrasheath ICA Pathway and Parasheath Structures

The internal carotid artery is a principal content of the carotid sheath. It is directly exposed to changes in sheath-wall tension and in the intrasheath mechanical microenvironment. The sheath also contains the internal jugular vein and the vagus nerve. The cervical sympathetic trunk is tightly adherent to the posterior sheath wall; its anatomical relationship and quantitative surgical anatomy are detailed in Section 2.4 and Section 5.3. Abnormal stress transmitted from the investing fascia through the deep fascial network to the sheath wall can therefore exert a direct intrasheath mechanical effect on the internal carotid artery.

The following three reports are all single-case feasibility reports. They are used solely to demonstrate the mechanical feasibility that soft tissue or fibrous bands can directly compress extracranial large vessels in humans. They do not demonstrate that investing fascial fibrosis can cause comparable compression, nor do they carry causal weight. They are uniformly referred to as single-case feasibility reports, meaning reports that demonstrate mechanical feasibility only. The single-case feasibility report of Hou et al. [20] suggests that perivascular soft-tissue thickening can compress the C1 segment of the internal carotid artery in vivo. Its inflammatory etiology and atypical hemodynamics require boundary qualifications (see Section 9). The single-case feasibility reports of Dadsetan et al. [21] and Nguyen et al. [22] suggest that fibrous-band-type soft tissues can compress the V2 segment of the vertebral artery in vivo. Their local fibrous band or fibro-osseous band etiology requires boundary qualifications (see Section 9).

2.3.3. Extrasheath VA V2 Pathway and Perivascular Environment

The V2 segment of the vertebral artery courses through the transverse foramina and is not within the carotid sheath. Its mechanical environment is determined by the fascia and osseous constraints of the transverse foramen region. The ex vivo mechanical study by Zhou et al. [23] provides key evidence for the extrasheath indirect pathway of the vertebral artery. The study compared the passive mechanical responses of porcine vertebral artery V2 and V3 segments. These segments differed significantly in wall geometry and structure. However, the wall stress distributions of adjacent segments could be predicted to be similar only when their specific perivascular environments were incorporated into the computation. The mechanical state of the vertebral artery cannot therefore be explained by wall material properties alone. The constraint conditions imposed by the surrounding fascia and osseous structures are an irreducible variable.

This finding directly supports the assertion of the hypothesis that the mechanical state of fascial attachments in the transverse foramen region determines vertebral artery strain. However, the study itself did not involve the investing fascia. The conduction chain from the investing fascia to the transverse foramen fascia therefore remains a step to be verified. The hemodynamic dissociation between the V2 and V3 segments observed by Meng et al. [24] in rotational vertebral artery syndrome (RVAS) further supports analyzing the V2 segment separately as the peripheral mechanical input site of the posterior circulation.

2.4. Anatomical Relationship Between the Cervical Sympathetic Trunk and the Carotid Sheath

2.4.1. Qualitative Evidence of Tight Adherence

The spatial relationship between the cervical sympathetic trunk and the carotid sheath is the anatomical premise for whether the sympathetic pathway in the hypothesis can be established. Kiray et al. [25], in dissections of 42 adult cadavers, confirmed that the cervical sympathetic trunk was tightly adherent to the posterior wall of the carotid sheath in all specimens. They also found an anatomical variant in which the sympathetic trunk lies within the sheath. This tight adherence directly addresses the spatial feasibility required by the hypothesis. Deformation or tension change of the sheath wall can be transmitted to the sympathetic trunk along the shortest path with minimal attenuation. No intermediate structure with mechanical buffering significance exists between the two.

2.4.2. Quantitative Surgical Anatomy

Civelek et al. [26], based on surgical dissection of 30 fresh cadavers, further provided quantitative parameters. The cervical sympathetic trunk is located posteromedially to the carotid sheath. At the C6 level, it is approximately 11.6 ± 1.6 mm from the medial border of the longus colli muscle. Its own diameter is approximately 3.3 ± 0.6 mm. It courses throughout deep to the prevertebral fascia. These quantitative data provide a direct anatomical reference for the geometric positioning and size settings of the sympathetic trunk in the computational simulation in Section 8.2.

2.4.3. Implications for Spatial Feasibility of Sympathetic Coupling

Kiray et al. [25] established the qualitative fact of tight adherence. Civelek et al. [26] provided precise quantitative coordinates. Together they constitute the spatial argumentation basis for the mechanical coupling of the sympathetic pathway. Anatomical tight adherence alone does not, however, establish mechanical coupling efficiency under fibrotic conditions. That remains a step to be measured.

3. Asymmetric Vulnerability of the Anterior and Posterior Circulations

3.1. Structural Parameters and Compensation Threshold of the Anterior Circulation

3.1.1. Perfusion Share and Autoregulatory Reserve

The anterior circulation carries approximately 82% of cerebral blood flow. Its key compensatory feature is relatively strong cerebral autoregulation. During blood pressure fluctuations, it maintains relatively stable perfusion by adjusting vascular resistance.

3.1.2. The Compensation–Decompensation Threshold as the Characteristic Node

In the hypothesis framework, when investing fascial tension is mildly elevated, the anterior circulation may absorb a considerable portion of the mechanical perturbation through resistance vessel adjustment without producing immediate symptoms. Compensation is not unlimited. When mechanical stress persists or is superimposed on the shear load of head rotation, compensation may be breached. The characteristic node of the anterior circulation is the transition threshold between compensation and decompensation.

3.1.3. Minimal Internal Stratification and Its Consequence for the Characteristic Node

The characteristic node of the anterior circulation proposed in 3.1.2 — the compensation–decompensation transition — is a statement about the ICA system as a whole. Under the three-node stratification of 2.3.1, this node must be qualified in two ways. First, the compensation threshold is not a single value across the anterior circulation. The cervical ICA node is the segment at which external mechanical constraint is most directly imposed and at which SWE is technically feasible [18,27]; its compensation is therefore mainly geometric and perivascular. The MCA node is the intracranially accessible regulatory node. No MCA-specific dCA evidence is cited in this article, and the MCA node is therefore a design node rather than an evidence-anchored node. Its compensation is expected to be mainly resistance-vessel mediated. The ACA node is exploratory and is not assigned a characteristic node in the present hypothesis. Second, the two characteristic nodes proposed for the anterior and posterior circulations are not of the same dimensional type. The anterior node is a transition threshold between compensation and decompensation; the posterior node is the slope of a dose–response relation among fascial attachment stiffness, effective luminal radius, and perfusion fluctuation (3.2.6). These are not directly commensurable. The asymmetry proposition of this article therefore does not assert that the two circulations have "different thresholds" in a single common metric. It asserts that the anterior circulation is characterized by a breakpoint in a compensatory capacity, whereas the posterior circulation is characterized by a steeper slope in a passive transmission relation. Any quantitative contrast between the two must therefore be specified either as a slope ratio at matched effective-radius perturbation or as a breakpoint-versus-slope comparison, and the two must not be collapsed into a single "threshold" statement. This distinction is carried into the cohort design of 8.3.1 and the refutation criteria of 7.5.

3.2. Low Reserve and Perfusion-Fluctuation Dose–Response of the Posterior Circulation

3.2.1. Direct In Vivo dCA Evidence for Posterior Circulation Pressure Passivity

Dynamic cerebral autoregulation in the posterior circulation has been shown in vivo to be more pressure-passive, possibly because of lower baseline vascular tone. Friend et al. [19] provide the most direct differentiation data to date. Under acute hypoxia, vertebral artery rate of regulation (RoR) in male subjects decreased significantly (−0.15 s⁻¹; P = 0.012). Internal carotid artery RoR was not significantly affected (−0.07 s⁻¹; P = 0.264). Hypoxia induced internal carotid artery dilation (+0.30 mm; P = 0.009), whereas vertebral artery diameter remained unchanged (+0.08 mm; P = 0.398). This pattern is the direct human manifestation of the posterior circulation V2 segment as a mechanical input site exhibiting low reserve and high passivity. The posterior circulation showed decreased regulatory capacity with absent compensatory dilation. The anterior circulation showed maintained regulatory capacity with preserved dilation reserve.

This pattern is directionally consistent with the systematic review by Srichawla et al. [28] of regional differences in anterior and posterior circulation autoregulation. That review confirmed that anterior circulation autoregulation is relatively robust, whereas the posterior circulation may have lower autoregulatory reserve. It listed reduced sympathetic innervation, endothelial dysfunction, and neurovascular uncoupling as candidate mechanisms. The review is a narrative qualitative synthesis and provides no new quantitative effect sizes. Its value lies in providing a broader regional evidence context for the single-point in vivo data of Friend et al. [19]. It does not replace those data as the most direct empirical support.

3.2.2. Regional Background and Internal Heterogeneity of the Posterior Circulation

The posterior circulation is not homogeneous internally. Under isometric exercise in healthy young men, Washio et al. [2] found that the posterior cerebral artery cerebrovascular conductance index decreased significantly while the vertebral artery was unchanged. Posterior cerebral artery CO₂ reactivity increased significantly while the vertebral artery was unchanged. Individual exercise-induced changes in partial end-tidal CO₂ correlated only with changes in posterior cerebral artery blood flow velocity. The study accordingly emphasized that posterior circulation cerebral blood flow assessment should take site specificity into account.

The implication for this article is that the posterior circulation low reserve and high passivity asserted by the hypothesis should not be extrapolated as a uniform property of the entire vertebrobasilar system. It should be localized to the vertebral artery V2 segment as the peripheral input site. The intracranial posterior cerebral artery segment may exhibit different regulatory behavior. This qualification does not weaken the core proposition of the hypothesis. Rather, it makes the anatomical localization of the posterior circulation as a mechanical amplification site more precise.

3.2.3. In Vivo Wall-Stiffness Evidence and SWE Reliability

The difference in arterial wall response to mechanical loading between the anterior and posterior circulations has been directly measured at the in vivo mechanical level in humans. Using shear-wave elastography (SWE), Thomas et al. [18] found that in the neutral head position, vertebral artery wall stiffness was already significantly higher than that of the internal carotid artery. During contralateral head rotation, the vertebral artery wall became significantly stiffer (P = 0.05), whereas the internal carotid artery wall became significantly softer (P = 0.01). This pattern is highly consistent in direction with the physiological asymmetry demonstrated by Friend et al. [19]. The posterior circulation artery tends toward passive stiffening under mechanical loading. The anterior circulation artery exhibits compensatory softening. This provides in vivo mechanical evidence for the posterior circulation low reserve and high passivity asserted by the hypothesis.

The methodological basis of this study follows the validation of SWE reliability for cervical arterial walls by Thomas et al. [27]. In 30 healthy subjects, the intraclass correlation coefficient was 0.81 for the internal carotid artery wall and 0.76 for the vertebral artery wall. The standard errors of measurement were 0.16 m/s and 0.34 m/s, respectively. This reliability evidence indicates that SWE measurement in the perivascular region of the cervical vessels is technically feasible and reliable. It provides a methodological premise for the head-rotation study of Thomas et al. [18] and for the in vivo validation of this hypothesis.

3.2.4. Sex, Stress Condition, and Site Boundaries

The sex differentiation of posterior circulation dynamic cerebral autoregulation reported by Friend et al. [19] needs to be defined in conjunction with evidence under baseline conditions. Chen et al. [29] found under normoxic resting conditions that posterior cerebral artery dynamic cerebral autoregulation and neurovascular coupling did not differ significantly between healthy men and women. Friend et al. [19] observed a male-specific posterior circulation dynamic cerebral autoregulation reduction under hypoxic conditions. This may be stress-condition-dependent rather than a general sex difference in posterior circulation dynamic cerebral autoregulation.

This boundary definition does not affect the core proposition of the hypothesis regarding posterior circulation low reserve and high passivity. It does limit the possibility that the expression strength of this proposition may differ across sex and stress conditions. Chen et al. [29] measured the posterior cerebral artery rather than the vertebral artery. Their negative finding supports sex similarity of the intracranial posterior circulation segment under baseline conditions, rather than direct extrapolation to the vertebral artery. Washio et al. [2] and Chen et al. [29] constrain the expression of posterior circulation asymmetry by site and by sex under baseline conditions. Friend et al. [19] and Thomas et al. [18] further constrain it by sex-specific hypoxia response and by unstratified SWE findings. Together these findings indicate that the expression of asymmetry depends on sex, stress condition, and measurement site, and that these dimensions should be prespecified rather than collapsed into one boundary statement.

3.2.5. VAH as Logical Corroboration and Structural Reference

The VAH literature provides parallel corroboration for posterior circulation vulnerability. The systematic review by Mirkovic et al. [30] included 46 studies and evaluated the epidemiological, hemodynamic, and clinical evidence for VAH. The review reported that patients with VAH exhibit reduced vertebral artery blood flow, increased vascular resistance, impaired cerebrovascular reactivity, and altered collateral flow redistribution. VAH is frequently associated with posterior circulation ischemic stroke, transient ischemic attack, and vertebrobasilar insufficiency. It is also associated with vestibular dysfunction and non-stroke neurological syndromes such as migraine and episodic vertigo. The review accordingly proposed a context-dependent low-flow vascular vulnerability phenotype model. It emphasized that clinical significance depends on the interaction among reduced vertebral artery blood flow, collateral compensatory capacity, and acquired cerebrovascular risk factors.

This structural model is logically parallel to the functional evidence of Friend et al. [19]. Regardless of whether the low reserve originates from a congenital vessel-diameter variant or from an acquired mechanical constraint, the posterior circulation may exhibit a similar vulnerability phenotype. VAH is a congenitally fixed anatomical variant. This hypothesis concerns an acquired fascial mechanical constraint. What the two share is the logic of low reserve leading to vulnerability, not a specific pathological mechanism.

In this hypothesis, VAH evidence is not used for causal inference about the fascial-constraint-to-low-reserve chain. It does provide argumentative support for the low-reserve-to-vulnerability logic. This distinction is stated explicitly to align the declared evidence positioning with the actual argumentative weight. VAH evidence supports the proposition that low reserve predisposes to vulnerability. It does not support the proposition that acquired fascial constraint causes low reserve.

The systematic review and meta-analysis by Triantafyllou et al. [3] further provided prevalence data for vertebral artery anatomical variants. The pooled prevalence of vertebral artery dominance was 27.45%, hypoplasia 13.41%, and aplasia 5.39%. This indicates that congenital posterior circulation vessel-diameter variants are not rare in the population. If the validation cohort for the hypothesis is to include individuals with VAH, its sample size is estimable. Triantafyllou et al. [3] provided purely anatomical epidemiological data, not involving hemodynamics, dynamic cerebral autoregulation, or clinical phenotypes. These data cannot replace the functional and clinical argumentation of Mirkovic et al. [30] regarding low reserve leading to vulnerability.

Gaigalaite et al. [5] further showed that the effect of VAH on posterior circulation stroke or transient ischemic attack risk is significantly modified by the posterior configuration of the circle of Willis. Among individuals with VAH, the adult-type configuration with bilateral absence of the posterior communicating arteries was associated with higher risk (odds ratio 2.2). The fetal-type configuration was associated with significantly lower risk (odds ratio 0.35). Among individuals without VAH, however, the fetal-type configuration was instead associated with increased risk (odds ratio 1.76). This suggests that the structural corroboration of VAH cannot remain at the single dimension of vessel-diameter variant leading to low reserve. It should incorporate collateral configuration as an effect-modifying variable. In a validation cohort, if individuals with VAH are included, the posterior configuration of the circle of Willis should therefore be treated as a prespecified stratification variable or covariate.

Sato et al. [6] further supplemented the functional consequences of VAH from the perspective of dynamic cerebral blood flow regulation. In patients with unilateral VAH, CO₂ reactivity on the hypoplastic vertebral artery side was significantly reduced. In those with vertebral artery insufficiency, bilateral reduction resulted in an inability to maintain normal net vertebral artery CO₂ reactivity. Posterior circulation low reserve therefore no longer relies solely on the single functional dimension of dynamic cerebral autoregulation rate. It is also supported by evidence from the cerebrovascular reactivity dimension.

Between structural variants and clinical phenotypes, Neumann et al. [4] provided direct human evidence of functional compensatory strategies under stress conditions. In that study, central hypovolemia was simulated by lower-body negative pressure in hypertensive patients. Cerebral blood flow regulatory responses were compared between groups with and without VAH. Hypertensive patients without VAH showed significant decreases in basilar artery blood flow and total cerebral blood flow during hypovolemia. This suggests that their posterior circulation tolerates hypoperfusion under stress. In contrast, patients with VAH, although their resting posterior circulation blood flow was already lower, showed unchanged basilar artery blood flow and total cerebral blood flow during hypovolemia. However, the magnitude of increase in total peripheral resistance was significantly greater than in the group without hypoplasia. The authors accordingly proposed that patients with VAH do not lack regulatory capacity. Rather, they mobilize a stronger systemic pressor response to protect the already hypoperfused posterior circulation.

The implication for the hypothesis is that the low-reserve state of the posterior circulation does not necessarily manifest as passive decline under stress. It may be accompanied by a systematic shift in compensatory strategy, from local cerebrovascular regulatory reserve to systemic pressor mobilization. This is logically concordant with the positioning adopted by the hypothesis for posterior circulation low reserve as dependent on compensation. It also extends the operational definition of low reserve from a resting-state attribute to a stress-state difference in regulatory strategy. Its hypertensive population background and congenital vessel-diameter variant nature must be noted, and full boundaries are given in Section 9. This study cannot replace the dynamic cerebral autoregulation differentiation data of Friend et al. [19]. It also cannot serve as direct evidence that acquired fascial mechanical constraint leads to posterior circulation vulnerability.

3.2.6. Candidate Biological Mechanism and the Dose–Response Amplification Site

Beyond the functional and mechanical evidence, Wang et al. [7] proposed a candidate explanation at the biological mechanism level. Long-term changes in the vertebral artery hemodynamic environment accompanying cervical degeneration can progressively remodel the local vascular wall microenvironment through changes in vascular geometry and shear stress. This promotes endothelial stress, metabolic imbalance, and inflammatory signaling. Ultimately, it drives vertebral artery endothelial senescence and posterior circulation functional instability. This process can occur in the absence of overt arterial stenosis. The review positioned vertebral artery endothelial senescence as the central biological process linking chronic mechanical stress, disturbed blood flow, and posterior circulation perfusion instability.

The low reserve and high passivity of the posterior circulation may therefore be not merely a passive functional attribute. They may possibly reflect an active biological process driven by chronic changes in the mechanical environment. If the mechanical constraint from the investing fascia to the transverse foramen fascia does alter the local mechanical environment of the vertebral artery, then endothelial senescence markers may become candidate biomarkers of posterior circulation vulnerability. The study by Wang et al. [7] is a narrative review and provides no new original data or quantitative effect sizes. Its discussion context is cervical degeneration, not the mechanical constraint of investing fascial fibrosis transmitted through the deep fascial network. Direct mechanistic evidence linking endothelial senescence to the PIEZO–CGRP pathway addressed by the hypothesis is currently lacking. It therefore provides only a candidate biological mechanism background for posterior circulation vulnerability. It cannot serve as direct support for the hypothesis chain, nor can it replace the structural corroboration of Mirkovic et al. [30].

Combined with the extrasheath indirect pathway of the vertebral artery, the posterior circulation serves as a low-threshold, high-sensitivity mechanical amplification site in the hypothesis. The posterior circulation lacks the compensatory dilation reserve demonstrated by Friend et al. [19]. Any minor constraint on its effective luminal radius is more likely to produce perfusion perturbation directly, rather than first being buffered by vascular resistance adjustment as in the anterior circulation. The characteristic output of the posterior circulation is therefore not decompensation after compensation. It is rather a dose–response relationship among fascial attachment stiffness, effective luminal radius, and perfusion fluctuation. Its slope should theoretically be steeper than the corresponding relationship in the anterior circulation.

The V2-segment flow velocity reduction, V3-segment flow velocity and resistance index elevation, and their receiver operating characteristic thresholds reported by Meng et al. [24] in RVAS provide a clinically detectable threshold reference for this dose–response relationship. However, their etiology is osseous or anatomical compression and the stimulus is acute head rotation. These cannot be directly equated with chronic fascial constraint. The single-case feasibility reports of Dadsetan et al. [21] and Nguyen et al. [22] suggest that the V2 segment can be a target of fibrous-band-type soft-tissue mechanical compression. Their full boundaries are given in Section 2.3 and Section 9.

3.3. Predictive Significance of the Asymmetry

3.3.1. Directional Phenotypic Predominance as Prediction, Not Causation

The early marker of the anterior circulation is more likely to be a vascular reactivity change detectable even in the compensated state. Its phenotype tends toward headache and ocular symptoms. The early marker of the posterior circulation is more likely to be perfusion instability at rest or with postural change. Its phenotype tends toward vertigo and balance disturbance.

This article distinguishes three related terms. Phenotypic predominance denotes a directional phenotypic tendency. Phenotypic differentiation denotes the process by which phenotypes diverge. Phenotypic spectrum denotes the overall range of phenotypes. The phenotypic predominance described here is a directional prediction rather than established causality. Friend et al. [19] and Thomas et al. [18] demonstrated differentiation at the dynamic cerebral autoregulation and wall-stiffness levels. An independent phenotype–perfusion association study is still needed to bridge from this differentiation to clinical phenotypic differentiation. This article uses predominance rather than determination to indicate that this link has not been established. This phenotype mapping is currently based mainly on the indirect association between dynamic cerebral autoregulation or wall-stiffness differentiation and clinical phenotypes. Direct mediation studies of wall-stiffness differentiation leading to phenotypic differentiation are still lacking. This article lists such studies as the third-priority validation task.

When discussing individual-level expression of this predominance, the term "individual susceptibility state" is used, not "regional mechanical vulnerability." Regional mechanical vulnerability denotes the differential response among circulatory regions. Individual susceptibility state denotes the threshold lowering of a given person. The two terms are not interchangeable. The phenotypic tendencies described here concern the individual susceptibility state, not the regional mechanical vulnerability itself.

3.3.2. Operable In Vivo Testing Paradigm and Mediation Design

An operable bridging design can draw on the acute hypoxic dynamic cerebral autoregulation paradigm of Friend et al. [19] and the head-rotation SWE paradigm of Thomas et al. [18]. It would jointly measure the vertebral artery–to–internal carotid artery RoR ratio, wall-stiffness ratio, and vertigo or headache phenotype in the same subjects. This design would test whether dynamic cerebral autoregulation or wall-stiffness differentiation mediates or modifies phenotypic differentiation. The design is not bound to a specific mediation model. It can test the direction and magnitude of two pathways. The first is posterior circulation low reserve leading to perfusion fluctuation leading to vertigo predominance. The second is anterior circulation compensation threshold leading to headache or ocular predominance.

An operable in vivo testing paradigm can directly draw on the designs of Friend et al. [19] and Thomas et al. [18]. It would use the vertebral artery–to–internal carotid artery RoR ratio and wall-stiffness ratio as dual quantitative indicators of asymmetry. It would then test their association with investing fascial mechanical parameters and posterior circulation perfusion outcomes. If effective luminal radius is used as a mediating variable, the effective radius-to-flow path coefficient (equivalently, the effective radius-to-flow slope) in the posterior circulation should be steeper than in the anterior circulation. The anterior circulation should exhibit reduced apparent sensitivity owing to the intervention of autoregulation. This prediction is not bound to a specific power-law form. Its direction and magnitude can still be tested through mediation analysis. The term "path coefficient" and the term "slope" are used interchangeably in this article.

Washio et al. [2] suggested that the vertebral artery and posterior cerebral artery within the posterior circulation should be treated as separate analytical nodes. They should not be merged into a single posterior circulation outcome. The negative findings of Chen et al. [29] suggest that the effect size of this mediation pathway may differ between men and women. In women, a different effect pattern may be exhibited owing to sex similarity of baseline dynamic cerebral autoregulation. This sex difference should be pre-stratified in cohort design.

The VAH model summarized by Mirkovic et al. [30] suggests that, in a validation cohort, if individuals with congenital posterior circulation vessel-diameter variants are included, the slope of their effective radius-to-flow relationship may be steeper. This structural factor should be considered as a stratification variable or covariate. The prevalence data provided by Triantafyllou et al. [3] provide epidemiological parameters for sample-size estimation of this subgroup. Neumann et al. [4] suggested that, under stress conditions, individuals with VAH may exhibit compensatory strategies different from those without. Therefore, if such individuals are included in a validation cohort, the systemic hemodynamic response under stress, such as change in total peripheral resistance, should be treated as a prespecified effect-modifying variable. Gaigalaite et al. [5] further suggested that the posterior configuration of the circle of Willis should be a prespecified stratification variable or covariate within the VAH subgroup. This avoids confounding of the VAH–outcome association by configurational differences. Sato et al. [6] suggested that CO₂ reactivity can provide a supplementary functional endpoint of posterior circulation reserve. It can be used to characterize the cerebrovascular reactivity dimension of VAH-related low reserve.

The endothelial senescence mechanism proposed by Wang et al. [7] suggests that candidate biomarkers of posterior circulation vulnerability may include endothelial senescence-related indicators. However, their association with investing fascial mechanical parameters still requires independent testing. The RVAS dynamic ultrasound thresholds provided by Meng et al. [24], namely V3 flow velocity greater than 74.68 cm/s, V3 resistance index greater than 0.71, and area under the curve of 0.80, provide a binary outcome reference for posterior circulation hemodynamic consequences. They can be used to test the association between investing fascial mechanical parameters and V2 or V3 flow changes. However, these thresholds derive from patients with suspected RVAS and an acute head-rotation paradigm. Extrapolation to physiological or subclinical populations requires independent calibration.

4. Five-Stage Causal Chain of the Hypothesis

4.1. Investing Fascial Fibrosis

4.1.1. TGF-β–Myofibroblast Activation as the Core Fibrotic Logic

When chronic forward head posture, prolonged stress, aging, and chronic low-grade inflammation act synergistically, normal fascial remodeling may shift toward pathological fibrosis. Its molecular core is TGF-β–driven myofibroblast activation. Using palmar fascial fibrosis, or Dupuytren disease, as a referable fascia-specific pathological model, Pirri [31] integrated the intrinsic molecular logic of this process. Genetic susceptibility and cytogenetic instability converge on extracellular matrix remodeling, aberrant TGF-β and Wnt/β-catenin signaling, cytoskeletal stabilization, and immune-inflammatory amplification. Epigenetic dysregulation further maintains fibroblasts in a persistent myofibroblast state. The increase in tissue stiffness itself can, through mechanochemical positive feedback, render the fibrotic state self-sustaining.

Under chronic mechanical loading and a low-grade inflammatory environment, the investing fascia may therefore follow a similar intrinsic fascial pathological logic. It may shift from reversible adaptive remodeling to a fibrotic self-sustaining state maintained by TGF-β–myofibroblast positive feedback. The result is thickening of the investing fascia, loss of elastic buffering capacity, and elevated residual passive tension.

4.1.2. Ang II–AT1R–YAP as a Candidate Biochemical Branch

A second fibrotic branch, Ang II–AT1R–YAP, exists in human deep fascial fibroblasts. Caroccia et al. [32] demonstrated in fibroblasts derived from human deep fascia, specifically thoracolumbar fascia, that Ang II activates YAP via AT1R through dephosphorylation and nuclear translocation. Sustained exposure upregulates type I and type III collagen and hyaluronan-binding protein 2 (HABP2). It also promotes fibroblast proliferation and migration. This effect can be blocked by the AT1R antagonist irbesartan or the YAP inhibitor verteporfin.

This evidence comes from the thoracolumbar fascia rather than the cervical investing fascia. It cannot be directly equated with the conclusion that investing fascial fibrosis is already driven by this pathway. Its implication is that fibroblasts of the deep fascial family possess the molecular capacity to converge biochemical signals onto the YAP mechanotranscriptional program. This provides a candidate branch to be verified for a fibrotic self-sustaining state of the investing fascia under conditions of chronic low-grade inflammation or local renin–angiotensin system activation. The branch can be linked mechanistically to PIEZO mechanotransduction. YAP, as a mechanosensitive transcription factor, may converge chronic mechanical loading and biochemical fibrotic signals into a self-reinforcing circuit of collagen deposition and increased tissue stiffness. However, this linkage remains inferential in the cervical investing fascia–sheath wall continuum and requires independent verification.

4.1.3. PIEZO1–Wnt2/Wnt11–CCL24 Fibrotic Loop as a Transferable Candidate Loop

He et al. [33] provided a third candidate branch in human fibrotic skin and mouse skin fibrosis models. In this model, PIEZO1 directly drives a fibrotic positive feedback loop. According to the original report, matrix stiffening activates PIEZO1. PIEZO1 activation then drives Wnt2/Wnt11–CCL24 signaling, fibroblast activation, extracellular matrix secretion, and further stiffening. This forms a positive feedback loop. In mouse models, Piezo1 is upregulated in fibrotic skin. Piezo1 knockdown causes skin fibroblasts to lose their fibroproliferative phenotype on stiff substrates. Adeno-associated virus (AAV)–mediated Piezo1 knockdown improves skin fibrosis and skin stiffness in mice.

This evidence transforms the fibrotic self-sustaining state of Section 4.1 and the PIEZO1 mechanotransduction of Section 4.4 from two parallel steps into a candidate self-sustaining loop transferable from a human fibrotic connective-tissue precedent. The loop is designated the PIEZO1–Wnt2/Wnt11–CCL24 fibrotic loop. Investing fascial fibrosis leads to increased stiffness, further PIEZO1 activation, and more fibrosis. Strict qualifications must be retained. The model is skin fibrosis, not fascia, and certainly not the investing fascia. The species evidence consists mainly of mouse in vivo studies and human skin fibroblasts or tissue, not human cervical fascia in vivo. It does not involve the PIEZO1-to-CGRP neural side. It cannot replace the functions of Mikhailov et al. [34], Liao et al. [35], and Gkouzioti et al. [36] in the core molecular transduction chain. It also does not close Gap 1, the investing fascia-to-sheath wall transmission coefficient, or Gap 2, the reliability of sheath-wall mechanical measurement.

This evidence therefore provides only a PIEZO1 positive-feedback precedent in human fibrotic skin. This precedent is transferable as a candidate mechanism for investing fascial fibrosis. It cannot be interpreted as evidence that investing fascial fibrosis is already driven by PIEZO1 positive feedback.

4.1.4. Boundaries and Unresolved Gaps

The three branches above provide candidate molecular logic for a fibrotic self-sustaining state of the investing fascia. These branches are TGF-β–myofibroblast, Ang II–AT1R–YAP, and the PIEZO1–Wnt2/Wnt11–CCL24 fibrotic loop. However, none has been directly verified in human cervical investing fascia under chronic mechanical loading. The TGF-β framework is derived from palmar fascial fibrosis as an analogical model. The Ang II–AT1R–YAP evidence derives from thoracolumbar fascia and biochemical stimulation. The PIEZO1 positive-feedback evidence derives from skin fibrosis. All three are transferable references, not direct evidence for the cervical investing fascia. The fibrotic initiation side therefore remains at a candidate-mechanism stage. Its direct verification in the target tissue is part of the first-priority histological set, as discussed in Section 8.1 and Section 8.4.3.

4.2. Mechanical Conduction Through the Deep Fascial Network

4.2.1. Conduction Route and Mechanical Feasibility

After fibrosis, the investing fascia can no longer effectively dissipate shear stress. Abnormal stress is transmitted deepward through the interlaminar pathway of investing fascia, pretracheal fascia, prevertebral fascia, and carotid sheath wall. It may also pass through the prevertebral fascia to the transverse foramen region. The actual efficiency of cross-layer conduction has not yet been directly measured. Existing evidence supports only mechanical feasibility.

Pires et al. [1] demonstrated that the pressure required to reach the deep fascia is approximately 6 N/cm² and is unaffected by individual mechanosensitivity or fat thickness. However, this threshold describes the feasibility of surface pressure reaching the deep fascia. It does not describe the conduction efficiency from the investing fascia's own tension to the sheath wall (Gap 1). Four candidate implementation mechanisms for functional continuity without anatomical fusion are listed in Section 2.2.2. They are radial constraint by perivascular connective tissue, intrasheath pressure transmission through sheath-wall deformation, shear transmission at the fascia–vessel interface, and an indirect route through the prevertebral fascia to the transverse foramen fascia. None of these mechanisms has been directly measured for efficiency in the investing fascia–sheath wall pathway.

4.2.2. Magnitude References from Perivascular Constraint and Fascial Force Transmission

At the mechanical magnitude level, Liu et al. [37] demonstrated in an ex vivo carotid artery model that the radial constraint imposed by perivascular connective tissue on the vessel wall can be quantified as an effective perivascular pressure. This pressure amounted to more than 50% of the intravascular pressure. At 100 mmHg, the fully constrained state reduced the circumferential wall stretch ratio by approximately 20%. It also reduced the mean circumferential stress by approximately 70% compared with the unconstrained state. That study was not specific to the carotid sheath or investing fascia. It cannot replace direct measurement of the investing fascia–sheath wall transmission coefficient. However, its magnitude indicates that if sheath-wall fibrosis does alter the perivascular constraint conditions of intrasheath vessels, the mechanical consequences may be substantial. It provides a magnitude reference for computational simulation.

Querry et al. [12] showed, using balloon catheter measurements in humans, that the transmission efficiency from external cervical pressure to the carotid sinus can reach 82%–89%. This indicates that external cervical mechanical input is not substantially attenuated. However, this pathway is external pressure to the carotid sinus, not investing fascia to sheath wall. It provides only a magnitude reference for the transmission coefficient (see Section 2.1.2). Wilke et al. [8] systematically reviewed the anatomical and functional evidence for fascial force transmission. They noted that fascia can participate in mechanical force transmission. However, the evidence they integrated includes cadaveric, animal, and in vivo studies. It cannot be directly equated with the transmission coefficient of the cervical investing fascia–sheath wall pathway. Kalichman [9] further integrated ex vivo myofascial force transmission studies, suggesting that the fascial network can transmit up to approximately 30% of mechanical force. However, these data do not derive from the specific cervical investing fascia-to-sheath wall pathway. In vivo human evidence is limited. This value provides only a magnitude upper-bound reference for the fascial network itself.

4.2.3. Anisotropy, Layered Sheath Wall, and Segmental Heterogeneity

Aparici-Gil et al. [10], through uniaxial, biaxial, and planar tensile testing, directly quantified the nonlinearity, incompressibility, and marked anisotropy of deep fascia. Under uniaxial tension, longitudinal stress was approximately 6.6 times the transverse stress, with longitudinal stress of 3.96 MPa versus transverse stress of 0.6 MPa. Under planar tension the longitudinal-to-transverse ratio was approximately 3.9 to 1. Under equibiaxial tension it was approximately 2.6 to 1. The authors further proposed a coupled exponential constitutive model capable of simultaneously reproducing all three strain states.

The implication is that the stress conduction efficiency from the investing fascia to the sheath wall may be significantly higher along the fiber direction than perpendicular to it. If a finite element model treats the deep fascia as an isotropic material, the estimation of the transmission coefficient will show systematic bias. Sensitivity analysis of the transmission coefficient should therefore be performed separately along the fiber direction and perpendicular to the fiber direction. The anisotropy ratio should be treated as a parameter to be tested.

The layered histology of the rabbit carotid sheath by Zhao et al. [14] suggests that the sheath wall itself is mechanically layered and heterogeneous. The arterial inner fascial layer thickened earliest and most markedly. In computational simulation, the sheath wall should therefore be modeled in layers rather than treated as a single homogeneous material. The histology of adult human cervical segments by Hayashi [15] further suggests that the sheath wall exhibits laminar adventitia plus common sheath layering with inter-individual and site variation in thickness. However, the finding that the common sheath does not fuse with the superficial fascia requires that the functional continuity from the investing fascia to the sheath wall be treated as a premise to be verified (see Section 2.2.2).

The human embryological evidence of Martínez-Sanz et al. [11] suggests a developmental anatomical association between the middle deep cervical fascia and lymphatic or venous structures. It also suggests that the deep fascial network may participate in regulating the drainage environment of these conduits. However, this evidence is developmental-stage and descriptive or associative. It cannot be directly equated with mechanical transmission under adult pathological conditions. Schachtel et al. [16] studied the fascial composition of the anterior sheath wall in the skull-base segment and suggested that the fascial source of the sheath wall may differ across segments. This further supports the view of the sheath wall as a regionally heterogeneous mechanical interface.

4.2.4. Unified Gap Statement

The single-case feasibility reports of Hou et al. [20], Dadsetan et al. [21], and Nguyen et al. [22] suggest that perivascular or fibrous-band soft tissues can compress extracranial large vessels in vivo. Their full boundaries are given in Section 2.3 and Section 9. This section presents a unified gap statement for the full text. First, Gap 1: the investing fascia-to-sheath wall transmission coefficient has not yet been directly measured. Second, Gap 2: human measurement reliability studies of the mechanical properties of the sheath wall itself are lacking. Third, all currently available direct human evidence consists of single-case feasibility reports and provides only single-case feasibility evidence.

A transitional measurement framework for the Gap 1–Gap 2 pair is specified in 8.4.2a. It does not close Gap 2; it defines what can be measured, and with what uncertainty, before Gap 2 is closed, and what can be estimated about Gap 1 in that interim.

4.3. Involvement of Intrasheath and Parasheath Structures

4.3.1. Intrasheath Structures and Unequal Mechanical Loading

Within the sheath, the adventitia of the internal carotid artery and internal jugular vein bears traction. The vagus nerve bears mechanical deformation. Adjacent to the sheath, the cervical sympathetic trunk is tightly adherent to the posterior sheath wall, as described by Kiray et al. [25] and Civelek et al. [26]. It is a mechanically highly coupled adjacent structure. The carotid sheath region is adjacent to the cervical terminal of the meningeal lymphatic drainage and glymphatic outflow pathways. Clapham et al. [38] confirmed that some deep cervical lymph nodes are located directly within the carotid sheath, as detailed in Section 5.2.1. A change in sheath-wall mechanical state may affect their drainage efficiency through the mechanical sensitivity of the lymphatic pathway.

4.3.2. Parasheath Structures and the Intrasheath Lymphatic Terminal

The distribution of sympathetic fibers on the target vessel side and its boundaries are given in Section 5.3.2, based on Struthoff et al. [39]. The single-case feasibility report of Hou et al. [20] suggests that perivascular soft-tissue thickening can compress the C1 segment of the internal carotid artery in vivo.

4.3.3. Layered and Segment-Dependent Involvement

The three-layered fascial architecture in the rabbit described by Zhao et al. [14] and the laminar adventitia plus common sheath layered description of adult human cervical segments by Hayashi [15] suggest that the fascial layer thickness and mechanical response on the arterial side, venous side, and nerve side within the sheath may differ (see Section 2.2.1). The effect of a change in sheath-wall mechanical state on each content is therefore not equal. The arterial side may be the interface with the most active mechanical response. Hayashi [15] also reported that the thickness of the two layers exhibits inter-individual and site-dependent variation. This suggests that this inequality is further modified by individual and segmental factors. Schachtel et al. [16] studied the fascial composition of the anterior sheath wall in the skull-base segment and suggested that the fascial source and number of layers of the sheath wall may differ across segments. The effect of a change in sheath-wall mechanical state on each content may therefore also be segment-dependent.

This inequality is not a weakness of the hypothesis but a testable prediction. The arterial and lymphatic terminals may exhibit the most active mechanical response. The venous side may be the earliest indicator of measurable hemodynamic consequences, as discussed in Section 5.1.3. The sympathetic trunk may require threshold crossing before its distal effects become hemodynamically significant, as discussed in Section 5.3.3. Validation design should therefore assign separate endpoints to each component rather than treating sheath-wall mechanics as a single exposure.

4.4. PIEZO–CGRP Signaling

This section uses PIEZO–CGRP signaling as an umbrella term but distinguishes three loops: the PIEZO2–CGRP neural loop, the PIEZO1–Wnt2/Wnt11–CCL24 fibrotic loop, and CGRP–PIEZO1 endothelial crosstalk. The three loops share the same upstream mechanical environment but are not the same molecular pathway.

4.4.1. Default PIEZO1 and PIEZO2 Division of Labor

To avoid ambiguity arising from alternating PIEZO1 and PIEZO2 terminology, the default division of labor is stated first. The following evidence is stratified accordingly.

First, CGRP release at the nerve terminal side is mediated mainly by PIEZO2 by default. This default is based on Tereshenko et al. [40], who reported predominance of PIEZO2 in human cervical fascia, and on Liao et al. [35], who reported PIEZO2–CGRP or substance P positive feedback.

Second, structural remodeling, stiffening, and mechanosensation of the vascular wall and sheath wall are mediated mainly by PIEZO1 by default. This default is based on Zhang J et al. [41], Zhang FR et al. [42], Luu et al. [43], and Beech [44].

Third, lymphatic endothelial mechanotransduction and meningeal lymphatic drainage are mediated by PIEZO1 by default. This default is based on Nonomura et al. [45], Choi et al. [46], and Matrongolo et al. [47].

Fourth, PIEZO1 and PIEZO2 coexist at the sympathetic ganglion level, based on Alba et al. [48]. Their functional division remains undetermined.

Fifth, PIEZO1 activation can trigger CGRP release in experimental preparations. This demonstrates the capacity for PIEZO1-mediated CGRP release. It does not establish physiological predominance at the neural terminal, where the default mediator remains PIEZO2. CGRP–PIEZO1 crosstalk is placed mainly at the endothelial or vascular level, based on Gkouzioti et al. [36]. It is not asserted as the same neural-terminal loop.

If subsequent validation in the target tissue shows the opposite or substantially overlapping division, the corresponding step of the hypothesis chain should be revised rather than the whole hypothesis rejected. An error in the neural-terminal division would directly affect the CGRP signaling step in Section 4.4. An error in the vascular-wall division would mainly affect the arterial remodeling step in Section 5.1.

4.4.2. Neural-Terminal Evidence: PIEZO2–CGRP and Substance P Positive Feedback

On the nerve terminal side, the evidence for PIEZO1 derives mainly from the functional experiments of Mikhailov et al. [34], the human endothelial cell crosstalk experiments of Gkouzioti et al. [36], and the PIEZO1-dependent structural remodeling and mechanosensory evidence established by Zhang J et al. [41], Zhang FR et al. [42], and Luu et al. [43] in vascular smooth muscle cells. The evidence for PIEZO2 is concentrated in the trigeminal nerve root compression model of Liao et al. [35] and the human cervical fascia histological evidence of Tereshenko et al. [40].

At the level of synergy and cross-validation, Alba et al. [48] and Zeng et al. [49] address the synergistic action of PIEZO1 and PIEZO2 from the perspectives of human histological distribution and functional necessity for the baroreceptor reflex, respectively. Nonomura et al. [45], Choi et al. [46], and Matrongolo et al. [47] provide evidence for PIEZO1 at the level of lymphatic endothelial cells and meningeal lymphatic drainage function. Konishi et al. [50] provide a pathological reference for upregulated PIEZO1 expression in myofibroblasts of human carotid plaques. He et al. [33] provide a mechanistic reference for the PIEZO1–Wnt2/Wnt11–CCL24 fibrotic loop in fibroblasts from human fibrotic skin and mouse skin fibrosis models, as discussed in Section 4.1.3. The term PIEZO channel is used only in framework-level statements. At each specific argumentation step, the subtype is specified according to the above stratification.

The evidence that PIEZO1 activation can trigger CGRP release is derived from the original experimental study of Mikhailov et al. [34]. That study demonstrated that the PIEZO1-specific agonist Yoda1 can activate approximately 35% of isolated trigeminal neurons and promote CGRP release in a hemiskull preparation. PIEZO1 was expressed in the nociceptive fibers of the trigeminal nerve. This is evidence that PIEZO1 activation can trigger CGRP release. It does not establish that PIEZO1 is the predominant physiological mediator at the neural terminal, where the default mediator remains PIEZO2. Della Pietra et al. [51] systematically argued, on this basis, for the overall role of PIEZO channels in migraine mechanotransduction. PIEZO channels are distributed in key elements of the trigeminovascular nociceptive system. PIEZO channels on meningeal afferent fibers exhibit prolonged functional activity. PIEZO channels can be activated by shear stress and pulsatile blood flow. The pulsatile character of migraine pain points precisely to this mechanical detection mechanism. CGRP release and PIEZO activation can form a positive feedback at the neural-terminal level, with the PIEZO2–CGRP neural loop as the default principal mediator. CGRP can in turn enhance mechanosensitivity.

At the level of PIEZO subtype division of labor, Zhao L et al. [52] noted that PIEZO1 and PIEZO2 are not interchangeable mechanosensors in neurological disorders. The two differ in expression distribution, activation threshold, and downstream signaling. PIEZO1 is more associated with neuroinflammation, blood–brain barrier disruption, and vascular or endothelial mechanical responses. PIEZO2 is more associated with mechanotransduction at sensory nerve terminals and with mechanical allodynia. That review covers generalized neurological disorders such as stroke, Alzheimer disease, traumatic brain injury, and glioma. It does not specifically address migraine or cervical fascial mechanotransduction. It therefore provides only a general background for the PIEZO subtype division of labor in the hypothesis. It cannot replace direct evidence for the target tissue.

Liao et al. [35] provided direct experimental evidence for PIEZO2–CGRP or substance P positive feedback in a rat model of chronic trigeminal nerve root compression. Nerve root compression induces neuroinflammation, which upregulates Piezo2 and CGRP or substance P expression through ATP–Ca²⁺–PKC signaling. This forms a Ca²⁺–CGRP or substance P–Piezo2 positive feedback loop that drives orofacial mechanical allodynia. Inhibition of cAMP or knockdown of Piezo2 can alleviate mechanical allodynia. This evidence places PIEZO2 at the core of the neural-terminal positive feedback loop. In subtype terms, it echoes the histological evidence of Tereshenko et al. [40], who detected predominantly PIEZO2 in human cervical fascia.

The initiating event of that study was direct mechanical compression of the nerve root. The disease model was trigeminal neuralgia rather than investing fascial fibrosis or functional headache. The tissue localization was the trigeminal ganglion–Merkel cell axis rather than the cervical fascia–sheath wall continuum. It supports the mechanistic capacity that PIEZO2–CGRP or substance P positive feedback can drive mechanical pain. It does not support the specific causal chain that investing fascial fibrosis drives CGRP release via PIEZO2.

At the reverse limb of CGRP–PIEZO1 crosstalk, Gkouzioti et al. [36] provided human cell evidence for indirect crosstalk between CGRP and PIEZO1 in human induced pluripotent stem cell–derived endothelial cells. When PIEZO1 was activated after 1 hour of CGRP pretreatment, the evoked intracellular Ca²⁺ response was higher than with acute co-exposure to the two. This effect could be blocked by a CGRP receptor antagonist. Moreover, CGRP sensitization combined with PIEZO1 activation also promoted remodeling of the focal adhesion protein paxillin. At the vascular endothelial level, CGRP signaling can therefore indirectly enhance the responsiveness of the PIEZO1 pathway. This provides a mechanistic reference for the endothelial limb of CGRP–PIEZO1 crosstalk in the hypothesis. It involves acute pharmacological stimulation in vitro, not chronic in vivo constraint. Full boundaries are given in Section 9.

4.4.3. Vascular-Wall, Lymphatic-Endothelial, and Sympathetic-Ganglion Evidence

Within the specific division of labor of PIEZO1, the lymphatic endothelial cell level provides a complete causal chain. Nonomura et al. [45] provided causal evidence for PIEZO1 mechanotransduction. Endothelial-specific PIEZO1 deletion severely impaired lymphatic valve formation. Yoda1 activation of lymphatic endothelial PIEZO1 induced cytoskeletal and intercellular adhesion remodeling. Choi et al. [46] further advanced this molecular step to the level of meningeal lymphatic drainage function. In mice, Piezo1 is required for the normal development and function of meningeal lymphatic vessels. Transgenic overexpression or Yoda1 activation of Piezo1 is sufficient to increase cerebrospinal fluid outflow and to alleviate pathological cerebrospinal fluid accumulation in mouse models of hydrocephalus and Down syndrome. Matrongolo et al. [47] further demonstrated that, in mice with craniosynostosis and in aged mice, Yoda1 treatment reduced intracranial pressure and improved cerebrospinal fluid flow. It also restored meningeal lymphangiogenesis, drainage to deep cervical lymph nodes, and brain–cerebrospinal fluid perfusion. The study accordingly proposed that cerebrospinal fluid provides a mechanical force for the growth and maintenance of meningeal lymphatic vessels. It also proposed that Piezo1 agonists are a potential therapeutic option under conditions of elevated intracranial pressure or reduced cerebrospinal fluid flow.

This provides a cell-type-matched molecular interface at the in vivo functional level for the lymphatic pathway's mechanical force leading to PIEZO1 leading to altered meningeal lymphatic drainage function. It further demonstrates that PIEZO1 is not only a functionally essential node but also an intervenable node whose drainage function can be restored by pharmacological activation. This evidence involves mouse models and Yoda1 activation, not chronic fibrotic constraint. Full boundaries are given in Section 9.

At the human histological level, Alba et al. [48] detected PIEZO1 and PIEZO2 expression in the sensory nerve structures of the wall of the normal human carotid sinus. PIEZO2 was present in type I glomus cells of the carotid body, whereas PIEZO1 was found in only a small subset of type I cells. Notably, approximately 71% of neurons in the superior cervical ganglion were PIEZO1-positive and 86% were PIEZO2-positive. This provides a human histological landmark for the propositions of the hypothesis that a PIEZO molecular apparatus exists in the carotid region and that the sympathetic ganglion itself may possess direct mechanosensitivity. The study describes protein distribution in normal tissue. It does not demonstrate that PIEZO expression would change or be activated under conditions of investing fascial fibrosis or abnormal mechanical loading. Presence does not establish functionality. It cannot replace functional experiments such as electrophysiology or calcium imaging.

Konishi et al. [50] detected upregulated PIEZO1 expression in myofibroblasts of the plaque fibrous cap in human carotid endarterectomy specimens. The PIEZO1 score of symptomatic plaques was significantly higher than that of asymptomatic plaques. It was associated with plaque rupture, thin fibrous cap, and microcalcification. This provides a preliminary histological reference for the detectability of PIEZO1 in human carotid wall pathology. It is a pilot study with an atherosclerotic background and a cross-sectional design. Full boundaries are given in Section 9. He et al. [33] provided human tissue or cell and mouse in vivo evidence for the PIEZO1–Wnt2/Wnt11–CCL24 fibrotic loop in fibroblasts, as discussed in Section 4.1.3. This cannot replace direct verification in investing fascial fibroblasts.

At the level of PIEZO functional causality, Zeng et al. [49] demonstrated by genetic means that PIEZO1 and PIEZO2 are mechanosensors essential for the baroreceptor reflex. The full argumentation of this evidence in the vagal pathway is given in Section 5.3.

The spatial presence of this molecular logic in human cervical fascia has been directly demonstrated by Tereshenko et al. [40]. That study detected Piezo2 expression in the trapezius fascia of patients with occipital neuralgia. Like the investing fascia, the trapezius fascia belongs to the superficial layer of the deep cervical fascia. The study observed significant upregulation of CGRP in the fascial nerve components and a 71% increase in the nociceptive axon population. Sympathetic nerve fibers that were tyrosine hydroxylase (TH)–positive were significantly present in the patients' fascia but absent in controls. This provides a direct human tissue-level landmark for the proposition of the hypothesis that the investing fascia–carotid sheath continuum possesses PIEZO–CGRP mechanotransduction capacity and sympathetic pathway involvement.

On the arterial side, the in vivo causal role of PIEZO1 in the mechanical response of the carotid artery has been supported by two independent lines of evidence. Zhang J et al. [41] demonstrated that vascular smooth muscle cell–specific knockout of Piezo1 can attenuate simulated microgravity–induced increases in carotid artery stiffness, wall thickening, and fibrosis. Zhang FR et al. [42] further demonstrated, in a carotid guidewire injury and partial ligation–induced hemodynamic disturbance or luminal stenosis model, that smooth muscle cell Piezo1 is essential for neointimal hyperplasia and phenotypic switching. In rodent models, Piezo1 expression in the neointima was upregulated approximately 500-fold relative to Piezo2. The authors positioned it as the principal ion channel in the carotid wall response to mechanical stimuli. Together, the two provide in vivo support for the hypothesis that PIEZO1-dependent structural remodeling occurs in the carotid wall under abnormal mechanical loading. The mechanical paradigm of the second line of evidence is closer to the luminal constraint scenario of interest to the hypothesis.

At the functional level, Luu et al. [43] further demonstrated that mechanosensation in vascular smooth muscle cells depends on the rapid opening of Piezo1. Piezo1 inhibition can attenuate vascular aging and partially restore the dynamic contractile properties lost in senescent cells. This extends the role of PIEZO1 in the arterial wall from an effector of structural remodeling to a sensor of mechanical perception. It directly links in logic with the positioning adopted by the hypothesis that PIEZO1 activation triggers downstream signaling. Its aging background shares a temporal dimension with the age-related remodeling of investing fascial fibrosis. Beech [44] further integrated and noted that the role of PIEZO1 in vascular smooth muscle is not limited to acute mechanotransduction. It extends to aging-related structural stiffening of the vascular wall. This is logically concordant with the aging-related decline in mechanosensation demonstrated by Luu et al. [43] and the PIEZO1-dependent carotid structural remodeling demonstrated by Zhang et al. [41,42]. Together they support the dual positioning of PIEZO1 in the arterial wall as both a sensor and a node that drives remodeling.

The cell types in the above studies are all vascular smooth muscle cells or ex vivo vascular smooth muscle cell models. Their phenotypes include fibrosis or increased stiffness, neointimal hyperplasia or phenotypic switching, aging-related decline in mechanosensation, and aging-related vascular wall stiffening. These phenotypes differ from CGRP release from trigeminal nerve terminals. They support the causal role of PIEZO1 in the mechanical response of the arterial wall, rather than the neural step from PIEZO2 to CGRP to nociceptive signaling. The latter is still borne by Liao et al. [35], Gkouzioti et al. [36], and Della Pietra et al. [51]. Mikhailov et al. [34] provide evidence that PIEZO1 activation can trigger CGRP release. Full boundaries are given in Section 9.

4.4.4. Synthesis, Boundaries, and Upstream Step Awaiting Verification

On the neural-terminal side, Liao et al. [35] demonstrated the mechanistic capacity of PIEZO2–CGRP or substance P positive feedback in a neuropathic pain model. Mikhailov et al. [34] demonstrated the capacity of PIEZO1 activation to trigger CGRP release. Neither establishes physiological predominance at the neural terminal. At the endothelial and carotid-region level, Gkouzioti et al. [36] demonstrated CGRP–PIEZO1 crosstalk in endothelial cells. Alba et al. [48] documented the normal protein distribution of PIEZO1 and PIEZO2 in the carotid sinus and superior cervical ganglion. For the vagal or baroreceptor pathway, Zeng et al. [49] demonstrated the functional necessity of PIEZO1 and PIEZO2 in the baroreceptor reflex. In human cervical fascia and carotid pathology, Tereshenko et al. [40] demonstrated the presence and pathological upregulation of this molecular apparatus. Konishi et al. [50] demonstrated upregulated PIEZO1 expression in myofibroblasts of human carotid plaques. For lymphatic endothelial cells, Nonomura et al. [45] demonstrated the mechanotransduction necessity of PIEZO1. Choi et al. [46] and Matrongolo et al. [47] demonstrated in vivo regulation and pharmacological restoration of meningeal lymphatic drainage.

The upstream step nevertheless remains an inference requiring independent verification. This step links the mechanical tension generated by investing fascial fibrosis to the in vivo activation of PIEZO1 or PIEZO2.

At the level of the field evidence landscape, Della Pietra et al. [53] included 39 studies using PRISMA standards. The review confirmed the field evidence base for the involvement of mechanosensitive channels, including PIEZO, TRP, K2P, ASICs, and NMDA receptors, in migraine mechanotransduction. It simultaneously noted that this evidence system consists predominantly of preclinical animal models with scarce clinical research. The PIEZO branch accounts for only a minority of all included studies. Its functional direction has not yet fully converged in the existing literature. This evidence landscape provides a methodological background for positioning the PIEZO step of the hypothesis as having mechanistic capacity but requiring tiered validation. It does not constitute direct support for the specific investing fascia–sheath wall causal chain.

4.5. Threshold Lowering and Phenotypic Differentiation

4.5.1. Threshold-Dependent Five-Pathway Superposition

The abnormal signals amplified through the molecular steps produce superimposed and mutually amplifying effects along the five pathways: arterial, venous, lymphatic/glymphatic, sympathetic, and vagal. The result is a systematic lowering of the attack threshold. The five-pathway superposition is threshold-dependent rather than simultaneous, with the threshold defined by the sympathetic hemodynamic threshold. Below that threshold, the superposition mainly involves the arterial, venous, lymphatic/glymphatic, and vagal pathways. The sympathetic pathway joins only after its threshold is crossed. Its direction of contribution is modified by stimulation type, intensity, and anterior or posterior circulation site, as described by Kaur et al. [54], Meyer et al. [55], and Washio et al. [2]. The hypothesis does not claim that all five pathways are equally superimposed simultaneously. It claims a basal four-pathway superposition with threshold-dependent sympathetic recruitment. Basal denotes participation under the same upstream mechanical perturbation, not simultaneous activation at identical thresholds. Pathway-specific activation thresholds remain to be measured. The evidence tiers of the four non-sympathetic pathways are graded separately in Section 5.

4.5.2. Dimensions of Phenotypic Differentiation

The direction of phenotypic differentiation is jointly determined by the asymmetric vulnerability of the anterior and posterior circulations and the selective weights of the five pathways. The anterior circulation is predominantly associated with headache and ocular phenotypes. The posterior circulation is predominantly associated with vertigo and balance phenotypes.

Washio et al. [2] suggested that the regulatory behavior of the vertebral artery and posterior cerebral artery within the posterior circulation differs. The posterior circulation–predominant phenotype should therefore be further divided into a vertebral artery input site–dominated perfusion fluctuation phenotype and a posterior cerebral artery end–dominated cortical reactivity phenotype. Gaigalaite et al. [5] suggested that the posterior configuration of the circle of Willis can modify VAH-related posterior circulation vulnerability. The posterior circulation–predominant phenotype may therefore also include a collateral configuration–dependent effect-modifying dimension. Sato et al. [6] suggested that VAH-related low reserve also includes the functional dimension of impaired cerebrovascular CO₂ reactivity. The posterior circulation–predominant phenotype can therefore be further divided into a dynamic cerebral autoregulation rate reduction type and a CO₂ reactivity impairment type. The vertebral artery endothelial senescence mechanism proposed by Wang et al. [7] suggests that the posterior circulation–predominant phenotype may also include a chronic biological remodeling dimension, rather than only acute perfusion fluctuation. Neumann et al. [4] suggested that, in individuals with VAH, the posterior circulation–predominant phenotype may also include a stress-state systemic compensatory mobilization dimension. That is, the maintenance of posterior circulation perfusion may depend more on the systemic pressor response than on local cerebrovascular regulation.

5. Pathological Mechanisms and Phenotypic Spectrum of the Five Pathways

The five pathways are graded here by evidence density and causal-chain completeness. The arterial and venous pathways differ in their cited compression etiologies, so etiological distance is not the grading criterion. The arterial pathway's first-tier positioning rests mainly on the density of PIEZO1 vascular-wall evidence, not on the single-case compression reports. The venous pathway's second-tier positioning reflects the absence of a comparable mechanistic evidence density, not merely the osseous nature of its compression etiology.

First-tier candidate pathways are the arterial and lymphatic/glymphatic pathways. Both have multilevel evidence from human tissue or in vivo function to animal causality. Both have clear spatial interfaces with altered sheath-wall mechanics. However, the upstream step from sheath-wall mechanics to pathway function remains to be verified.

The second-tier candidate pathway is the venous pathway. It has a clinical phenomenological basis, provided by Ma et al. [56], and developmental anatomical background, provided by Martínez-Sanz et al. [11]. However, its mechanistic evidence density is lower than that of the arterial pathway. Extrapolation from osseous compression to fascial or sheath-wall mechanics requires independent argumentation.

The sympathetic pathway is threshold-dependent; the vagal pathway is exploratory. The distal effects of the sympathetic pathway are modified by hemodynamic threshold, stimulation type, and anterior or posterior circulation site. The upstream step from sheath wall to vagus nerve remains inferential. Both are included as exploratory modules in validation, not as first-tier endpoints of the main hypothesis test. The evidence tiers of the two pathways are graded separately in 8.4.1.

This grading does not change the five-pathway framework. It only clarifies the current argumentative weight of each pathway. Five pathways denote biological routes. Five evidence tiers denote argumentative weight. The two are not equivalent.

5.1. Arterial and Venous Pathways

5.1.1. Arterial Pathway: Two Sub-Pathways and Their Convergence

The arterial pathway should be divided into two sub-pathways that converge at an altered sheath-wall mechanical environment but are not part of the same molecular chain.

The first is the vascular-wall sub-pathway. Altered sheath-wall or perivascular mechanical environment acts on vascular smooth muscle cell PIEZO1. This drives structural remodeling, stiffening, and decreased mechanosensation, as described by Zhang J et al. [41], Zhang FR et al. [42], Luu et al. [43], and Beech [44].

The second is the perivascular nerve-terminal sub-pathway. Mechanical stimulation acts on perivascular or sheath-wall nerve terminal PIEZO1 or PIEZO2. This triggers CGRP release and positive feedback, as described by Mikhailov et al. [34], Liao et al. [35], Gkouzioti et al. [36], and Tereshenko et al. [40]. The default mediator at the neural terminal is PIEZO2. Evidence that PIEZO1 activation can trigger CGRP release is retained as evidence of experimental capacity, not of physiological predominance.

The vascular-wall sub-pathway does not directly cause CGRP release. The nerve-terminal sub-pathway does not directly explain vascular-wall structural remodeling. They may superimpose phenotypically, but should be assigned separate endpoints in validation design.

The adventitia of the intrasheath internal carotid artery, in the anterior circulation, bears abnormal traction. Alternatively, the mechanosensitive channels of the vessel wall are affected through changes in the intrasheath mechanical environment. On the vertebral artery side, in the posterior circulation, mechanical changes in the fascial attachments of the transverse foramen region affect its strain. The lower reserve of the posterior circulation makes these strains more likely to produce perfusion consequences.

The first-tier positioning of the arterial pathway in Section 5 is a positioning of the pathway as a whole. Within the pathway, the vascular-wall sub-pathway carries the PIEZO1 structural-remodeling evidence, and the perivascular nerve-terminal sub-pathway carries the PIEZO2–CGRP neural evidence. The two sub-pathways are not graded separately here, but validation design assigns them separate endpoints (7.1, 8.1.1).

5.1.2. Arterial Pathway: Evidence Base and Boundaries

The in vivo SWE data of Thomas et al. [18] show that the vertebral artery wall stiffness increases during head rotation whereas the internal carotid artery softens. This provides in vivo mechanical evidence for the differentiation of this pathway. The technical feasibility of this measurement is supported by the reliability study of Thomas et al. [27].

Zhang J et al. [41] demonstrated that PIEZO1 has an in vivo causal role in carotid artery structural remodeling induced by mechanical loading. Zhang FR et al. [42] further demonstrated in a hemodynamic disturbance or luminal stenosis model that smooth muscle cell Piezo1 is essential for neointimal hyperplasia and phenotypic switching. Piezo1 upregulation in the neointima far exceeds that of Piezo2. Luu et al. [43] demonstrated that Piezo1 mediates the mechanosensation of vascular smooth muscle cells. This function declines with aging, and Piezo1 inhibition can partially restore the dynamic contractile properties of senescent cells. Beech [44] further suggested that the role of PIEZO1 in vascular smooth muscle extends to aging-related structural stiffening of the vascular wall. Together, the four provide complementary in vivo, ex vivo, or review evidence for PIEZO1-dependent structural remodeling, mechanical perception, and aging-related stiffening of the arterial wall. However, their cell types are all vascular smooth muscle cells or vascular smooth muscle cell models. They cannot replace evidence for the PIEZO–CGRP neural step.

Alba et al. [48] detected PIEZO1 and PIEZO2 protein expression in the human carotid sinus and carotid body. This provides a histological reference for the presence of a mechanosensitive molecular apparatus on the arterial side. It does not demonstrate that this apparatus is activated under the hypothesized conditions. Konishi et al. [50] detected upregulated PIEZO1 expression in myofibroblasts of human carotid plaques. This provides a preliminary histological reference for the detectability of PIEZO1 in human carotid wall pathology. It is a pilot study with an atherosclerotic background. Full boundaries are given in Section 9.

The V2-segment flow velocity reduction and V3-segment flow velocity or resistance index elevation reported by Meng et al. [24] in the RVAS model provide a clinical threshold reference for the arterial-side luminal constraint leading to hemodynamic consequences. However, their osseous or acute compression etiology needs to be distinguished from chronic fascial constraint. The single-case feasibility report of Hou et al. [20] suggests that perivascular soft-tissue thickening can compress the C1 segment of the internal carotid artery in vivo. The single-case feasibility reports of Dadsetan et al. [21] and Nguyen et al. [22] suggest that fibrous-band-type soft tissues can compress the vertebral artery V2 segment in vivo.

Wang et al. [7] further suggested, at the biological mechanism level, that chronic mechanical environment changes on the arterial side of the posterior circulation may produce cumulative consequences through the endothelial senescence pathway. This adds a candidate chronic remodeling dimension to the arterial pathway. However, this mechanistic hypothesis still lacks direct evidence linking it to investing fascial mechanical constraint.

The layered histology of the rabbit carotid sheath by Zhao et al. [14] suggests that the arterial inner fascial layer thickens earliest and most markedly. This supports the assertion that the arterial side is the interface with the most active mechanical response in the sheath wall. The histology of adult human cervical segments by Hayashi [15] further suggests that the laminar adventitia remains continuously intact whereas the common sheath may be interrupted or indistinct. The thickness of the two layers exhibits inter-individual and site-dependent variation. This supports the view that the arterial-side sheath-wall mechanical response is layered and subject to individual and segmental dependence (see Section 2.2.2). Schachtel et al. [16] studied the fascial composition of the anterior sheath wall in the skull-base segment and suggested that the fascial source and number of layers of the sheath wall may differ across segments. The arterial-side sheath-wall mechanical response may therefore also be segment-dependent.

5.1.3. Venous Pathway

The internal jugular vein has a thin wall and high compliance. It is particularly sensitive to elevated intrasheath pressure. It may be one of the earliest links to produce measurable hemodynamic consequences. Internal jugular vein compression is sufficient to drive the core phenotype of interest to the hypothesis. This has been provided by the case series of Ma et al. [56] as direct clinical causal evidence. That study reported 3 patients with symptomatic internal jugular vein stenosis caused by external compression by osseous structures, mainly the C1 transverse process. All achieved significant symptom relief, including disappearance of head noise, and imaging-confirmed improvement in venous outflow after isolated surgical decompression. This reversibility, namely symptom relief upon simple removal of external compression, provides a clinical phenomenological basis for the assertion of the hypothesis that the internal jugular vein acts as an early indicator structure. Its compression can lead to cerebral venous outflow obstruction and related symptoms.

The compression etiology in that study was osseous structures rather than fascia or sheath-wall mechanical change. Etiological extrapolation from osseous compression to fascial compression remains a step that the hypothesis needs to argue independently. The human embryological evidence of Martínez-Sanz et al. [11] suggests a developmental anatomical association between the middle deep cervical fascia and venous structures. It also suggests that the deep fascial network may participate in regulating the venous drainage environment. This adds a developmental histological background to the venous pathway.

5.2. Lymphatic and Glymphatic Pathway

5.2.1. Spatial Landmarks and Pathway Coordinates

This section combines the lymphatic and glymphatic pathways into a single pathway. The glymphatic system is regarded as the parenchymal extension of the lymphatic pathway and is not listed as an independent pathway. The existence and function of the meningeal lymphatic drainage terminal have been independently demonstrated by Louveau et al. [57] and Aspelund et al. [58]. Functional lymphatic vessels expressing lymphatic endothelial markers exist adjacent to the dural sinuses. This pathway is functionally essential for draining cerebrospinal fluid and brain interstitial macromolecules to the deep cervical lymph nodes. Yoon et al. [17] further demonstrated that meningeal lymph fluid converges via the skull-base nasopharyngeal lymphatic plexus, an anatomical hub, into the deep cervical lymphatic vessels. These vessels are innervated by adrenergic nerves and undergo degenerative changes with aging. This anatomical hub is distinct from the functional convergence point of the carotid sheath.

At the cervical terminal of this drainage pathway, the human anatomical study of Clapham et al. [38] confirmed that some deep cervical lymph nodes are located directly within the carotid sheath, directly adjacent to the internal carotid artery. Their mean diameter was 13.5 × 4.8 mm. This pathway, from meningeal lymphatic vessels to the nasopharyngeal lymphatic plexus, to deep cervical lymph nodes, and to the intrasheath terminal, provides spatial landmarks from the anatomical starting point and pathway hub to the cervical terminal. It supports the proposition of the hypothesis that a change in sheath-wall mechanical state can act on the lymphatic drainage terminal. A terminal structure for cerebral lymphatic drainage is present within the sheath. This means that no anatomical intermediate barrier exists between these lymph nodes and either sheath-wall fibrosis or an altered intrasheath mechanical environment. The human embryological evidence of Martínez-Sanz et al. [11] suggests a developmental anatomical association between the middle deep cervical fascia and lymphatic structures. It also suggests that the deep fascial network may participate in regulating the lymphatic drainage environment. This adds a developmental histological background to the lymphatic pathway.

5.2.2. Mechanical Function and Tissue-Level Mechanics

On this spatial basis, Nelson-Maney et al. [59] provided direct evidence for the functional causality of this pathway. In CGRP receptor–deficient mice, the nitroglycerin-induced chronic migraine model exhibited reduced pain and light-avoidance behavior. In vitro, CGRP treatment of cultured human lymphatic endothelial cells caused VE-cadherin rearrangement and reduced functional permeability. In vivo injection of CGRP resulted in reduced uptake of cerebrospinal fluid by meningeal lymphatic vessels and impaired drainage. The authors concluded that CGRP signaling acts in chronic migraine by initiating meningeal lymphatic–immune interactions and reducing cerebrospinal fluid outflow. This revealed a previously unrecognized role of the lymphatic system.

Between the spatial landmark and the molecular functional evidence, Jin et al. [60] provided direct experimental evidence for the mechanical functional step of mechanical force leading to lymphatic drainage efficiency. Using a force-regulated mechanical device, that study applied mechanical compression to the superficial cervical lymphatic vessels of mice from outside intact skin. This manipulation doubled cerebrospinal fluid outflow. In aged mice, it corrected the drainage impairment, with tracer accumulation increasing 2.81-fold in specific lymph nodes and 4.7-fold in superficial lymphatic vessels. The mechanism was attributed to compression of the superficial lymphatic vessels themselves rather than to alteration of the spontaneous contractile function of the lymphatic vessels. This demonstrates that external mechanical constraint is sufficient to substantially and directionally consistently alter cervical lymphatic drainage efficiency. This evidence involves mouse superficial lymphatic vessels and short-term external compression. Full boundaries are given in Section 9. Jin et al. [60] therefore provide a functional causal reference and a magnitude reference for mechanical constraint leading to lymphatic drainage efficiency. They do not provide direct proof that sheath-wall fibrosis causes lymphatic drainage impairment.

Kim and Tithof [61] constructed the first two-dimensional fluid–structure interaction model of cervical lymphatic vessels, parameterized on the basis of in vivo imaging data. The model characterizes lymphatic pumping dynamics and valve function. It analyzes the regulation of cerebrospinal fluid transport by circular sinus geometry under favorable and unfavorable pressure gradients. This provides a parameterizable tissue-level mechanical framework for mechanical constraint leading to lymphatic drainage efficiency. If sheath-wall mechanical constraint is translated into changes in cervical lymphatic vessel transmural pressure or wall pretension, this fluid–structure interaction framework can be referenced to predict the direction and magnitude of the pumping efficiency response. This evidence is mouse-derived, two-dimensional, and uses single or three-unit simplification. Full boundaries are given in Section 9.

Du et al. [62] were the first to quantify in vivo the fluid dynamic characteristics of cervical lymphatic vessels. They demonstrated that the primary driving force of drainage is intrinsic pumping driven by wall contraction. In aged mice, contraction frequency and flow velocity both decreased, accompanied by decreased alpha-smooth muscle actin (α-SMA) expression. Local application of prostaglandin F2α restored lymphatic function and enhanced central nervous system clearance in aged mice. This provides original in vivo evidence independent of PIEZO1 for aging leading to decreased cervical lymphatic pumping function leading to reduced drainage efficiency. It also indicates that this functional decline is pharmacologically reversible. This evidence involves a mouse aging model and local prostaglandin F2α application. Full boundaries are given in Section 9.

5.2.3. Molecular Interface and Functional Causality

At the level of molecular transduction and meningeal lymphatic drainage function, Nonomura et al. [45] demonstrated that PIEZO1 is an essential channel for lymphatic endothelial cells to sense mechanical force. Endothelial-specific PIEZO1 deletion severely impaired lymphatic valve formation. Yoda1 activation of lymphatic endothelial PIEZO1 directly induced remodeling of the cytoskeleton and intercellular adhesion. Choi et al. [46] further demonstrated that Piezo1 is required for the normal development and function of meningeal lymphatic vessels. Transgenic overexpression or Yoda1 activation of Piezo1 is sufficient to increase cerebrospinal fluid outflow and to alleviate pathological cerebrospinal fluid accumulation in mouse models of hydrocephalus and Down syndrome. Matrongolo et al. [47] further demonstrated that, in mice with craniosynostosis and in aged mice, Yoda1 treatment reduced intracranial pressure and improved cerebrospinal fluid flow. It also restored meningeal lymphangiogenesis, drainage to deep cervical lymph nodes, and brain–cerebrospinal fluid perfusion. The study accordingly proposed that cerebrospinal fluid provides a mechanical force for the growth and maintenance of meningeal lymphatic vessels. It also proposed that Piezo1 agonists are a potential therapeutic option under conditions of elevated intracranial pressure or reduced cerebrospinal fluid flow.

This provides a cell-type-matched candidate molecular interface at the in vivo functional level. The chain runs from sheath-wall mechanics to lymphatic endothelial PIEZO1, and from there to altered meningeal lymphatic drainage function. It further indicates that PIEZO1 is not only a functionally essential node but also an intervenable node whose drainage function can be restored by pharmacological activation. This evidence involves mouse models and Yoda1 activation, not chronic fibrotic constraint. Full boundaries are given in Section 9. Du et al. [62] showed, through another pathway independent of PIEZO1, that the intrinsic pumping function of cervical lymphatic vessels declines with aging. It can be restored by pharmacological means that increase smooth muscle contractility. This provides a second node of functional recoverability for the lymphatic pathway that is independent of PIEZO1.

5.2.4. Clinical Association and Evidence Boundaries

At the human clinical level, Ren et al. [63] used dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) in 38 patients with episodic migraine and 22 healthy controls to directly quantify meningeal lymphatic drainage kinetics. In patients with episodic migraine, the wash-in rate of meningeal lymphatic vessels adjacent to the superior sagittal sinus and sigmoid sinus was reduced, and time to peak was prolonged. The initial area under the curve and the volume transfer constant (Ktrans) were also decreased. These kinetic abnormalities correlated with higher visual analog scale headache intensity. This provides in vivo evidence for the clinical relevance of impaired meningeal lymphatic drainage in human episodic migraine. It fills the associative step from animal mechanisms to human disease phenotypes.

The study measured meningeal lymphatic vessels adjacent to the superior sagittal sinus and sigmoid sinus, rather than the intrasheath lymphatic terminal. The study was a prospective design but lacked longitudinal follow-up. It still cannot establish the causal direction between lymphatic impairment and headache. Scanning was performed during the interictal period and cannot reflect dynamic changes during attacks. It supports the statement that impaired meningeal lymphatic drainage truly exists in human migraine and correlates with symptom severity. It does not support the statement that sheath-wall mechanical constraint caused this impairment.

At the chronic migraine stage, Wu et al. [64] further evaluated the glymphatic system using the diffusion tensor imaging along perivascular spaces index (DTI-ALPS) and meningeal lymphatic drainage function. Patients with chronic migraine had significant glymphatic and meningeal lymphatic dysfunction. This was particularly prominent in the medication-overuse headache subgroup. The authors accordingly proposed that this dysfunction is highly clinically relevant and may participate in the pathological mechanism of migraine chronification. This finding extends the clinical association evidence of the lymphatic pathway from episodic migraine, as reported by Ren et al. [63], to chronic migraine and the medication-overuse headache subgroup. It forms a continuous evidence chain along the disease-severity gradient. At the same time, it covers both glymphatic and meningeal lymphatic drainage. This is consistent with the approach in this section that combines the lymphatic and glymphatic pathways into a single pathway.

This study likewise cannot establish the causal direction between lymphatic impairment and headache. It measured glymphatic and meningeal lymphatic drainage rather than the intrasheath lymphatic terminal. Moreover, the pathological background of chronic migraine, such as medication overuse and central sensitization, is substantially distant from the investing fascial mechanical constraint of interest to the hypothesis. It therefore cannot be interpreted as evidence that fascial mechanics causes lymphatic impairment.

At the level of the field evidence landscape, Bagheri et al. [65] evaluated, using PRISMA standards, the role of the meningeal lymphatic and glymphatic system in the pathophysiology of migraine. The review reported that DCE-MRI shows altered lymphatic enhancement features in both episodic and chronic migraine. DTI-ALPS abnormalities are more consistent in chronic and high-frequency migraine. Animal models confirmed that cortical spreading depression, nitroglycerin exposure, and familial hemiplegic migraine mutations all impair glymphatic influx and reduce cerebrospinal fluid outflow through meningeal lymphatic vessels. However, the review also explicitly noted that no literature currently supports these changes as causal drivers of migraine. It called for longitudinal imaging studies. This evidence landscape is consistent with the conditional positioning of the lymphatic pathway adopted by the hypothesis. The direction is consistent, but causal evidence is scarce, and clinical studies remain predominantly cross-sectional DCE-MRI.

The review by Bagheri et al. [65] provides no new quantitative effect sizes. It cannot replace Ren et al. [63] as the most direct human clinical evidence of the lymphatic pathway. The human studies it included measured meningeal lymphatic vessels rather than the intrasheath terminal. They also lacked longitudinal follow-up. Therefore, they cannot establish the causal direction. Choi B et al. [66] further integrated the glymphatic system as a mechanistic bridge linking migraine and sleep. This provides a framework-level background for the field recognition of the lymphatic pathway. However, its upstream driver is sleep state rather than cervical mechanics. It cannot replace direct verification of the specific causal chain of this hypothesis.

Anatomical, molecular, mechanical, and clinical evidence supports the lymphatic pathway. This evidence falls into four groups. The anatomical starting point and pathway hub are provided by Louveau et al. [57], Aspelund et al. [58], Yoon et al. [17], and Clapham et al. [38]. The molecular and mechanical interface is provided by Nelson-Maney et al. [59], Jin et al. [60], Kim and Tithof [61], Du et al. [62], Nonomura et al. [45], Choi et al. [46], and Matrongolo et al. [47]. Human clinical association evidence is provided by Ren et al. [63] and Wu et al. [64]. Field-level support and framework integration are provided by Bagheri et al. [65] and Choi B et al. [66].

These evidence layers address the location of relevant structures, molecular feasibility, mechanical modifiability, human disease relevance, and therapeutic reversibility. None of these layers establishes whether sheath-wall mechanics causes this impairment. The upstream mechanical step from a change in sheath-wall mechanics to a decline in lymphatic drainage efficiency remains the part of the hypothesis awaiting verification. Clapham et al. [38] demonstrated spatial adjacency, not mechanical constraint or drainage function.

5.3. Sympathetic and Vagal Pathways

5.3.1. Sympathetic Pathway: Starting Point and Structure

The tight spatial relationship between the cervical sympathetic trunk and the carotid sheath has been directly confirmed by the cadaveric anatomical study of Kiray et al. [25]. The cervical sympathetic trunk was tightly adherent to the posterior wall of the carotid sheath in all specimens. An anatomical variant exists in which the sympathetic trunk lies within the sheath. Civelek et al. [26] further provided surgical anatomical quantitative data for this region. The cervical sympathetic trunk is located posteromedially to the carotid sheath. At the C6 level, it is approximately 11.6 ± 1.6 mm from the medial border of the longus colli muscle. Its own diameter is approximately 3.3 ± 0.6 mm. It courses throughout deep to the prevertebral fascia. These anatomical parameters provide a spatial coordinate system for the histological evidence on the target vessel side and at the ganglion level.

5.3.2. Target Vessel Side and Ganglion Level

Tereshenko et al. [40] observed in human cervical fascia that sympathetic nerve fibers that were TH-positive were significantly present in patients with occipital neuralgia but absent in controls. This suggests that sympathetic innervation may undergo phenotypic switching or aberrant sprouting under cervical fascial pathological conditions. The human immunofluorescence study of Struthoff et al. [39] in the human carotid bifurcation region further showed that perivascular nerve fibers progressively approach the lumen along the internal carotid artery segment. The closest point was approximately 2.5 mm from the intimal surface. The vast majority were TH-positive sympathetic efferent fibers.

This provides a target vessel–side human histological reference for the transmission of sheath-wall mechanical perturbation through the prevertebral fascia to the sympathetic terminals of the arterial adventitia. However, this evidence describes nerve distribution and spatial adjacency in normal human specimens. It does not demonstrate mechanical coupling efficiency under fibrotic conditions. It also cannot replace distal hemodynamic measurement of the sympathetic pathway.

Alba et al. [48] detected high proportions of PIEZO1 and PIEZO2 expression in the normal human superior cervical ganglion, approximately 71% and 86%, respectively. This suggests that the sympathetic ganglion itself may possess direct mechanosensitivity. It provides a ganglion-level histological reference for the transmission of sheath-wall mechanical deformation through the prevertebral fascia to the sympathetic pathway. However, this evidence describes normal distribution and has not yet demonstrated functional activation under fibrotic conditions. Yoon et al. [17] further demonstrated that deep cervical lymphatic vessels are innervated by adrenergic nerves. This suggests that the sympathetic pathway and the lymphatic pathway share an anatomical and functional coupling node at the cervical terminal.

5.3.3. Distal Effects and Threshold Dependence

The distal vascular effects of sympathetic trunk activation are the necessary closing link for the sympathetic pathway as one of the five pathways of the hypothesis. Kaur et al. [54] directly measured the hemodynamic responses of the internal carotid artery and the vertebral artery during graded lower-body negative pressure–induced reflex sympathetic activation. Low-to-moderate levels of sympathetic activation, from −10 to −40 Torr lower-body negative pressure, had no significant effect on the blood flow, diameter, or flow velocity of the internal carotid artery or the vertebral artery. Only at moderate-to-high levels, from −50 to −70 Torr, did blood flow decline appear. This decline was mediated jointly by luminal narrowing and reduced flow velocity.

This provides functional-level evidence for the distal limb of the sympathetic chain: sympathetic activation leading to distal vasoconstriction and insufficient blood supply. It does not provide evidence for the complete sheath-wall-to-sympathetic pathway. Its upstream step remains to be verified. The effect has a clear threshold dependence. The pathological effect of the sympathetic pathway should be understood as having hemodynamic significance only after sympathetic tone crosses a certain threshold. It is not the case that any slight mechanical stimulation of the sympathetic trunk is directly translated into perfusion decline.

5.3.4. Anterior Versus Posterior Circulation Divergence

The relative effects of sympathetic activation on the anterior and posterior circulations have not yet converged in the existing literature. The hypothesis should not assume a particular direction. The reflex physiological activation study of Kaur et al. [54] explicitly reported that, at moderate-to-high levels of activation, the constriction magnitudes of the internal carotid artery and the vertebral artery were comparable. The internal carotid artery was not preferentially constricted. In contrast, Meyer et al. [55] observed a significant reduction in internal carotid artery blood flow through direct stimulation of the superior cervical sympathetic ganglion in a monkey model. This animal experimental evidence under direct electrical stimulation suggests that the hemodynamic consequences of sympathetic activation may be highly dependent on the type and intensity of stimulation. Direct, nonphysiological electrical stimulation of the ganglion and reflex physiological activation may produce different circulatory effect patterns.

In addition, animal experiments with direct electrical stimulation of the superior cervical sympathetic ganglion also reported significant luminal narrowing of the extracranial and proximal intracranial arteries of the anterior circulation. The diameter changes of posterior circulation arteries were minimal. Anatomically, the sympathetic innervation density of the anterior circulation is indeed higher than that of the posterior circulation. However, a higher innervation density and stronger constriction under physiological activation are not a simple linear relationship. Washio et al. [2] further suggested that, even under physiological sympathetic activation such as isometric exercise, the vertebral artery and posterior cerebral artery within the posterior circulation are not consistent in cerebrovascular reactivity. This indicates that site specificity exists within the posterior circulation itself.

The hypothesis therefore treats four factors as moderating variables to be tested: whether sympathetic tone crosses the threshold; the relative effects of sympathetic activation on the anterior and posterior circulations; the site differences within the posterior circulation; and the stimulation type and intensity. It does not assume that the sympathetic pathway necessarily has a predominant effect on the posterior circulation. The data of Meyer et al. [55] should be strictly limited to a supplementary reference under direct electrical stimulation and animal model conditions. They cannot be directly extrapolated to the mechanical, chronic sympathetic trunk stimulation scenario described by the hypothesis.

5.3.5. Vagal Pathway

The vagus nerve undergoes mechanical deformation within the sheath. The expression of PIEZO in the carotid sinus and carotid body suggests that vagal mechanotransduction is not limited to baroreceptors. More direct functional evidence comes from Zeng et al. [49]. That study demonstrated by genetic means that PIEZO1 and PIEZO2 are mechanosensors essential for the baroreceptor reflex. Simultaneous knockout of Piezo1 and Piezo2 in the nodose and petrosal ganglia abolished the baroreceptor reflex and aortic depressor nerve activity. Optogenetic activation of PIEZO2-positive afferent fibers was sufficient to initiate the reflex. Animals lacking PIEZO exhibited unstable hypertension and increased blood pressure variability. This upgrades the mechanical sensitivity of the vagal pathway from expression correlation to functional necessity.

Alba et al. [48] detected PIEZO1 and PIEZO2 expression in the human petrosal ganglion and carotid sinus. This echoes the functional evidence of Zeng et al. [49] in tissue localization. Together they constitute a two-layer basis of functional necessity plus human distribution for the vagal pathway. The study of Zeng et al. [49] was completed in mice and demonstrated mechanotransduction at the baroreceptor level, rather than the specific pathway from carotid sheath wall mechanics to the vagus nerve. The upstream step from a change in sheath-wall mechanics to vagal PIEZO activation remains an inference requiring independent verification by the hypothesis. Altered vagal afferent activity can affect parasympathetic tone and the efficacy of the cholinergic anti-inflammatory pathway.

5.4. Interactions Among Pathways and the Phenotypic Spectrum

5.4.1. Spatial Convergence and the Phenotypic Predominance Framework

The five pathways share the carotid sheath as a spatial convergence point and mutually amplify one another in function. Combined with the asymmetry between the anterior and posterior circulations, a preliminary phenotypic predominance framework can be proposed. Phenotypes predominantly associated with the anterior circulation are predicted to be dominated by headache and ocular symptoms. Phenotypes predominantly associated with the posterior circulation are predicted to be dominated by vertigo and balance disturbance. The differentiation between the vertebral artery and the internal carotid artery in dynamic cerebral autoregulation demonstrated by Friend et al. [19], and the differentiation between the vertebral artery and the internal carotid artery in wall-stiffness response demonstrated by Thomas et al. [18], jointly provide a quantifiable physiological and mechanical basis for this phenotypic predominance framework.

Washio et al. [2] suggested that the vertebral artery and posterior cerebral artery within the posterior circulation should be analyzed separately. The posterior circulation–predominant phenotype can therefore be further divided into vertebral artery input site–dominated perfusion fluctuation and posterior cerebral artery end–dominated cortical reactivity. The review by Srichawla et al. [28] provides broader background confirmation of the regional characteristics of posterior circulation low reserve. The negative findings of Chen et al. [29] suggest that the sex expression of this predominance may depend on stress conditions. The case series of Ma et al. [56] provides a clinical phenomenological reference for the phenotypic output of the venous pathway.

When discussing individual-level expression of this predominance, the term "individual susceptibility state" is used, not "regional mechanical vulnerability." Regional mechanical vulnerability denotes the differential response among circulatory regions. Individual susceptibility state denotes the threshold lowering of a given person.

5.4.2. Posterior Circulation Phenotypic Dimensions

The clinical evidence for VAH summarized by Mirkovic et al. [30] suggests that individuals with congenital posterior circulation low reserve may exhibit similar vertigo or migraine phenotypes. This provides a structural reference for the phenotypic predominance proposed by the hypothesis. The prevalence data of Triantafyllou et al. [3] suggest that this structural reference has an estimable base in the population. Neumann et al. [4] further suggested that individuals with VAH may exhibit stronger systemic compensatory mobilization under stress conditions. The posterior circulation–predominant phenotype may therefore also include a stress-state compensatory strategy dimension. Gaigalaite et al. [5] suggested that the posterior circulation–predominant phenotype may also include a Willis circle posterior configuration–dependent effect-modifying dimension. That is, the same VAH may exhibit clinically vulnerable phenotypes in different directions under different collateral configurations. Sato et al. [6] suggested that the posterior circulation–predominant phenotype may also include a cerebrovascular CO₂ reactivity impairment dimension. That is, VAH-related low reserve is manifested not only as a reduction in dynamic cerebral autoregulation rate but also as a decline in local vascular reactivity reserve. The vertebral artery endothelial senescence mechanism proposed by Wang et al. [7] suggests that the posterior circulation–predominant phenotype may also include a chronic biological remodeling dimension, rather than only acute perfusion fluctuation.

The phenotypic output of the lymphatic pathway rests on anatomical, molecular, mechanical, and clinical evidence summarized in Section 5.2.4. Its detailed boundaries have been elaborated item by item there and are not repeated here.

5.4.3. Unequal Pathway Inputs and Current Explanatory Limits

Zeng et al. [49] showed that PIEZO1 and PIEZO2 are essential sensors for the baroreceptor reflex. This finding provides functional evidence for the mechanical sensitivity of the vagal pathway. It suggests that, against the background of sympathetic–vagal tone interaction, baroreceptor reflex sensitivity may constitute another moderating dimension of phenotypic predominance. The distal effects of the sympathetic pathway introduce an additional moderating dimension. When sympathetic tone has not crossed the threshold, phenotypic predominance is mainly determined by the asymmetry of the anterior and posterior circulations. After the threshold is crossed, the type and intensity of stimulation may change the direction of the predominance. This interaction effect currently lacks in vivo data. It is therefore not treated as a main prediction of the hypothesis but only as an exploratory endpoint.

The RVAS thresholds of Meng et al. [24] provide an operable clinical hemodynamic reference for the phenotypic output of the posterior circulation. The layered sheath-wall histology of Zhao et al. [14], the layered description of adult human cervical segments by Hayashi [15], and the sheath-wall regional heterogeneity evidence of Schachtel et al. [16] suggest that the fascial layer thickness and mechanical response on the arterial, venous, and nerve sides within the sheath may differ. They also suggest that the fascial source and number of layers of the sheath wall may differ across segments. The local input of the five pathways under the same sheath-wall mechanical perturbation is therefore not equal. The distribution of TH-positive sympathetic fibers in the human carotid adventitia provided by Struthoff et al. [39] suggests that the local input density of the sympathetic pathway on the target vessel side may itself exhibit segmental and site differences. The single-case feasibility reports of Hou et al. [20], Dadsetan et al. [21], and Nguyen et al. [22] suggest that soft tissues possess the capacity for direct mechanical action on extracranial large vessels. Their full boundaries are given in Section 2.3 and Section 9. This framework is currently explanatory. The improvement of its predictive capacity awaits the establishment of pathway-specific biomarkers.

6. Variants and Scope of Application of the Hypothesis

This hypothesis is a variant group sharing a core skeleton. The four variants below share the mechanical coupling of the investing fascia and carotid sheath as their shared skeleton. They differ in the dimensions each extends and the outputs each foregrounds.

6.1. Five-Pathway and Multifactorial Superposition Variant

This is the version developed in the main text. It explains the systematic lowering of the headache threshold through the parallel coupling of five pathways: arterial, venous, lymphatic/glymphatic, sympathetic, and vagal. Its extension point lies in treating multiple factors, namely chronic forward head posture, aging, and low-grade inflammation, as synergistic drivers of upstream fibrosis. Its most critical step awaiting verification is whether the relative weights of the five pathways under the same sheath-wall mechanical perturbation can be distinguished by pathway-specific biomarkers.

6.2. Five-Pathway Plus Emotional–Autonomic Modulation Variant

This variant extends the five pathways by adding an emotional–autonomic regulatory dimension. The added dimension is not a sixth anatomical pathway. It refers to the overall tone state of sympathetic and vagal pathways, treated as a moderating variable for pathway weights. Its most critical step awaiting verification is whether emotional or autonomic indicators modify the association between investing fascial mechanical parameters and downstream phenotypes, and whether this modification is threshold-dependent.

6.3. Vagal Mechanical Coupling Variant

This variant focuses on PIEZO transduction and parasympathetic output. It shares the molecular interface of sheath-wall mechanics leading to PIEZO activation. Its extension point lies in advancing the distal effects of the vagal pathway from framework-level inference to measurable parasympathetic tone and cholinergic anti-inflammatory pathway indicators. Its most critical step awaiting verification is whether baroreceptor reflex sensitivity or blood pressure variability is associated with investing fascial mechanical parameters, and whether PIEZO expression in the petrosal or nodose ganglia can provide a functional reference for this pathway.

6.4. Cerebral Perfusion and Ocular Pathology Mechanical Node Variant

This variant foregrounds the anterior circulation arterial pathway and ocular consequences. It shares the prediction of anterior circulation–predominant headache and ocular phenotypes. Its extension point lies in treating ocular pathology, such as dry eye and altered retinal perfusion, as an independent output dimension of anterior circulation mechanical vulnerability. Its most critical step awaiting verification is whether anterior circulation arterial mechanical parameters are associated with ocular perfusion indicators or ocular surface symptoms, and whether this association is mediated by autoregulatory reserve.

6.5. Scope of Application

The hypothesis addresses the cervicogenic mechanical component. It does not claim to replace central mechanisms, genetic mechanisms, or the independent pathophysiology of medication-overuse headache. It does not equate anatomical adjacency with causal proof. Its claims are conditional on three requirements.

First, cross-layer mechanical conduction efficiency must reach a certain magnitude. Pires et al. [1] have demonstrated that mechanical accessibility exists. Liu et al. [37], Wilke et al. [8], and Kalichman [9] provide magnitude references. Querry et al. [12] provide a human magnitude reference for the external cervical pressure–to–carotid sinus pathway. Aparici-Gil et al. [10] provide anisotropic constitutive parameters. Zhao et al. [14] provide histological evidence of layered rabbit sheath wall. Hayashi [15] provides histological evidence of layered adult human cervical sheath wall. Its non-fusion finding requires that functional continuity be treated as a premise to be verified (see Section 2.2.2). Schachtel et al. [16] provide evidence of regional heterogeneity.

Second, the change in sheath-wall mechanics must be sufficient to activate PIEZO1 or PIEZO2 in the investing fascia–sheath wall continuum. Evidence at the ex vivo and animal in vivo levels is provided by refs [34,35,36,37,38,39,40,41,42,43,44,45,46,47]. Evidence at the human histological and human pathological levels is provided by refs [40,48,50]. He et al. [33] provide a transferable precedent of the PIEZO1–Wnt2/Wnt11–CCL24 fibrotic loop in human fibrotic skin (see Section 4.1.3).

Third, mechanical stimulation of the sympathetic trunk must be sufficient to drive its tone across the hemodynamic threshold. Kaur et al. [54] have demonstrated that this threshold exists. Struthoff et al. [39] provide a histological basis for the distribution of sympathetic fibers on the target vessel side.

If all three requirements are met, the downstream effects of the five pathways acquire pathological significance. These three requirements are precisely where the testability of the hypothesis lies.

At the level of validation design, Gaigalaite et al. [5] suggest that the clinical significance of the posterior circulation structural corroboration is significantly modified by the posterior configuration of the circle of Willis. Sato et al. [6] suggest that VAH-related low reserve also includes the functional dimension of impaired cerebrovascular CO₂ reactivity. Kim and Tithof [61] and Du et al. [62] provide a parameterizable and recoverable tissue-level mechanical framework for the pumping efficiency of the lymphatic pathway and for aging-related decline in pumping. If a validation cohort includes individuals with VAH, collateral configuration and CO₂ reactivity should therefore be treated as prespecified effect-modifying variables or functional endpoints. If the lymphatic pathway is to be modeled, the fluid–structure interaction framework and aging-related pumping decline parameters should serve as references for parameterizing pumping efficiency.

The single-case feasibility reports of Hou et al. [20], Dadsetan et al. [21], and Nguyen et al. [22] merely suggest that soft tissue or fibrous bands can directly compress extracranial large vessels in humans. Their single-case evidence level and inflammatory or local fibrous-band etiologies are substantially distant from the chronic investing fascial fibrosis scenario of interest to the hypothesis. They cannot serve as a basis for relaxing the scope of application.

7. Testable Predictions

7.1. Histological and Molecular Predictions

The spatial colocalization of PIEZO1, PIEZO2, and CGRP in the investing fascia–sheath wall continuum should be detectable. Tereshenko et al. [40] have already observed the co-distribution of PIEZO2, CGRP, and TH in the trapezius fascia. This observation should be reproducible in the investing fascia itself.

Mikhailov et al. [34] showed that the PIEZO1 agonist Yoda1 can trigger CGRP release. This capacity should be verifiable in ex vivo tissue preparations obtained from the investing fascia. This capacity test should be distinguished from the default physiological predominance of PIEZO2 at the neural terminal. Liao et al. [35] demonstrated the mechanistic capacity of PIEZO2–CGRP or substance P positive feedback in a neuropathic pain model. Future studies should test whether PIEZO2 participates in local CGRP or substance P positive feedback in the investing fascia–sheath wall continuum. They should also test whether mechanical loading can trigger this positive feedback.

Gkouzioti et al. [36] provided human endothelial cell evidence that CGRP pretreatment enhances PIEZO1 responsiveness. Future studies should test whether CGRP sensitization alters PIEZO1-mediated calcium responses and cytoskeletal remodeling in endothelial cells of the investing fascia–sheath wall continuum. This should be labeled as endothelial CGRP–PIEZO1 crosstalk, not as the neural-terminal loop. The functional differentiation of PIEZO1 and PIEZO2 subtypes summarized by Zhao L et al. [52] provides a background framework for the default subtype division-of-labor model. It cannot replace direct verification in the target tissue.

Alba et al. [48] established the PIEZO1 and PIEZO2 immunohistochemical distribution in the carotid sinus, carotid body, and superior cervical ganglion. This distribution provides a positive sampling reference and a histological reference for antibody validation. However, its normal distribution data cannot replace expression and functional testing under pathological conditions. Konishi et al. [50] detected upregulated PIEZO1 expression in myofibroblasts of human carotid plaques. This provides a preliminary histological reference for the detectability of PIEZO1 in human carotid wall pathology. This observation should be testable in pathological specimens from the investing fascia–sheath wall–vascular adventitia continuum. This is a pilot study with an atherosclerotic background. Full boundaries are given in Section 9.

He et al. [33] established the PIEZO1–Wnt2/Wnt11–CCL24 fibrotic loop. This loop suggests that, in histological validation of the investing fascia–sheath wall continuum, the spatial relationships among PIEZO1, Wnt2/Wnt11, CCL24, and α-SMA can be detected simultaneously. Future studies should also test whether mechanical loading drives the same positive feedback program. This prediction should be labeled as a cross-tissue mechanistic test, meaning that a mechanism established in one tissue is tested in a different target tissue (see Section 4.1.3).

Zeng et al. [49] demonstrated the functional necessity of PIEZO1 and PIEZO2 in the baroreceptor reflex. Future studies should test whether baroreceptor reflex sensitivity or blood pressure variability is associated with investing fascial mechanical parameters. PIEZO expression in the petrosal or nodose ganglia can provide a histological reference for the functional pathway. Nonomura et al. [45] demonstrated the mechanotransduction necessity of PIEZO1 in lymphatic endothelial cells. In specimens from the intrasheath lymphatic terminal, the spatial relationship between PIEZO1 expression and lymphatic valve or endothelial adhesion markers should be tested. Ex vivo lymphatic endothelial preparations can be used to test whether mechanical loading or Yoda1 induces cytoskeletal and VE-cadherin remodeling.

Choi et al. [46] and Matrongolo et al. [47] demonstrated in vivo that PIEZO1 regulates and restores meningeal lymphatic drainage function. In functional validation of the lymphatic pathway, future studies should test whether PIEZO1 functional status is associated with cerebrospinal fluid drainage efficiency, meningeal lymphatic vessel morphology, and tracer accumulation in deep cervical lymph nodes. This evidence involves mouse models and Yoda1 activation, not chronic fibrotic constraint. Full boundaries are given in Section 9. Du et al. [62] showed that aging leads to decreased contraction frequency and flow velocity of cervical lymphatic vessels and decreased α-SMA expression. They also showed that prostaglandin F2α can restore pumping function. In histological validation of the lymphatic pathway, α-SMA, valve function, and pumping frequency should therefore be assessed in parallel. Aging-related pumping decline should be incorporated as a functional dimension independent of PIEZO1. This evidence involves a mouse aging model and local prostaglandin F2α application. Full boundaries are given in Section 9.

Zhang J et al. [41], Zhang FR et al. [42], Luu et al. [43], and Beech [44] demonstrated or integrated PIEZO1-dependent carotid structural remodeling, neointimal hyperplasia, aging-related decline in mechanosensation, and aging-related vascular wall stiffening. On the arterial side, future studies should test whether these phenotypes are triggered downstream of investing fascia–sheath wall mechanical conduction. Della Pietra et al. [53] confirmed a landscape of predominantly preclinical evidence and scarce clinical research. The above predictions should therefore be prioritized in human tissue and in vivo models in a tiered manner, rather than relying solely on extrapolation from animal models.

Zhao et al. [14] described a three-layered fascial architecture in the rabbit sheath wall, with earliest and most marked thickening of the arterial inner fascial layer. Hayashi [15] described laminar adventitia plus common sheath layering in adult human cervical segments, with inter-individual and site variation in thickness (see Section 2.2.1). These findings suggest that histological validation should sample each layer of the sheath wall separately and record individual and segmental factors. Schachtel et al. [16] reported sheath-wall regional heterogeneity. Histological validation should therefore consider the sampling segment. Martínez-Sanz et al. [11] reported a developmental anatomical association between the middle deep cervical fascia and lymphatic or venous structures. At the deep fascia–lymphatic or venous interface, the spatial relationship between fascia and conduits should therefore be assessed simultaneously. Struthoff et al. [39] established the quantitative distribution of human perivascular nerve fibers around the carotid bifurcation. Histological validation of the sympathetic pathway should therefore treat adventitial TH-positive fiber density and distance as independent analytical nodes. These nodes should be analyzed jointly with specimens of the sheath wall and sympathetic trunk.

7.2. Imaging and Mechanical Predictions

Investing fascial SWE values should be positively correlated with carotid sheath-wall thickness or stiffness. After fascial release, the intrasheath arterial cross-sectional area, internal jugular vein flow velocity, or autonomic indicators should show measurable immediate changes.

The methodological premise of this prediction has received three independent supports. Zandi et al. [67] demonstrated excellent-to-good test–retest reliability for cervical fascial SWE, with a stiffness intraclass correlation coefficient of at least 0.90. They also provided standard error of measurement and minimal detectable change. Their negative finding, however, suggests that cervical fascial stiffness in patients with chronic neck pain does not differ significantly from healthy controls. Validation design therefore cannot rely solely on a single resting-state stiffness indicator. It should prespecify multidimensional parameters such as thickness, fascia-to-fat stiffness ratio, and loaded-state stiffness.

Thomas et al. [27] demonstrated in 30 healthy subjects that SWE measurement of the internal carotid artery and vertebral artery walls has acceptable intra-rater reliability. Intraclass correlation coefficients were 0.81 and 0.76, respectively. They also provided standard error of measurement. Ghahremani et al. [68] demonstrated in 136 subjects that the shear modulus, circumferential stress, longitudinal stress, and stiffness index of the common carotid artery can be calculated in a conventional ultrasound system. These parameters systematically discriminate the severity of arterial disease. When SWE is used to assess the mechanical state of the sheath wall or arterial wall, geometric parameters must be included as covariates in the analysis or model calibration, as described by Capron et al. [69]. Device implementation and imaging depth should be treated as prespecified covariates (see Section 8.4.2).

7.2.1. SWE Effect-Size Reference for Cervical Fascial Fibrosis

Dapper et al. [70] reported in 16 patients with bilateral post-radiotherapy cervical fibrosis and 16 matched controls that the SWE stiffness of the sternocleidomastoid-region fascia increased from approximately 22.7–25.5 kPa to 44.3–46.4 kPa. There was a synchronous increase in muscle stiffness, whereas subcutaneous tissue showed no significant change. Tissue thickness decreased but cervical range of motion was largely preserved. This suggests a dissociation between structural change and functional limitation.

This evidence provides the most direct in vivo effect-size reference to date for cervical fascial fibrosis leading to increased SWE stiffness. It can be used for directional prediction and sample-size estimation of investing fascial SWE. Two qualifications must be retained. First, the etiology is post-radiotherapy fibrosis and the site is the sternocleidomastoid-region fascia, not the investing fascia itself, and certainly not the carotid sheath wall. Second, the synchronous increase in fascial and muscle stiffness with no significant change in subcutaneous tissue indicates that increased fascial stiffness cannot be attributed to the fascia alone. Synchronous muscle fibrosis means that the stiffness change measured in that study may be an overall effect of the fascia–muscle complex. Investing fascia specificity still requires independent verification in the target tissue.

This evidence does not close Gap 2, because reliability of sheath-wall SWE remains lacking. It also does not close Gap 1, because the transmission coefficient still has no direct measurement. It provides only a transferable reference for the in vivo SWE effect size of investing fascial fibrosis, not validated SWE of investing fascial fibrosis.

The predictions for the anterior and posterior circulations should be differentiated. The effective radius-to-flow path coefficient of the posterior circulation should be significantly greater than that of the anterior circulation. The anterior circulation path coefficient should be attenuated by the mediation of autoregulation. Operable in vivo testing can use the vertebral artery–to–internal carotid artery RoR ratio, based on the Friend et al. [19] paradigm, and the wall-stiffness ratio, based on the Thomas et al. [18] paradigm, as dual quantitative indicators of asymmetry. It can then test their association with investing fascial mechanical parameters and posterior circulation perfusion outcomes. The change in this ratio under stress conditions or mechanical loading may be sex-specific, as suggested by Friend et al. [19] and Chen et al. [29]. Cohort design should therefore be pre-stratified by sex and stress condition.

Washio et al. [2] suggested that the vertebral artery and posterior cerebral artery should be treated as separate outcome variables. The thresholds provided by Meng et al. [24], namely V2-segment flow velocity reduction, V3 flow velocity greater than 74.68 cm/s, and V3 resistance index greater than 0.71, provide candidate quantitative indicators for posterior circulation hemodynamic endpoints. Attention must be paid to their RVAS osseous or acute compression background. The device implementation and imaging depth of the above SWE and ultrasound biomechanical endpoints should be treated as prespecified covariates, as described by Pruijssen et al. [71] and discussed in Section 8.4.2.

7.3. Pathway-Specific Predictions

The sympathetic pathway makes a specific prediction. In individuals with jointly elevated investing fascial mechanical parameters and sympathetic tone indicators, the direction and magnitude of change in the vertebral artery–to–internal carotid artery blood flow ratio should differ from those in individuals with elevated fascial mechanical parameters alone. This interaction effect should exhibit nonlinearity before and after sympathetic tone crosses the threshold, based on the Kaur et al. [54] paradigm. The direct electrical stimulation data of Meyer et al. [55] suggest that the type and intensity of sympathetic stimulation should be treated as independent moderating variables. Washio et al. [2] suggest that the differential responses of the vertebral artery and posterior cerebral artery within the posterior circulation should also be incorporated into the outcome stratification of the sympathetic module. The high PIEZO1 and PIEZO2 expression in the superior cervical ganglion reported by Alba et al. [48] suggests that the mechanosensitivity of the sympathetic ganglion itself can be tested. The distribution of TH-positive fibers in the carotid adventitia reported by Struthoff et al. [39] suggests that the sympathetic module should treat target vessel–side nerve density and distance as prespecified covariates. The adrenergic innervation of deep cervical lymphatic vessels reported by Yoon et al. [17] suggests that the sympathetic module should be analyzed jointly with the lymphatic module at the cervical terminal.

For the vagal pathway, if sheath-wall mechanical conduction affects the carotid sinus region, then baroreceptor reflex sensitivity or blood pressure variability should be associated with investing fascial mechanical parameters. Zeng et al. [49] showed that PIEZO1 and PIEZO2 are essential sensors for the baroreceptor reflex. This finding provides a functional mechanistic basis. The human PIEZO1 and PIEZO2 expression reported by Alba et al. [48] provides a histological reference. This evidence involves a mouse baroreceptor model, not the sheath wall–to–vagus nerve pathway. Full boundaries are given in Section 9.

For the lymphatic pathway, if sheath-wall mechanical constraint does indeed affect the intrasheath lymphatic terminal, then investing fascial mechanical parameters should be associated with internal jugular vein flow velocity or cerebrospinal fluid drainage surrogate indicators. The mechanical compression–induced doubling of drainage efficiency reported by Jin et al. [60] provides an effect-magnitude reference. This evidence involves mouse superficial lymphatic vessels and short-term external compression. Full boundaries are given in Section 9. Louveau et al. [57], Aspelund et al. [58], and Yoon et al. [17] established the meningeal lymphatic vessel–nasopharyngeal lymphatic plexus–deep cervical lymph node pathway. This pathway suggests that human endpoints should include meningeal lymphatic drainage kinetics, nasopharyngeal or deep cervical lymphatic vessel imaging features, and deep cervical lymph node imaging features. Nonomura et al. [45], Choi et al. [46], and Matrongolo et al. [47] suggest that molecular endpoints should include lymphatic endothelial PIEZO1 expression, valve or adhesion markers, cerebrospinal fluid drainage function indicators, and the recoverability of PIEZO1 pharmacological modulation. Kim and Tithof [61] and Du et al. [62] suggest that mechanical endpoints should include the parameterized relationships among cervical lymphatic vessel transmural pressure, wall pretension, pumping frequency, α-SMA, and pumping efficiency. Ren et al. [63] and Wu et al. [64] demonstrated impaired meningeal lymphatic or glymphatic drainage in human episodic and chronic migraine. Their findings suggest that human endpoints should include meningeal lymphatic drainage kinetics, glymphatic function indicators, and deep cervical lymph node imaging features. Disease-severity stratification should be prespecified between episodic and chronic migraine. However, what they measured was meningeal lymphatic vessels and glymphatic function rather than the intrasheath terminal. Both lacked longitudinal follow-up. Full boundaries are given in Section 9. The systematic reviews of Bagheri et al. [65] and Choi B et al. [66] suggest that human evidence for the lymphatic or glymphatic pathway should test causal direction using longitudinal or interventional designs.

For cross-layer transmission, the primary sensitivity upper bound is approximately 30%, based on the ex vivo fascial force transmission ratio reviewed by Wilke et al. [8] and Kalichman [9]. A secondary sensitivity scenario uses the human magnitude reference of 82%–89% from Querry et al. [12]. This refers to external cervical pressure to the carotid sinus, not to the investing fascia–to–sheath wall pathway (see Section 4.2.2). If the transmission ratio reaches these bounds, a change in investing fascial stiffness should produce detectable dose-related changes in the mechanical indicators of the sheath wall or intrasheath structures. The anisotropic data of Aparici-Gil et al. [10] suggest that the transmission ratio should be direction-dependent. The model should be scanned separately along the fiber direction and perpendicular to the fiber direction. Zhao et al. [14] and Hayashi [15] suggest that the transmission function should be parameterized separately for each layer of the sheath wall and that individual and segmental variation should be recorded (see Section 2.2.1). Schachtel et al. [16] suggest that the transmission function should take the sampling segment into account.

For posterior circulation biological mechanisms and compensatory strategies, the vertebral artery endothelial senescence hypothesis proposed by Wang et al. [7] suggests that future studies should test whether endothelial senescence markers are associated with local vertebral artery mechanical parameters and posterior circulation perfusion indicators. This is a mechanistic hypothesis test with a cervical degeneration background. Full boundaries are given in Section 9. Neumann et al. [4] suggest that, in individuals with VAH, the change in total peripheral resistance under stress should be measured in advance. Future studies should test whether this change modifies the association between investing fascial mechanical parameters and posterior circulation perfusion outcomes. This involves a hypertensive population background and a congenital vessel-diameter variant. Full boundaries are given in Section 9. Gaigalaite et al. [5] suggest that the posterior configuration of the circle of Willis should be treated as a prespecified stratification variable or covariate within the VAH subgroup. Sato et al. [6] suggest that the CO₂ reactivity of the vertebral artery and basilar artery can provide a supplementary functional endpoint of posterior circulation reserve.

7.4. Clinical Intervention Predictions

The meta-analysis of Lu et al. [72] reported that myofascial release significantly reduced pain intensity in cervicogenic headache. The standardized mean difference was −2.01, with a 95% confidence interval from −2.73 to −1.29. It improved disability, with a standardized mean difference of −1.45 and a 95% confidence interval from −2.07 to −0.83, with an I² of 0%. It had a moderate effect on disability in tension-type headache, with a standardized mean difference of −0.98 and a 95% confidence interval from −1.32 to −0.65, with an I² of 0%. It did not significantly improve disability in migraine, with a standardized mean difference of −2.44 and a 95% confidence interval from −6.04 to 1.16, with an I² of 97%. Overall evidence quality was rated low to very low. This effect gradient is compatible with the expectation of the hypothesis that the cervicogenic mechanical component accounts for the largest proportion in cervicogenic headache and the smallest proportion in migraine. However, the myofascial release in that meta-analysis was a generalized cervical myofascial intervention and was not specifically targeted at the investing fascia–carotid sheath interface. It therefore constitutes only compatible evidence rather than confirmatory support.

The randomized controlled trial of Uzun et al. [73] showed that cervical mobilization combined with clinical Pilates exercises can increase internal carotid artery and vertebral artery flow velocities in patients with cervicogenic headache. This suggests that cervical mechanical intervention has the potential to alter downstream cerebral circulation. However, the intervention targets were cervical joints and muscles. Investing fascial or sheath-wall mechanical properties were not measured. The sample size was only 25, and the between-group difference in blood flow indicators did not reach significance. It provides only upper-level corroboration that cervical mechanical input can modulate cerebral circulation. It cannot replace direct verification of the investing fascia–sheath wall conduction pathway.

At the level of PIEZO1-targeted intervention, Matrongolo et al. [47] demonstrated that Yoda1 treatment can restore meningeal lymphangiogenesis, drainage, and brain–cerebrospinal fluid perfusion in mice with craniosynostosis and in aged mice. Du et al. [62] demonstrated that prostaglandin F2α can restore cervical lymphatic pumping function and cerebrospinal fluid clearance capacity in aged mice by increasing smooth muscle contractility. He et al. [33] demonstrated that AAV-mediated Piezo1 knockdown can improve skin fibrosis and skin stiffness in mice (see Section 4.1.3). All three are mouse models, specific disease or aging or skin fibrosis backgrounds, and pharmacological or genetic intervention approaches. They cannot be directly equated with the chronic fibrotic sheath-wall constraint scenario of interest to the hypothesis. They provide only a mechanistic background for exploratory intervention endpoints. The systematic review of Della Pietra et al. [53] indicates that clinical evidence in the field of mechanosensitive receptors in migraine is overall scarce. The intervention predictions proposed by this hypothesis should therefore be positioned as exploratory endpoints within a tiered validation framework.

7.5. Refutation Criteria

This hypothesis should be weakened or refuted at least under the following circumstances.

First, for primary endpoints, after controlling for age, sex, body mass index, hypertension, cervical degeneration, and medication overuse, investing fascial mechanical parameters show no association with internal jugular vein flow velocity, vertebral artery V2-segment strain, or the vertebral artery–to–internal carotid artery wall-stiffness ratio. Sheath-wall thickness is treated as an exploratory endpoint. A null association with sheath-wall thickness is reported as an exploratory negative finding, not as a primary refutation criterion. Sheath-wall SWE is not used for refutation until its reliability has been validated. After validation, a null association with sheath-wall SWE is treated as an exploratory negative finding, not as a primary refutation criterion (see Section 8.3.1).

Second, computational simulation shows that, within the physiological range of head and neck movement, even when the investing fascia–to–sheath wall transmission coefficient is taken at the primary sensitivity upper bound of 30%, and additionally in a sensitivity scenario at the human magnitude reference of 82%–89%, it cannot produce detectable stress or strain increments in the sheath wall or intrasheath structures. The 30% bound is for fascial network force transmission. The 82%–89% bound is external cervical pressure to the carotid sinus and is not the same pathway (see Section 4.2.2).

Third, PIEZO1 or PIEZO2 and CGRP colocalization exists in the target tissue, but mechanical loading cannot trigger CGRP release or PIEZO-dependent calcium responses.

Fourth, in a prospective cohort, the vertebral artery–to–internal carotid artery RoR ratio and wall-stiffness ratio cannot modify the association between investing fascial mechanical parameters and posterior circulation perfusion outcomes.

Fifth, after fascial release or postural intervention, there is no immediate change in intrasheath arterial cross-sectional area, internal jugular vein flow velocity, or autonomic indicators.

The establishment of any one of the above constitutes refutation of the corresponding step of the hypothesis. If multiple are established simultaneously, the core chain of the hypothesis does not hold.

8. Validation Strategy

8.1. Histological and Immunohistochemical Validation

8.1.1. Sampling Targets and Multi-Marker Colocalization

Sampling should cover the investing fascia itself, the common layer of the sheath wall, and the adventitial layers of each vessel. TGF-β–driven myofibroblast activation and collagen deposition should be used as the histological reference for the degree of fibrosis. The molecular pathological framework of palmar fascial fibrosis reviewed by Pirri [31] can be referenced.

The multi-marker colocalization paradigm of PIEZO2, CGRP, and TH established by Tereshenko et al. [40] in the trapezius fascia should be directly transferred to investing fascial sampling. Liao et al. [35] demonstrated the PIEZO2–CGRP or substance P positive feedback loop. In histological analysis of the investing fascia–sheath wall continuum, the spatial relationships among PIEZO2, CGRP, substance P, and their receptors should therefore be detected simultaneously. These receptors include the calcitonin receptor-like receptor (CRLR), receptor activity-modifying protein 1 (RAMP1), and neurokinin 1 receptor (NK1R). Gkouzioti et al. [36] demonstrated indirect CGRP-to-PIEZO1 crosstalk. At the endothelial cell layer, the spatial relationships among CGRP receptors, PIEZO1, and endothelial markers should therefore be jointly detected. Ex vivo tissue preparations can be used to test whether CGRP pretreatment enhances PIEZO1 agonist–induced calcium responses.

The PIEZO1 and PIEZO2 subtype functional differentiation summarized by Zhao L et al. [52] provides a background framework for the default subtype division-of-labor model in colocalization analysis. Final confirmation of subtype division of labor still requires direct functional verification in the target tissue. Alba et al. [48] established the PIEZO1 and PIEZO2 immunohistochemical distribution in the carotid sinus, carotid body, and superior cervical ganglion. This provides a reference for antibody specificity and positive control tissues. It also suggests that the superior cervical ganglion should be treated as an independent analytical node in sampling of the investing fascia–sheath wall–sympathetic pathway. Konishi et al. [50] reported upregulated PIEZO1 expression in myofibroblasts of human carotid plaques. This provides a human positive reference for PIEZO1 detection in the investing fascia–sheath wall–vascular adventitia continuum. This is a pilot study with an atherosclerotic background and a cross-sectional design. Full boundaries are given in Section 9.

He et al. [33] established the PIEZO1–Wnt2/Wnt11–CCL24 fibrotic loop. In histological analysis of the investing fascia–sheath wall continuum, the spatial relationships among PIEZO1, Wnt2/Wnt11, CCL24, and α-SMA should therefore be detected simultaneously. Future studies should also test whether mechanical loading drives the same positive feedback program. This prediction should be labeled as a cross-tissue mechanistic test (see Section 4.1.3). Zeng et al. [49] demonstrated the functional necessity of PIEZO1 and PIEZO2 in the baroreceptor reflex. The petrosal and nodose ganglia should therefore serve as positive control tissues for PIEZO functional validation of the vagal pathway.

8.1.2. Pathway Landmarks, Lymphatic Terminal Markers, and Sampling Records

Nonomura et al. [45] established the PIEZO1 mechanotransduction paradigm in lymphatic endothelial cells. In sampling of the intrasheath lymphatic terminal and deep cervical lymph nodes, the spatial relationships among PIEZO1, lymphatic endothelial markers, and valve or adhesion markers should therefore be detected simultaneously. These markers include lymphatic vessel endothelial hyaluronan receptor 1 (LYVE1), prospero homeobox protein 1 (PROX1), VE-cadherin, and integrin α9. Choi et al. [46] and Matrongolo et al. [47] demonstrated in vivo that PIEZO1 regulates and restores meningeal lymphatic drainage function. On this basis, functional validation of the lymphatic pathway can jointly analyze PIEZO1 expression or activation status with cerebrospinal fluid drainage efficiency, meningeal lymphatic vessel morphology, and tracer accumulation in deep cervical lymph nodes. This evidence involves mouse models, Yoda1 activation, and craniosynostosis or aging background. Full boundaries are given in Section 9. Du et al. [62] showed that aging leads to decreased α-SMA expression, contraction frequency, and flow velocity in cervical lymphatic vessels. In histological validation of lymphatic vessels, α-SMA, valve function, and pumping frequency should therefore be assessed in parallel. This evidence involves a mouse aging model and local prostaglandin F2α application. Full boundaries are given in Section 9.

The intrasheath location of deep cervical lymph nodes confirmed by Clapham et al. [38] should be marked at the time of sampling. The meningeal lymphatic vessel–nasopharyngeal lymphatic plexus–deep cervical lymph node pathway established by Louveau et al. [57], Aspelund et al. [58], and Yoon et al. [17] should be recorded as an upstream anatomical reference during lymphatic pathway sampling. The tight adherence relationship between the sympathetic trunk and the sheath wall, and the quantitative anatomical parameters confirmed by Kiray et al. [25] and Civelek et al. [26], should be recorded at the time of sampling.

Struthoff et al. [39] established the quantitative distribution of human perivascular nerve fibers around the carotid bifurcation. Histological validation of the sympathetic pathway should therefore simultaneously record the density of adventitial TH-positive fibers and their distance from the intimal surface. This should serve as an independent indicator of target vessel–side sympathetic input. Zhang J et al. [41], Zhang FR et al. [42], Luu et al. [43], and Beech [44] demonstrated or integrated PIEZO1-dependent carotid structural remodeling, neointimal hyperplasia, aging-related decline in mechanosensation, and aging-related vascular wall stiffening. In arterial-side sampling, PIEZO1 expression, the degree of vascular wall fibrosis, neointimal hyperplasia markers, mechanosensory or aging-related markers, and aging-related stiffening indicators can therefore be assessed in parallel. The vertebral artery endothelial senescence mechanism proposed by Wang et al. [7] suggests that posterior circulation arterial-side sampling can explore the spatial relationship between endothelial senescence markers and local mechanical parameters. This should be labeled as a mechanistic hypothesis test.

8.1.3. Layered or Segmental Sampling and Validation Priority

Zhao et al. [14] described a three-layered fascial architecture in the rabbit sheath wall, with earliest and most marked thickening of the arterial inner fascial layer. Hayashi [15] described laminar adventitia plus common sheath layering in adult human cervical segments, with inter-individual and site variation in thickness (see Section 2.2.1). Sheath-wall sampling should therefore be performed layer by layer. Individual and sampling segment should be recorded. Schachtel et al. [16] reported regional heterogeneity of fascial sources and layer numbers of the sheath wall. Sheath-wall sampling should therefore consider segmental factors and record the fascial composition of the sampled segment. Martínez-Sanz et al. [11] reported a developmental anatomical association between the middle deep cervical fascia and lymphatic or venous structures. In histological analysis of the deep fascia–lymphatic or venous interface, the spatial relationship between fascial layers and conduits should therefore be assessed simultaneously.

The systematic reviews of Della Pietra et al. [53] and Bagheri et al. [65] noted the scarcity of human evidence and the inadequacy of causal designs. Histological validation should therefore prioritize human tissue and in vivo reproducible marker combinations, rather than relying solely on inference from animal models.

8.2. Computational Simulation Validation

8.2.1. Model Geometry and Material Inputs

A fluid–structure interaction finite element model directly uses the geometry and material parameters of the investing fascia–sheath wall–vessel as input. The model quantifies the transmission efficiency of investing fascial tension to the sheath wall and intrasheath structures. The model should incorporate the heterogeneous material properties of each layer of the deep fascia, namely investing, pretracheal, prevertebral, and alar fascia. It should include the geometric configuration of the carotid sheath, including its anatomical variants whose prevalence Bond et al. [13] have noted. It should also include the fluid–structure interaction boundaries of the internal carotid artery, internal jugular vein, and surrounding tissues.

At the material property level, the anisotropic constitutive parameters of deep fascia provided by Aparici-Gil et al. [10] should serve as model input. Under uniaxial tension, longitudinal stress is approximately 6.6 times the transverse stress. Under planar tension, the ratio is approximately 3.9 to 1. Under equibiaxial tension, it is approximately 2.6 to 1. The coupled exponential model can simultaneously reproduce all three strain states. The model should perform parameter sweeps separately along the fiber direction and perpendicular to the fiber direction. It should treat the anisotropy ratio as a parameter to be tested.

Zhao et al. [14] described a three-layered fascial architecture in the rabbit sheath wall, with earliest and most marked thickening of the arterial inner fascial layer. Hayashi [15] described laminar adventitia plus common sheath layering in adult human cervical segments, with inter-individual and site variation in thickness (see Section 2.2.1). These findings require the model to parameterize the sheath wall by layer. Inter-individual and segmental variation should be treated as sensitivity analysis dimensions. Schachtel et al. [16] reported regional heterogeneity of fascial sources and layer numbers of the sheath wall. The model should therefore consider segmental factors or explicitly annotate the sampling segment during parameterization.

The intrasheath lymph nodes localized by Clapham et al. [38], with a mean diameter of 13.5 × 4.8 mm, should be incorporated into the model as independent mechanical entities. The sympathetic trunk anatomical parameters provided by Kiray et al. [25] and Civelek et al. [26] should serve as anatomical input for the geometric positioning and mechanical coupling modeling of the sympathetic trunk. These parameters include a distance of approximately 11.6 mm from the medial border of the longus colli muscle at the C6 level, a diameter of approximately 3.3 mm, and a location deep to the prevertebral fascia. The distance of human carotid adventitial sympathetic fibers provided by Struthoff et al. [39], approximately 2.5 mm at the closest point, can serve as an independent parameter for the geometric boundary of the sympathetic terminal side in the model.

8.2.2. Transfer Function and Parameter Sweeps

The key output is the transfer function. Within the physiological range of head and neck movement, the transfer function describes the stress and strain increments produced by investing fascial tension changes at the sheath wall, contents, intrasheath lymph nodes, and sympathetic trunk surface. The effective perivascular pressure and circumferential stress reduction magnitudes provided by Liu et al. [37] can serve as initial calibration references for the boundary conditions of this transfer function. They must be treated in the model as parameters to be tested rather than as fixed conclusions.

Querry et al. [12] reported a transmission efficiency from external cervical pressure to the carotid sinus of 82%–89%. This can serve as a secondary sensitivity scenario and human magnitude reference. However, this pathway is not the investing fascia–to–sheath wall pathway. The model should not treat it as a fixed transmission coefficient for this specific pathway (see Section 4.2.2). The fascial force transmission ratio upper bound of approximately 30%, reviewed by Wilke et al. [8] and Kalichman [9], can serve as the primary sensitivity upper bound for the investing fascia–to–sheath wall transmission coefficient. However, these data do not derive from the cervical investing fascia–sheath wall pathway. The model should perform parameter sweeps within the 0–30% primary range. It should additionally run a secondary sensitivity scenario at the 82%–89% human magnitude reference. The transmission coefficient should be treated as a parameter to be tested. Combined with the anisotropic data of Aparici-Gil et al. [10], these range sweeps should be performed separately along the fiber direction and perpendicular to the fiber direction.

8.2.3. Lymphatic, Vertebral Artery, and Systemic Modulating Variables

Jin et al. [60] reported effect magnitudes that can serve as initial references for the mechanical constraint–to–drainage efficiency step in the lymphatic pathway transfer function. These magnitudes include mechanical compression doubling cerebrospinal fluid outflow, a 2.81-fold increase in lymph node tracer accumulation, and a 4.7-fold increase in superficial lymphatic vessel accumulation. This evidence involves mouse superficial lymphatic vessels and short-term external compression. Full boundaries are given in Section 9. The cervical lymphatic vessel fluid–structure interaction framework established by Kim and Tithof [61] can serve as a tissue-level reference for parameterizing lymphatic pathway pumping efficiency. If sheath-wall mechanical constraint is translated into changes in cervical lymphatic vessel transmural pressure or wall pretension, this framework can be referenced to predict the direction and magnitude of the pumping efficiency response. This evidence is mouse-derived, two-dimensional, and uses single or three-unit simplification. Full boundaries are given in Section 9.

Du et al. [62] showed that aging decreases contraction frequency, flow velocity, and α-SMA expression in cervical lymphatic vessels. They also showed that prostaglandin F2α can restore pumping function. The lymphatic pathway model should therefore incorporate aging-related pumping decline as a parameter dimension independent of PIEZO1, using α-SMA and pumping frequency as calibratable parameters. This evidence involves a mouse aging model and pharmacological intervention. Full boundaries are given in Section 9. The meningeal lymphatic vessel–nasopharyngeal lymphatic plexus–deep cervical lymph node pathway established by Louveau et al. [57], Aspelund et al. [58], and Yoon et al. [17] can serve as the upstream boundary and pathway parameterization reference for the lymphatic pathway model.

Nonomura et al. [45], Choi et al. [46], and Matrongolo et al. [47] demonstrated lymphatic endothelial PIEZO1 mechanotransduction, PIEZO1 regulation of meningeal lymphatic drainage function, and pharmacological activation–restored drainage function. These findings can serve as parameterization hypotheses for the lymphatic endothelial–side mechanosensitive node and drainage function output in the model. This evidence involves developmental valve formation, acute pharmacological stimulation in vitro, mouse transgenic or Yoda1 activation, and craniosynostosis or aging backgrounds. Full boundaries are given in Section 9. Ren et al. [63] and Wu et al. [64] reported abnormal human meningeal lymphatic drainage kinetics and impaired glymphatic function. These findings can serve as directional references for human clinical endpoints of the lymphatic pathway. However, their measurement sites are meningeal lymphatic vessels and glymphatic function rather than the intrasheath terminal. They lack longitudinal follow-up and cannot provide causal parameters. Full boundaries are given in Section 9.

Vertebral artery modeling should additionally incorporate the constraint conditions of the perivascular fascial environment of the transverse foramen. Zhou et al. [23] showed in an ex vivo mechanical study that the wall stress distribution of vertebral artery segments cannot be predicted without considering the perivascular environment. The RVAS dynamic ultrasound thresholds of Meng et al. [24] can serve as a magnitude reference between model output and clinical hemodynamic endpoints. Their osseous or acute compression conditions should be annotated separately during parameterization. The single-case feasibility reports of Dadsetan et al. [21] and Nguyen et al. [22] suggest that the model can set parameterized scenarios of fibrous band or fascial band compression in the external soft-tissue constraint module. It should be noted that their etiology is local fibrous bands rather than investing fascial fibrosis.

The vertebral artery endothelial senescence mechanism proposed by Wang et al. [7] can serve as a candidate connecting hypothesis between model output and chronic biological endpoints. It is a narrative review and cannot provide directly parameterizable quantitative relationships. The stress-state systemic compensatory strategies revealed by Neumann et al. [4] suggest that a model simulating only local cerebrovascular regulation while ignoring the systemic pressor response may underestimate the posterior circulation perfusion maintenance capacity of individuals with VAH. However, this mechanism involves the systemic circulation and should be annotated separately as a moderating variable outside the model boundary. Gaigalaite et al. [5] revealed that the posterior configuration of the circle of Willis modifies the clinical vulnerability of VAH. A model simulating only local vertebral artery mechanics without incorporating collateral configuration parameters may therefore fail to fully explain the heterogeneity of posterior circulation outcomes among individuals with VAH. This factor is a structural variable at the vascular network level and should be annotated separately as an effect-modifying parameter outside the local finite element model. Sato et al. [6] revealed VAH-related impairment of CO₂ reactivity. A model simulating only passive luminal constraint while ignoring vascular reactivity reserve may therefore underestimate the functional consequences of posterior circulation low reserve. This should be annotated separately as a supplementary functional endpoint outside the local mechanical model.

Martínez-Sanz et al. [11] revealed a developmental anatomical association between the middle deep cervical fascia and lymphatic or venous structures. If the model incorporates the deep fascia–lymphatic or venous interface, developmental stage differences should be considered. However, this evidence is descriptive or associative and cannot provide directly parameterizable mechanical relationships. The single-case feasibility report of Hou et al. [20] suggests that the model can set a parameterized scenario of luminal compression caused by increased perivascular volume in the perivascular soft-tissue thickening module. Its inflammatory etiology and single-case evidence level limit its generalizability.

8.2.4. Downstream Mechanistic Hypotheses to Be Tested

Dapper et al. [70] reported SWE stiffness elevation of post-radiotherapy cervical fibrotic fascia. This can serve as a magnitude reference for the fibrotic fascia stiffness parameter in the model. Its etiological and site boundaries need to be recalibrated in the investing fascia and sheath-wall scenario (see Section 7.2.1). Hayashi [15] reported sheath-wall layering and inter-individual or site variation of adult human cervical segments. This can serve as a histological reference for the geometric and layered parameterization of the sheath wall in the model. It does not provide mechanical parameters (see Section 2.2.1).

The arterial-side mechanical endpoints of the model output can be defined and reported with reference to the ultrasound biomechanical parameters validated by Ghahremani et al. [68]. Capron et al. [69] demonstrated the systematic influence of vascular geometry on shear-wave velocity measurement. The model output should therefore simultaneously report geometric parameters and material parameters. As discussed in Section 8.4.2, device implementation and imaging depth should be treated as prespecified dimensions of sensitivity analysis.

Zhang J et al. [41], Zhang FR et al. [42], Luu et al. [43], and Beech [44] can serve as mechanistic connecting hypotheses between model output and histological or functional endpoints. They should be annotated in the simulation as downstream effects to be tested. Liao et al. [35] demonstrated the PIEZO2–CGRP or substance P positive feedback loop. This can serve as a parameterization hypothesis for mechanotransduction at the nerve terminal side and CGRP or substance P release coupling in the model. Its model background is trigeminal nerve root compression rather than fascial mechanical conduction. Gkouzioti et al. [36] demonstrated the CGRP sensitization–enhanced PIEZO1 calcium response. This can serve as a parameterization hypothesis for endothelial-side CGRP–PIEZO1 crosstalk in the model. It involves acute pharmacological stimulation in vitro.

Alba et al. [48] reported high PIEZO1 and PIEZO2 expression in the superior cervical ganglion. The model can therefore incorporate the sympathetic ganglion as an independent mechanosensitive node for parameterization exploration. Its normal distribution data are insufficient to determine functional parameters. Zeng et al. [49] demonstrated the functional necessity of PIEZO1 and PIEZO2 in the baroreceptor reflex. This can serve as a functional reference for parameterizing mechanotransduction of the vagal or baroreceptor pathway in the model. It is a mouse baroreceptor model. Meyer et al. [55] reported blood flow changes under direct sympathetic nerve stimulation. This can serve as an animal experimental reference for parameterizing the distal effects of the sympathetic pathway in the model. The stimulation type and intensity must be annotated as independent parameters. Washio et al. [2] reported the vertebral artery–posterior cerebral artery site difference. This suggests that separate distal effect nodes should be set within the posterior circulation.

8.3. Prospective Human Mechanistic Validation Cohort

8.3.1. Exposure Variables, Outcomes, and Mediation Design

The cohort should use imaging or ultrasound mechanical parameters as exposure variables. These include investing fascial SWE values, sheath-wall thickness, internal jugular vein flow velocity, vertebral artery transverse foramen segment strain, and the vertebral artery–to–internal carotid artery wall-stiffness ratio. Outcomes should include headache frequency, ocular indicators, autonomic function assessed by heart rate variability, sympathetic tone indicators, and posterior circulation perfusion indicators. The design should prespecify stratification of anterior circulation–predominant and posterior circulation–predominant phenotypes. It should also prespecify stratification by sex and stress condition, based on the differentiation findings of Friend et al. [19] and Chen et al. [29]. Posterior circulation outcomes should further analyze the vertebral artery and posterior cerebral artery separately, as suggested by Washio et al. [2]. Key variables in mediation analysis should include imaging-based estimation of effective luminal radius, CGRP levels, and sympathetic tone indicators. If the semi-quantitative predictions of the hypothesis hold, the effective radius-to-flow path coefficient in the posterior circulation mediation model should be significantly greater than that in the anterior circulation. The anterior circulation path coefficient should be attenuated by the mediation of autoregulation.

Anterior circulation outcomes are stratified at the three nodes specified in 2.3.1: cervical ICA wall stiffness by SWE as the peripheral mechanical node; MCA dCA or cerebrovascular conductance index as the regulatory node; and ACA indices as exploratory only. The VA-to-ICA wall-stiffness ratio is computed against the cervical ICA node, and this restriction is stated explicitly so that the ratio is not interpreted as an anterior-circulation-wide quantity. Where MCA dCA is available, a secondary VA-to-MCA ratio is reported as an exploratory contrast. The VA-to-ICA RoR ratio inherits the ICA-system denominator of Friend et al. [19], which is not segmentally resolved. This ratio is therefore reported as an ICA-system-level contrast, not as a cervical-ICA-node contrast. Where MCA dCA is available, the secondary VA-to-MCA ratio supplies the segmentally resolved counterpart. The two ratios are not interchangeable and are reported separately.

Human measurement reliability for sheath-wall SWE has not yet been established (see Section 4.2.4 and Section 9). Sheath-wall SWE in this cohort is therefore used only as an exploratory exposure variable. The main hypothesis tests are temporarily based on investing fascial SWE, internal jugular vein flow velocity, vertebral artery V2-segment strain, and the vertebral artery to internal carotid artery wall-stiffness ratio. Sheath-wall SWE can be promoted to a primary endpoint only after its reliability has been validated on the target device, at the target depth, and at the target segment.

8.3.2. Measurement Reliability and Covariate Calibration

The cervical arterial wall SWE reliability data of Thomas et al. [27] and the head-rotation paradigm of Thomas et al. [18] can be directly transferred to the arterial-side mechanical assessment of the cohort. The cervical fascial SWE reliability and minimal detectable change validated by Zandi et al. [67] can serve as the basis for sample-size estimation and measurement error control. The carotid ultrasound biomechanical parameters validated by Ghahremani et al. [68] can serve as candidate indicators for arterial-side mechanical endpoints. Capron et al. [69] revealed geometric dependence. Vessel diameter and wall thickness should therefore be included as covariates in the analysis or model calibration. As discussed in Section 8.4.2, device implementation and imaging depth should be treated as prespecified covariates. Reliability should be reported independently on the target device and at the target depth.

Dapper et al. [70] provided the cervical fascial fibrosis SWE effect magnitude. This can serve as a reference for the direction and sample-size estimation of the investing fascial SWE exposure variable. However, it involves post-radiotherapy fibrosis and the sternocleidomastoid-region fascia, with synchronous elevation of fascial and muscle stiffness. It requires recalibration at the target depth and in the target population of the investing fascia (see Section 7.2.1). Hayashi [15] reported sheath-wall layering and inter-individual or site variation of adult human cervical segments. Sheath-wall thickness or stiffness measurement should therefore be recorded layer by layer and should consider segmental and individual factors. However, this is histological evidence and does not provide in vivo measurement reliability (see Section 2.2.1). The internal jugular vein compression case series of Ma et al. [56] suggests that venous outflow indicators should be incorporated as pathway-specific outcomes of the venous pathway.

8.3.3. VAH Subgroup and Posterior Circulation Effect Modifiers

Mirkovic et al. [30] summarized VAH evidence. Individuals with congenital posterior circulation vessel-diameter variants should be included in the analysis as a prespecified subgroup or covariate. The prevalence data for vertebral artery dominance, hypoplasia, and aplasia provided by Triantafyllou et al. [3] can provide epidemiological parameters for sample-size estimation of this prespecified subgroup. Neumann et al. [4] suggest that, in this subgroup, the change in total peripheral resistance under stress should be measured simultaneously. This would test whether it constitutes an effect-modifying factor for posterior circulation perfusion outcomes. Gaigalaite et al. [5] further suggest that, in this subgroup, the posterior configuration of the circle of Willis should be treated as a prespecified stratification variable or covariate. Sato et al. [6] suggest that, in this subgroup, CO₂ reactivity can provide a supplementary functional endpoint of posterior circulation reserve.

The regional evidence synthesis of Srichawla et al. [28] can serve as background for the selection and directional prespecification of posterior circulation dynamic cerebral autoregulation outcome indicators. Its narrative review nature determines that it cannot replace the quantitative differentiation data of Friend et al. [19]. The RVAS dynamic ultrasound thresholds provided by Meng et al. [24] can serve as candidate binary indicators for posterior circulation hemodynamic endpoints. The difference between their osseous or acute compression source and the chronic fascial constraint scenario of the hypothesis should be made explicit. The vertebral artery endothelial senescence mechanism proposed by Wang et al. [7] suggests that the posterior circulation outcome module can incorporate endothelial senescence-related biomarkers as exploratory endpoints. This exploration should be explicitly labeled as a mechanistic hypothesis test.

8.3.4. Exploratory Pathway Modules

The distal effects of the sympathetic pathway should serve as an independent exploratory analysis module. It should test whether the direction and magnitude of change in the vertebral artery–to–internal carotid artery blood flow ratio exhibit a nonlinear inflection before and after sympathetic tone crosses the threshold, based on the Kaur et al. [54] paradigm. It should also test whether this inflection is associated with investing fascial mechanical parameters. The direct electrical stimulation data of Meyer et al. [55] suggest that this module should also record the type and intensity of sympathetic activation. The superior cervical ganglion PIEZO expression data of Alba et al. [48] suggest that mechanosensitivity markers at the sympathetic ganglion level can be explored. The adventitial TH-positive fiber distribution of Struthoff et al. [39] suggests that this module should treat target vessel–side nerve density and distance as prespecified covariates. The adrenergic innervation data of deep cervical lymphatic vessels of Yoon et al. (17) suggest that this module should be analyzed jointly with the lymphatic module at the cervical terminal. Washio et al. (2) suggest that this module should treat the vertebral artery and posterior cerebral artery as separate outcomes.

Mechanotransduction of the vagal pathway should serve as an independent exploratory module. This module should test whether baroreceptor reflex sensitivity, blood pressure variability, or the baroreflex component of heart rate variability is associated with investing fascial mechanical parameters. Zeng et al. [49] showed that PIEZO1 and PIEZO2 are essential sensors for the baroreceptor reflex. This finding provides the functional basis for the mechanistic hypothesis of this module. The PIEZO1 and PIEZO2 expression detected by Alba et al. [48] in the human petrosal ganglion and carotid sinus provides the histological reference for this module. This evidence involves a mouse baroreceptor model, not the sheath wall–to–vagus nerve pathway. Full boundaries are given in Section 9.

The mechanical step of the lymphatic pathway should serve as an independent exploratory module. It should test whether investing fascial mechanical parameters are associated with internal jugular vein flow velocity, cerebrospinal fluid drainage surrogate indicators, or intrasheath lymph node imaging features. The mechanical compression–induced doubling of drainage efficiency reported by Jin et al. [60] provides a reference for effect magnitude and directional prespecification of this module. This evidence involves mouse superficial lymphatic vessels and short-term external compression. Full boundaries are given in Section 9. The pathway established by Louveau et al. [57], Aspelund et al. [58], and Yoon et al. [17] suggests that this module should incorporate nasopharyngeal or deep cervical lymphatic vessel imaging features and deep cervical lymph node imaging features as human endpoints. Nonomura et al. [45], Choi et al. [46], and Matrongolo et al. [47] suggest that this module can incorporate lymphatic endothelial PIEZO1–related markers, cerebrospinal fluid drainage function indicators, and PIEZO1 pharmacological modulation response as molecular and functional endpoints. Kim and Tithof [61] and Du et al. [62] suggest that this module can incorporate parameterized analyses of cervical lymphatic vessel transmural pressure, wall pretension, pumping frequency, α-SMA, and pumping efficiency as tissue-level mechanical and functional endpoints. Ren et al. [63] and Wu et al. [64] suggest that the human outcomes of this module should include meningeal lymphatic drainage kinetic indicators, glymphatic function indicators, and deep cervical lymph node imaging features. Disease-severity stratification should be prespecified between episodic and chronic migraine. Measurement sites are meningeal lymphatic vessels and glymphatic function rather than the intrasheath terminal, and longitudinal follow-up is lacking. Full boundaries are given in Section 9.

Zhang J et al. [41], Zhang FR et al. [42], Luu et al. [43], and Beech [44] can serve as mechanistic hypotheses for arterial-side structural or functional endpoints. These can be explored in the cohort through joint measurement of PIEZO1-related biomarkers and imaging structural indicators. Konishi et al. [50] suggest that, in the arterial-side histological subgroup, PIEZO1 expression can be jointly analyzed with plaque or wall structural instability markers. This is a pilot study with an atherosclerotic background. Full boundaries are given in Section 9. Liao et al. [35] demonstrated PIEZO2–CGRP or substance P positive feedback. In the histological subgroup, the association of PIEZO2, CGRP, substance P, and their receptor co-expression with fascial mechanical parameters and pain phenotypes can therefore be explored. This should be explicitly labeled as a mechanistic hypothesis test. Gkouzioti et al. [36] demonstrated CGRP-to-PIEZO1 crosstalk. In the endothelial function subgroup, the interaction between CGRP levels and PIEZO1-dependent vascular reactivity can therefore be explored. This should be explicitly labeled as a human translational hypothesis of an in vitro mechanism. He et al. [33] demonstrated the PIEZO1–Wnt2/Wnt11–CCL24 fibrotic loop. In the histological subgroup, the co-expression of PIEZO1, Wnt2/Wnt11, CCL24, and α-SMA in the investing fascia–sheath wall continuum and its association with SWE mechanical parameters can therefore be explored. This should be explicitly labeled as a cross-tissue mechanistic test (see Section 4.1.3). The systematic review of Della Pietra et al. [53] confirmed the scarcity of clinical evidence. This cohort should therefore prioritize reproducible human mechanical and molecular markers. Positive results should be treated as a starting point for tiered validation rather than as an endpoint.

8.3.5. Proxy for Cross-Layer Transmission Efficiency

Inter-individual differences in cross-layer transmission efficiency should be treated as prespecified covariates or stratification factors. A proxy indicator of transmission efficiency can be constructed based on imaging-estimated investing fascial stiffness, sheath-wall thickness, and cervical geometric parameters. Future analyses should test whether this proxy modifies the association between investing fascial mechanical parameters and downstream outcomes. The ex vivo approximately 30% upper bound reviewed by Wilke et al. [8] and Kalichman [9] can serve as a background reference for the directional expectation of this proxy. It cannot serve as a direct individual-level conversion coefficient. The 82%–89% reported by Querry et al. [12] can serve as a magnitude upper-bound background reference for a different pathway. It likewise cannot provide a direct individual-level conversion coefficient (see Section 4.2.2). The anisotropic data of Aparici-Gil et al. [10] suggest that this proxy should consider the angle between fiber direction and measurement direction. The sheath-wall layering evidence of Zhao et al. [14] and Hayashi [15] suggests that this proxy should consider the mechanical contribution of each sheath-wall layer separately and record individual and segmental variation (see Section 2.2.1). The sheath-wall regional heterogeneity evidence of Schachtel et al. [16] suggests that this proxy should record the sampling segment. The middle deep cervical fascia–lymphatic or venous developmental anatomical association of Martínez-Sanz et al. [11] suggests that, if this proxy involves the deep fascia–lymphatic or venous interface, developmental stage differences should be considered.

8.4. Evidence Grading and Citation Principles

8.4.1. Four-Tier Evidence Grading

This article applies a two-axis classification of evidence source and design strength rather than a single three-tier ranking.

Human in vivo functional or mechanical evidence (primary) includes Friend et al. [19], Thomas et al. [18], Ren et al. [63], Wu et al. [64], Kaur et al. [54], Washio et al. [2], Meng et al. [24], Neumann et al. [4], Uzun et al. [73], Gaigalaite et al. [5], Querry et al. [12], and Sato et al. [6].

Human observational or transferable evidence includes two subtypes. The single-case feasibility subtype includes Hou et al. [20], Dadsetan et al. [21], and Nguyen et al. [22]. The quantitative transferable-reference subtype includes Dapper et al. [70]. These subtypes are distinguished because they carry different inferential weight. Single-case feasibility reports demonstrate that a mechanical event is possible. Quantitative transferable references provide an effect-size anchor for study design. Neither subtype provides primary causal evidence.

Human histological or human-derived ex vivo evidence includes Alba et al. [48], Tereshenko et al. [40], Gkouzioti et al. [36], Clapham et al. [38], Konishi et al. [50], Schachtel et al. [16], Martínez-Sanz et al. [11], Caroccia et al. [32], Struthoff et al. [39], and Hayashi [15].

Animal in vivo causal evidence includes Zeng et al. [49], Liao et al. [35], Zhang J et al. [41], Zhang FR et al. [42], Louveau et al. [57], Aspelund et al. [58], Yoon et al. [17], Nonomura et al. [45], Choi et al. [46], Matrongolo et al. [47], Zhao et al. [14], Kim and Tithof [61], Du et al. [62], and He et al. [33].

Animal or ex vivo functional evidence, material parameters, reviews, and magnitude references include Mikhailov et al. [34], Luu et al. [43], Beech [44], Aparici-Gil et al. [10], Liu et al. [37], Wilke et al. [8], Kalichman [9], Srichawla et al. [28], Della Pietra et al. [53], Bagheri et al. [65], Choi B et al. [66], Wang et al. [7], Triantafyllou et al. [3], and Pruijssen et al. [71].

Hou et al. [20], Dadsetan et al. [21], and Nguyen et al. [22] are all single-case feasibility reports and represent the lowest tier of in vivo human clinical observation. Their value is strictly limited to single-case feasibility evidence, and they carry no causal inference weight. Dapper et al. [70] is an in vivo human SWE quantitative study. Its etiology of post-radiotherapy fibrosis, its site in the sternocleidomastoid-region fascia, and its synchronous fascia–muscle involvement limit it to a transferable reference for the SWE effect size of cervical fascial fibrosis (see Section 7.2.1). Hayashi [15] is human histological evidence and does not provide mechanical properties. Its finding that the common sheath does not fuse with the superficial fascia requires that the functional continuity from the investing fascia to the sheath wall be treated as a premise to be verified (see Section 2.2.2). He et al. [33] is animal in vivo causal plus human cell or tissue mechanistic evidence. Its model is skin fibrosis and cannot replace direct evidence from fascia or the sheath wall (see Section 4.1.3).

8.4.2. Current Gaps

The unified statement of the gaps across the full text is given in Section 4.2.4. Section 9 elaborates their specific boundaries. Pruijssen et al. [71] provided a technical boundary reference for inter-device variability, depth dependence, and stiffness underestimation in SWE measurement of thin-walled structures. The object of that evidence is thin-walled vascular phantoms and the carotid artery, not the sheath wall. Reliability validation of sheath-wall SWE therefore cannot assume reproducible measurement of thin-walled structures. It must treat device implementation, imaging depth, and stiffness magnitude as prespecified covariates. This is the boundary qualification for Gap 2.

No study has yet directly simulated the complete chain of investing fascial fibrosis, intrasheath compression, and sympathetic or vagal dysfunction. The CGRP–meningeal lymphatic vessel–migraine model established by Nelson-Maney et al. [59] provides a referable experimental paradigm for downstream validation of the lymphatic pathway. The mechanical compression–lymphatic drainage efficiency paradigm established by Jin et al. [60] provides a transferable experimental strategy for the mechanical functional step of the lymphatic pathway. Superficial lymphatic vessels and mouse models need to be revalidated in the intrasheath terminal and human scenarios.

The cervical lymphatic vessel fluid–structure interaction framework established by Kim and Tithof [61] provides a transferable framework for tissue-level mechanical parameterization of the lymphatic pathway. The mouse source, two-dimensional nature, and single or three-unit simplification need to be re-examined in the complete human cervical lymphatic network and chronic fibrotic constraint scenarios. The aging-related cervical lymphatic pumping decline and prostaglandin F2α restoration paradigm established by Du et al. [62] provides a transferable experimental strategy for the aging dimension and recoverability of the lymphatic pathway. Mouse aging models and pharmacological interventions need to be re-examined in human and chronic fascial constraint scenarios.

Louveau et al. [57], Aspelund et al. [58], and Yoon et al. [17] established the meningeal lymphatic vessel–nasopharyngeal lymphatic plexus–deep cervical lymph node pathway. This pathway provides a transferable localization framework for the anatomical and functional starting point of the lymphatic pathway. Species and stimulation scenarios need to be re-examined in the human intrasheath terminal. Nonomura et al. [45], Choi et al. [46], and Matrongolo et al. [47] provide transferable experimental strategies for the molecular interface, functional causality, and intervenability of the lymphatic pathway. Developmental valve formation, acute pharmacological stimulation in vitro, mouse transgenic or Yoda1 activation, and craniosynostosis or aging backgrounds need to be re-examined in adult chronic mechanical constraint scenarios. Meningeal lymphatic vessel or glymphatic measurement sites and the lack of longitudinal follow-up need to be re-examined in the intrasheath terminal and in causal designs. The systematic reviews of Bagheri et al. [65] and Choi B et al. [66] provide methodological confirmation for the field evidence landscape and framework integration of the lymphatic or glymphatic pathway. They explicitly note the scarcity of causal designs.

The human DCE-MRI and DTI-ALPS assessment paradigms established by Ren et al. [63] and Wu et al. [64] provide transferable outcome indicators and effect directions for clinical in vivo validation of the lymphatic pathway. These paradigms measured meningeal lymphatic vessels and glymphatic function rather than the intrasheath terminal, and lacked longitudinal follow-up. Both limitations need to be re-examined in causal designs.

The human cervical fascia PIEZO2, CGRP, and TH colocalization paradigm established by Tereshenko et al. [40] provides a directly transferable methodological framework for upstream validation at the histological tier. The PIEZO2–CGRP or substance P positive feedback loop established by Liao et al. [35] provides a referable mechanistic framework for subtype division-of-labor validation at the molecular tier. The trigeminal nerve root compression model differs in initiation conditions from the fascial mechanical conduction scenario. The human induced pluripotent stem cell endothelial cell CGRP–PIEZO1 crosstalk paradigm established by Gkouzioti et al. [36] provides a transferable human cell validation strategy for the endothelial crosstalk step. Acute pharmacological stimulation in vitro needs to be re-examined in the target tissue and chronic mechanical scenarios. The PIEZO1 and PIEZO2 immunohistochemical distribution established by Alba et al. [48] in the human carotid sinus, carotid body, and superior cervical ganglion provides positive controls and antibody references for presence validation of the target region at the histological tier. Normal distribution data cannot replace functional validation under pathological conditions. The PIEZO1 immunohistochemical detection in myofibroblasts of human carotid plaques established by Konishi et al. [50] provides a preliminary reference for the detectability of PIEZO1 in human carotid wall pathology. The atherosclerotic background, pilot-study nature, and cross-sectional design need to be re-examined in the investing fascia–sheath wall–vascular adventitia continuum and chronic mechanical constraint scenarios. The baroreceptor reflex PIEZO functional necessity paradigm established by Zeng et al. [49] provides a functional causal reference for mechanotransduction validation of the vagal pathway. The mouse baroreceptor model needs to be re-examined in the specific sheath wall–to–vagus nerve pathway. The intrasheath lymph node location confirmed by Clapham et al. [38] provides anatomical coordinates for sampling and modeling in the spatial validation of the lymphatic pathway. The sympathetic trunk anatomical data provided by Kiray et al. [25] and Civelek et al. [26] provide spatial parameters for mechanical modeling of the sympathetic pathway. The quantitative distribution of human perivascular nerve fibers around the carotid bifurcation established by Struthoff et al. [39] provides a directly transferable quantitative framework for target vessel–side histological validation of the sympathetic pathway. Normal human specimens and non-fibrotic scenarios need to be re-examined in the investing fascia–sheath wall continuum and pathological states.

Measurement reliability on the two sides of the pathway has been anchored separately. Zandi et al. [67] and Thomas et al. [27] have validated the SWE reliability of the investing fascia and the arterial wall, respectively. Zhao et al. [14] provided histological evidence of layered rabbit sheath wall. Hayashi [15] provided histological evidence of layered adult human cervical sheath wall. Schachtel et al. [16] provided evidence of regional anatomical heterogeneity of the sheath wall. Dapper et al. [70] provided an SWE effect-size reference for post-radiotherapy cervical fascial fibrosis. He et al. [33] provided a mechanistic reference for the PIEZO1–Wnt2/Wnt11–CCL24 fibrotic loop in skin fibrosis. Pruijssen et al. [71] provided a technical boundary for SWE of thin-walled structures. However, this boundary does not change the conclusion that direct human measurement reliability studies of the mechanical properties of the carotid sheath wall itself remain absent (see Section 4.2.4). This is Gap 2.

Transitional Measurement Framework for the Gap 1–Gap 2 Pair

Gap 2 is logically prior to Gap 1 in measurement terms: without a reliability basis for sheath-wall mechanical measurement, a measured transmission coefficient cannot be interpreted. The two gaps are nevertheless not strictly sequential in practice, because a transmission-coefficient study can be designed around a specimen-level measurement that does not require in vivo sheath-wall SWE reliability. Three transitional components are specified. None closes Gap 2; each yields a bounded posterior or a feasibility bound that allows Gap 1 design to proceed.

T1. Cadaveric cervical-block indentation with deep ultrasound displacement mapping. In an intact human cervical block, a controlled surface indentation is applied over the investing fascia territory while high-frequency ultrasound maps the displacement field at the sheath wall and at the intrasheath arterial and venous walls. Two ratios are obtained. The stage-1 ratio is the sheath-wall displacement divided by the surface indentation. This stage-1 ratio is the specimen-level candidate for the investing fascia-to-sheath wall coefficient, that is, for Gap 1. The stage-2 ratio is the intrasheath vessel-wall displacement divided by the sheath-wall displacement. The product of the stage-1 and stage-2 ratios is a separate fascia-to-intrasheath vessel coefficient, not the Gap 1 coefficient, and is reported under that separate label. The indentation is displacement-controlled. Its depth is calibrated to an equivalent deep-fascia pressure of approximately 6 N/cm², with the 6.06 ± 0.186 N/cm² value of Pires et al. [1] as the reference; the equivalent pressure is reported alongside the indentation depth. The boundary condition and magnitude references for the displacement-mapping component are taken from Liu et al. [37], in which effective perivascular pressure exceeded 50% of intravascular pressure and circumferential stress was reduced by approximately 70% under full constraint. Those values are used as magnitude references, not as calibration targets. T1 yields a specimen-level coefficient with donor-level and segment-level variance, not an in vivo individual-level coefficient.

T2. Bayesian indirect estimation anchored on published effect sizes. A posterior interval for the investing fascia-to-sheath wall transmission coefficient is constructed from three published anchors. The fascial-side stiffness contrast is anchored on the post-radiotherapy cervical fibrosis SWE data of Dapper et al. [70], in which stiffness rose from approximately 22.7–25.5 kPa to 44.3–46.4 kPa. The perivascular constraint magnitude is anchored on Liu et al. [37]. The upper bound is anchored on the approximately 30% ex vivo fascial force-transmission ratio reviewed by Wilke et al. [8] and Kalichman [9], with the 82%–89% external-cervical-pressure-to-carotid-sinus value of Querry et al. [12] entering only as a secondary sensitivity scenario for a different pathway, as specified in 4.2.2. Each anchor is entered as a likelihood term with an explicitly stated transfer-distance penalty, that is, a penalty for the inferential distance between the anchor's measurement context and the target pathway. The 30% anchor enters as an upper truncation rather than as a likelihood contribution. The resulting posterior is reported as an interval, not as a point estimate. T2 yields a posterior interval that serves as the prior-informed input for computational parameter sweeps (8.2.2) and for cohort proxy construction (8.3.5). It does not yield an individual-level conversion coefficient, and it is explicitly labeled as indirect.

Because the 30% anchor is an upper truncation, the posterior cannot exceed 30% by construction. The relevant comparison is therefore whether the posterior's lower bound falls below 0, or whether its shape is bimodal. If the posterior falls wholly within 0–30% with a single mode and a lower bound at or above 0, the 0–30% primary sweep of 8.2.2 is retained, and the posterior is reported as its prior-informed refinement. If the posterior's lower bound falls below 0, the sweep is widened downward to the union of the two ranges, and the extension is reported as a sensitivity analysis. If the posterior is bimodal, the bimodality is reported as a sensitivity analysis, and the two modes are swept separately.

T3. Phantom–specimen two-tier calibration of device, depth, and stiffness bias. The three biases identified by Pruijssen et al. [71] for SWE of thin-walled structures — inter-device variability, depth dependence, and stiffness underestimation — are calibrated in two tiers. The phantom tier uses thin-walled vascular phantoms at the target stiffness range. The specimen tier uses the cervical blocks of T1 at the target imaging depth and target segment. The geometric dependence of shear-wave velocity demonstrated by Capron et al. [69] is calibrated in the same tiers, so that vessel diameter and wall thickness enter as covariates rather than as unmodeled error. T3 yields a bias-correction function and a minimal detectable change for sheath-wall measurement on the target device, at the target depth, and at the target segment. It does not yield a reliability coefficient, because reliability requires repeated in vivo measurement, which remains the content of Gap 2 proper.

The three components are ordered by inferential weight. T1 provides the strongest specimen-level evidence but is bounded by post-mortem tissue properties. T2 provides the widest coverage but is indirect by construction. T3 provides the bias correction on which any future in vivo reliability study will depend. A transmission-coefficient study can be launched on T1 plus T3, with T2 supplying the posterior input; the resulting coefficient is reported as specimen-level and prior-informed, and is explicitly distinguished from a directly measured in vivo coefficient. This distinction is carried into the refutation criteria: the second criterion of 7.5 is evaluated against the 0–30% primary range and the 82%–89% secondary scenario, and a T1-derived specimen-level coefficient falling outside both bounds is treated as evidence against the mechanical-conduction step regardless of whether Gap 2 has been formally closed.

8.4.3. Minimal Testable Hypothesis Set

Without changing the core propositions of this hypothesis, validation can be prioritized into a five-tier minimal testable hypothesis set, with tier-specific minimal test sets. These five tiers are validation-method tiers, distinct from the evidence tiers of Section 5.

The histological minimal test set comprises multi-marker colocalization of PIEZO1, PIEZO2, CGRP, substance P, TH, α-SMA, and collagen deposition in the human investing fascia, layered sheath wall, and vascular adventitia. The layered sheath wall should be sampled with layer number and individual or segmental variation recorded, as described by Zhao et al. [14] and Hayashi [15] (see Section 2.2.1). Tereshenko et al. [40] and Alba et al. [48] serve as positive controls. Konishi et al. [50] provides a human pathological detectability reference. Struthoff et al. [39] provides a target vessel–side sympathetic fiber quantitative reference. He et al. [33] provides a cross-tissue mechanistic reference for the PIEZO1–Wnt2/Wnt11–CCL24 fibrotic loop (see Section 4.1.3).

The mechanical minimal test set comprises the following primary mechanical endpoints: (i) investing fascial SWE in resting state and loaded state; (ii) fascial thickness; (iii) fascia-to-fat stiffness ratio; (iv) internal jugular vein flow velocity; (v) vertebral artery V2-segment strain; and (vi) vertebral artery to internal carotid artery wall-stiffness ratio. Sheath-wall thickness and sheath-wall SWE are exploratory endpoints only until sheath-wall SWE reliability has been validated. They are not used for the primary hypothesis test. Reliability references are Zandi et al. [67] and Thomas et al. [27]. Geometric covariate calibration is Capron et al. [69]. Device and depth covariate calibration is Pruijssen et al. [71]. The SWE effect-size reference is Dapper et al. [70] (see Section 7.2.1).

The computational simulation minimal test set comprises an anisotropic constitutive model based on Aparici-Gil et al. [10], layered sheath-wall parameterization based on Zhao et al. [14] and Hayashi [15], transmission coefficient sweeping over the 0–30% primary range with an additional 82%–89% sensitivity scenario, separate sweeps along the fiber direction and perpendicular to it, and intrasheath lymph nodes and the sympathetic trunk as independent mechanical entities.

The cohort minimal test set comprises prespecified stratification by sex, stress condition, vertebral artery or posterior cerebral artery site, and VAH status, with boundaries given in Section 3.2.5. The vertebral artery to internal carotid artery RoR ratio and wall-stiffness ratio serve as primary endpoints, with effective luminal radius, CGRP, and sympathetic tone as mediators, and with prespecified refutation criteria and sample-size estimation.

The intervention minimal test set comprises immediate changes in intrasheath arterial area, internal jugular vein flow velocity, and autonomic indicators after fascial release or postural intervention. These are exploratory endpoints of mechanical coupling rather than efficacy validation.

The five tier-specific minimal test sets correspond to the five tiers of histology, mechanics, computation, cohort, and intervention, respectively. They can be implemented independently and progressed tier by tier.

9. Limitations

9.1. The Anatomical Premise Has Not Yet Been Closed

The fascial composition of the carotid sheath remains a substantive academic controversy. Bond et al. [13] explicitly list it as a core unresolved issue. The hypothesis is conditional on the functional continuity of the deep fascial network rather than on direct anatomical composition. Hayashi [15] described laminar adventitia plus common sheath layering and inter-individual or site variation in adult human cervical segments. However, Hayashi also explicitly reported that the common sheath does not fuse with the superficial fascia or the pretracheal fascia. Direct anatomical fusion may therefore not exist between the investing fascia and the sheath wall. Functional continuity cannot be presumed. It must be treated as an unverified premise, not as a substitute for anatomical fusion (see Section 2.2.2).

Zhao et al. [14] provided histological evidence of layered sheath-wall heterogeneity in the rabbit. However, the species is the rabbit and the context is the developmental process. It cannot be directly equated with the adult human fibrotic sheath wall. Schachtel et al. [16] provided human evidence of regional heterogeneity in the skull-base segment. That study focused on the skull-base region, had a small sample size, and concerned normal adult anatomy. It cannot be directly extrapolated to the mid-to-lower cervical segments and pathological states. The four candidate implementation mechanisms listed in Section 2.2.2 are compatible with the non-fusion finding. None has been directly measured for efficiency.

9.2. The Cross-Layer Causal Chain Is Derived Mainly from the Literature

Pires et al. [1] support the premise of mechanical accessibility with the 6 N/cm² threshold. They do not address the conduction efficiency from the investing fascia to the sheath wall (Gap 1). Liu et al. [37] support the magnitude that perivascular tissue constraint is sufficient to significantly alter wall stress. They do not address the starting point of the investing fascia. Querry et al. [12] support efficient conduction from external cervical pressure to the carotid sinus, 82%–89%. Their pathway is external pressure to the carotid sinus, not investing fascia to sheath wall.

The ex vivo fascial force transmission ratio upper bound of approximately 30%, reviewed by Wilke et al. [8] and Kalichman [9], provides a citable magnitude boundary for transmission efficiency. However, these data do not derive from the cervical investing fascia–sheath wall pathway. In vivo human evidence is also limited. Aparici-Gil et al. [10] provided ex vivo material parameters for the anisotropic constitutive behavior of deep fascia. Their test object was the deep fascia, specifically fascia lata, rather than the cervical investing fascia. They did not involve the transmission coefficient of the specific investing fascia-to-sheath wall pathway. Zhou et al. [23] support the endpoint that perivascular fascial constraint affects arterial mechanics. They do not address the starting point of the investing fascia. Friend et al. [19], Srichawla et al. [28], and Thomas et al. [18] support the asymmetry of anterior and posterior circulation dynamic cerebral autoregulation and wall-stiffness responses. They do not address fascial mechanics.

Washio et al. [2] suggest that vertebral artery–posterior cerebral artery site differences exist within the posterior circulation. This further limits the ability to treat the posterior circulation as a homogeneous unit. Meng et al. [24] provide the V2 and V3 hemodynamic thresholds of RVAS. Their osseous or acute compression etiology differs from chronic fascial constraint. Hou et al. [20], Dadsetan et al. [21], and Nguyen et al. [22] are single-case feasibility reports. They support that soft tissue or fibrous bands can compress the internal carotid artery or vertebral artery in vivo. They have inflammatory or local fibrous-band etiologies. Hou et al. [20] is an atypical hemodynamic manifestation. Dadsetan et al. [21] and Nguyen et al. [22] are local fibrous bands rather than investing fascial fibrosis. They cannot be equated with the systemic chronic mechanical conduction of interest to this hypothesis.

Caroccia et al. [32] support the existence of the Ang II–AT1R–YAP fibrotic branch in human deep fascial fibroblasts. Their sampling is the thoracolumbar fascia and cannot be equated with the cervical investing fascia. Their upstream is Ang II biochemical signaling rather than chronic mechanical loading. He et al. [33] support the PIEZO1–Wnt2/Wnt11–CCL24 fibrotic loop in human fibrotic skin, but their tissue is skin rather than fascia (see Section 4.1.3). Dapper et al. [70] support a significant increase in SWE stiffness in post-radiotherapy cervical fascial fibrosis. Their etiology and site are both outside the investing fascia–sheath wall pathway, and the fascia and muscle are synchronously involved (see Section 7.2.1). Hayashi [15] supports the layered structure and inter-individual or site variation of the adult human cervical sheath wall. It does not provide mechanical properties, and the non-fusion finding requires that functional continuity be treated as a premise to be verified (see Section 2.2.2). The claims of the hypothesis are conditional.

9.3. Species and Disease-Model Limitations of the Molecular Evidence

The species and disease-model limitations of the molecular evidence can be divided into three tiers.

For neural-side evidence, Mikhailov et al. [34] demonstrated at the animal ex vivo and semi-intact levels the capacity of PIEZO1 activation to trigger CGRP release. They did not establish physiological predominance at the neural terminal. Liao et al. [35] demonstrated in a rat trigeminal nerve root compression model the mechanistic capacity of PIEZO2–CGRP or substance P positive feedback to drive mechanical allodynia. Gkouzioti et al. [36] demonstrated indirect CGRP-to-PIEZO1 crosstalk in human induced pluripotent stem cell endothelial cells. Tereshenko et al. [40] demonstrated the spatial colocalization and pathological upregulation of PIEZO2, CGRP, and TH in human cervical fascia. These lines of evidence are respectively limited as follows. The disease model of Tereshenko et al. [40] is occipital neuralgia rather than investing fascial fibrosis. It lacks longitudinal follow-up and cannot establish the causal direction between mechanical change and CGRP upregulation. The initiating event of Liao et al. [35] is direct nerve root compression rather than fascial mechanical conduction. The disease phenotype is trigeminal neuralgia rather than functional headache. The evidence of Gkouzioti et al. [36] derives from endothelial cells and acute pharmacological stimulation in vitro and cannot be equated with chronic in vivo mechanical constraint.

For histological and baroreceptor evidence, Alba et al. [48] demonstrated PIEZO1 and PIEZO2 protein expression in the normal human carotid sinus, carotid body, and superior cervical ganglion. They described normal distribution and did not demonstrate expression changes or functional activation under pathological conditions. Zeng et al. [49] demonstrated in mice that PIEZO1 and PIEZO2 are mechanosensors essential for the baroreceptor reflex. This is a mouse baroreceptor model demonstrating mechanotransduction at the baroreceptor level rather than the specific pathway from carotid sheath wall mechanics to the vagus nerve.

For arterial-side, fibrosis-side, and field evidence, Zhang J et al. [41] demonstrated the in vivo causal role of vascular smooth muscle cell PIEZO1 in simulated microgravity–induced carotid structural remodeling. Zhang FR et al. [42] demonstrated the necessity of smooth muscle cell Piezo1 for neointimal hyperplasia and phenotypic switching in a hemodynamic disturbance or luminal stenosis model. Luu et al. [43] demonstrated that Piezo1 mediates vascular smooth muscle cell mechanosensation and that this function declines with aging, using an ex vivo or animal vascular smooth muscle cell model and a phenotype of aging-related decline in mechanosensation. Beech [44] is a review, and the integrated evidence derives mainly from the aorta and cannot replace in vivo causal evidence from the carotid artery. Konishi et al. [50] demonstrated upregulated PIEZO1 expression in myofibroblasts of human carotid plaques. This is a pilot study with an atherosclerotic background and a cross-sectional design and cannot establish causal direction. He et al. [33] demonstrated the PIEZO1–Wnt2/Wnt11–CCL24 fibrotic loop in human fibrotic skin and mouse skin fibrosis, but the tissue is skin rather than fascia (see Section 4.1.3).

The cell types in the above studies are all vascular smooth muscle cells, fibroblasts, or vascular smooth muscle cell models. Their phenotypes include fibrosis or increased stiffness, neointimal hyperplasia, aging-related decline in mechanosensation, aging-related vascular wall stiffening, and skin fibrosis. All differ from the neural step from PIEZO2 to CGRP to nociceptive signaling. They cannot replace the functions of Liao et al. [35] and Della Pietra et al. [53] in the core molecular transduction chain. Mikhailov et al. [34] provide evidence that PIEZO1 activation can trigger CGRP release. The PIEZO1 and PIEZO2 subtype division of labor summarized by Zhao L et al. [52] derives from a generalized neurological disease background and cannot be directly extrapolated to the specific cell and tissue environment of the cervical fascia–sheath wall continuum. Della Pietra et al. [53] further confirm that evidence in the field of mechanosensitive receptors in migraine is predominantly preclinical animal models with scarce clinical research. They also confirm that the functional direction of the PIEZO branch has not yet fully converged in the existing literature. This limits the current evidence level of the molecular step of this hypothesis. The Ang II–AT1R–YAP branch demonstrated by Caroccia et al. [32] derives from thoracolumbar fascia fibroblasts and a biochemical stimulation background. It cannot replace evidence of investing fascial mechanical loading–driven fibrosis. It also cannot replace functional evidence for the PIEZO–CGRP neural step. The PIEZO1–Wnt2/Wnt11–CCL24 fibrotic loop demonstrated by He et al. [33] derives from a skin fibrosis model and cannot replace evidence of mechanical loading–driven fibrosis in fascia or the sheath wall.

9.4. Dissociation of Lymphatic Pathway Evidence

Louveau et al. [57] and Aspelund et al. [58] confirmed the existence and functional necessity of meningeal lymphatic vessels. Yoon et al. [17] confirmed the anatomical hub from the nasopharyngeal lymphatic plexus to the deep cervical lymphatic vessels and its adrenergic regulation. Clapham et al. [38] confirmed the existence of the intrasheath lymphatic terminal. Nelson-Maney et al. [59] demonstrated that CGRP–lymphatic drainage dysfunction drives pain. Jin et al. [60] demonstrated that mechanical compression of superficial lymphatic vessels can alter cerebrospinal fluid drainage efficiency in mouse superficial lymphatic vessels under short-term external compression. Kim and Tithof [61] provided a parameterizable tissue-level mechanical framework for cervical lymphatic vessel pumping efficiency, based on mouse source and a two-dimensional simplified model. Du et al. [62] demonstrated that aging leads to decreased cervical lymphatic pumping function and that prostaglandin F2α can restore it, providing aging and recoverability evidence in a mouse aging model with pharmacological intervention. Nonomura et al. [45] demonstrated the mechanotransduction necessity of PIEZO1 in lymphatic endothelial cells. Choi et al. [46] demonstrated the in vivo role of PIEZO1 in regulating meningeal lymphatic drainage function. Matrongolo et al. [47] demonstrated that pharmacological activation of PIEZO1 can restore meningeal lymphatic drainage in aging or pathological states. Ren et al. [63] and Wu et al. [64] respectively demonstrated impaired meningeal lymphatic or glymphatic drainage in human episodic and chronic migraine and its association with headache intensity or chronification. These clinical studies used meningeal lymphatic vessel or glymphatic measurement and lacked longitudinal follow-up. Bagheri et al. [65] and Choi B et al. [66] confirmed that the field evidence direction is consistent but that causal conclusions have not yet been formed.

These lines of evidence still lack an intermediate step that directly demonstrates sheath-wall mechanical change leading to decreased intrasheath lymphatic drainage efficiency. The differences in species, site, nature of stimulation, and disease background determine that they cannot be directly equated with the chronic fibrotic constraint of the intrasheath terminal. Martínez-Sanz et al. [11] provide a human embryological reference for the developmental anatomical association between the middle deep cervical fascia and lymphatic or venous structures. However, this is developmental-stage, descriptive or associative evidence and cannot be equated with the mechanical–drainage causal chain under adult pathological conditions.

9.5. Vagal Pathway Boundaries

Zeng et al. [49] demonstrated in mice that PIEZO1 and PIEZO2 are mechanosensors essential for the baroreceptor reflex. Alba et al. [48] detected PIEZO1 and PIEZO2 expression in the human petrosal ganglion and carotid sinus. Together they provide a two-layer basis of functional necessity plus human distribution for the mechanical sensitivity of the vagal pathway. However, Zeng et al. [49] demonstrated mechanotransduction at the baroreceptor level rather than the specific pathway from carotid sheath wall mechanics to the vagus nerve. The upstream step from a change in sheath-wall mechanics to vagal PIEZO activation remains an inference requiring independent verification. The distal effects of the vagal pathway, such as parasympathetic tone and the cholinergic anti-inflammatory pathway, remain mainly framework-level inferences in this hypothesis. They lack in vivo data directly linking them to sheath-wall mechanical parameters.

9.6. Sympathetic and Venous Pathway Etiological Specificity

Kiray et al. [25] and Civelek et al. [26] confirmed the close anatomical relationship between the sympathetic trunk and the sheath wall. They did not address coupling efficiency under fibrotic conditions. Kaur et al. [54] demonstrated that low-to-moderate levels of sympathetic activation do not alter internal carotid artery or vertebral artery blood flow. Significant blood flow decline occurs only at moderate-to-high levels. The pathological effect of the sympathetic pathway is therefore defined as threshold-dependent. The reflex activation study of Kaur et al. [54] did not find preferential constriction of the anterior circulation. Meyer et al. [55] observed a significant reduction in internal carotid artery blood flow through direct stimulation of the superior cervical sympathetic ganglion in a monkey model. Animal experiments with direct ganglion stimulation also reported significant anterior circulation arterial narrowing with minimal posterior circulation change. These two lines of evidence together suggest that the hemodynamic consequences of sympathetic activation may be highly dependent on the type and intensity of stimulation. The functional meaning of the anatomical fact that the anterior circulation has higher sympathetic innervation density remains to be clarified.

Washio et al. [2] further suggested that the vertebral artery and posterior cerebral artery within the posterior circulation respond differently under physiological sympathetic activation. The posterior circulation cannot therefore serve as a single sympathetic effect unit. The hypothesis does not assume that the sympathetic pathway necessarily has a predominant effect on the posterior circulation. It treats stimulation type, intensity, and posterior circulation site as moderating variables to be tested. The data of Meyer et al. [55] are limited by the animal model and direct electrical stimulation conditions and cannot be directly extrapolated to the mechanical, chronic sympathetic trunk stimulation scenario. Alba et al. [48] reported high PIEZO1 and PIEZO2 expression in the superior cervical ganglion. This suggests that the sympathetic ganglion may possess direct mechanosensitivity. This evidence is only normal distribution and does not demonstrate functional activation under fibrotic conditions. It also cannot replace distal hemodynamic measurement of the sympathetic pathway. Struthoff et al. [39] provided a human histological reference for the local density and distance of target vessel–side sympathetic input. However, this evidence derives from normal human specimens, does not address coupling efficiency under fibrotic conditions, and cannot replace measurement of the distal hemodynamic consequences after sympathetic tone crosses the threshold.

Yoon et al. [17] demonstrated that deep cervical lymphatic vessels are innervated by adrenergic nerves. This suggests that the sympathetic pathway and the lymphatic pathway share a functional coupling node at the cervical terminal. The quantitative contribution of this coupling under the hypothesized conditions remains unmeasured. The internal jugular vein compression case series reported by Ma et al. [56] was caused by osseous compression, specifically by the C1 transverse process, rather than by fascial or sheath-wall mechanical change. The RVAS thresholds reported by Meng et al. [24] likewise derive from osseous or anatomical compression and acute head rotation and cannot be directly equated with chronic fascial constraint. Dadsetan et al. [21] and Nguyen et al. [22] reported vertebral artery compression by the longus colli fibrous band or an oblique fibro-osseous band. These cases represent a local anatomical variant or a fibrotic band structure. They do not represent the systemic mechanical constraint of investing fascial fibrosis transmitted through the deep fascial network. The internal carotid artery compression reported by Hou et al. [20] has perivascular inflammatory thickening as its etiology. This likewise is not equivalent to chronic fascial mechanical constraint.

9.7. Sex, Stress, Site, and Structural or Mechanistic Boundaries of Posterior Circulation Asymmetry

Friend et al. [19] observed reduced vertebral artery dynamic cerebral autoregulation in men under hypoxic conditions. Chen et al. [29] found similar posterior cerebral artery dynamic cerebral autoregulation between men and women under normoxic resting conditions. Thomas et al. [18] did not report sex-stratified results. Washio et al. [2] found site-specific differences between the vertebral artery and posterior cerebral artery under isometric exercise conditions in healthy young men. These findings separately constrain the expression of posterior circulation asymmetry. They do not define a single unified boundary. Srichawla et al. [28] provided a regional background for posterior circulation low reserve. Their narrative review nature and the cautious wording that the posterior circulation may have a lower autoregulatory capacity determine that it can provide only directional confirmation rather than quantitative evidence.

The VAH evidence summarized by Mirkovic et al. [30] derives from congenital vessel-diameter variants. It shares with the acquired fascial mechanical constraint of interest to this hypothesis the logic of low reserve leading to vulnerability. However, the etiological sources differ and cannot be directly equated. This etiological distance is an inherent tension in the structure of this hypothesis. It can only be controlled by explicitly labeling the logical corroboration positioning of VAH evidence (see Section 3.2.5). It cannot be fully eliminated by wording. In validation design, the VAH subgroup should serve as an independent reference framework for the low reserve leading to vulnerability logic, rather than a direct evidence source for acquired fascial constraint leading to low reserve.

Triantafyllou et al. [3] provide anatomical variant prevalence data, not involving hemodynamics or clinical phenotypes. They cannot replace the functional and clinical argumentation of Mirkovic et al. [30]. Neumann et al. [4] provide evidence on compensatory strategies under stress in hypertensive patients. Their population background and congenital vessel-diameter variant nature likewise cannot be directly equated with the investing fascial mechanical constraint scenario. Gaigalaite et al. [5] provide evidence on the modifying effect of the interaction between VAH and the posterior configuration of the circle of Willis on posterior circulation stroke or transient ischemic attack risk. Their outcome is structural cerebrovascular events and their exposure is static anatomical configuration. This cannot be directly equated with the functional or perfusion outcomes and acquired fascial mechanical exposure of interest to this hypothesis. They did not measure investing fascial or sheath-wall mechanics. They can therefore provide only a reference for effect modification within the VAH subgroup and cannot provide direct support for the core chain of the hypothesis. Sato et al. [6] provide evidence of VAH-related impairment of cerebrovascular CO₂ reactivity. Their limited sample size and the congenital vessel-diameter variant nature of VAH mean they cannot be directly equated with the acquired fascial mechanical constraint scenario. They can provide a functional-dimension supplement to posterior circulation low reserve.

The vertebral artery endothelial senescence mechanism proposed by Wang et al. [7] derives from a cervical degeneration background. It likewise differs from the mechanical constraint of investing fascial fibrosis transmitted through the deep fascial network of interest to this hypothesis. Its review nature, the lack of a direct connection to the PIEZO–CGRP pathway, and the lack of empirical association with investing fascial mechanical parameters determine that it can provide only a candidate biological mechanism background. It cannot provide direct support for the posterior circulation proposition.

9.8. Measurement Reliability and Geometric Bias

Zandi et al. [67] demonstrated excellent cervical fascial SWE reliability. Their negative finding suggests that fascial stiffness in patients with chronic neck pain may not be significantly elevated in the resting state. Validation design for the hypothesis therefore needs to prespecify multidimensional parameters such as thickness, stiffness ratio, and loaded-state stiffness. Thomas et al. [27] demonstrated acceptable intra-rater reliability of cervical arterial wall SWE. Their measurement object is the arterial wall rather than the sheath wall. It cannot be directly extrapolated to measurement of the mechanical properties of the sheath wall.

The carotid ultrasound biomechanical parameters validated by Ghahremani et al. [68] derive from an atherosclerotic population. Whether they can capture arterial mechanical changes caused by external compressive fascial constraint still requires independent validation. Moreover, that study included only men, limiting sex generalizability. Capron et al. [69] further demonstrated that the measured value of carotid shear-wave velocity strongly depends on vascular geometric configuration. Ignoring geometric factors leads to severe underestimation of Young's modulus. Therefore, even if the parameters validated by Ghahremani et al. [68] are technically measurable, their interpretation under the hypothesized conditions must still include geometric parameters as covariates in the analysis or model calibration.

As discussed in Section 8.4.2, a structure at the thickness scale of the sheath wall faces a superposition of three biases in wave velocity measurement: inter-device variability, depth dependence, and stiffness underestimation. Reliability validation must be independently designed on the target device, at the target depth, and at the target segment. This evidence does not involve the sheath wall itself and cannot replace sheath-wall SWE reliability research, which is Gap 2.

9.9. Material Anisotropy and Pathological Signal Non-Uniqueness

Aparici-Gil et al. [10] demonstrated that deep fascia has marked anisotropy. Longitudinal stress under uniaxial tension is approximately 6.6 times the transverse stress. This means that even if a finite element model adopts the correct material constitutive law, inaccurate fiber orientation parameters will still produce directional bias in the estimation of the transmission coefficient. In vivo calibration of investing fascial fiber orientation should therefore be a prerequisite step for computational simulation validation.

The transmission efficiency data provided by Querry et al. [12] are human measurements. However, their stimulation site is the exterior of the carotid sinus, and their measurement method is balloon catheter. Both differ in mechanical boundary conditions from the investing fascia–to–sheath wall pathway. These data therefore cannot be used directly as the transmission coefficient for this pathway (see Section 4.2.2). Zhao et al. [14] demonstrated that the rabbit sheath wall has a three-layered fascial architecture with earliest and most marked thickening of the arterial inner fascial layer. Hayashi [15] demonstrated that the adult human cervical sheath wall has two layers, laminar adventitia plus common sheath, with inter-individual or site variation in thickness (see Section 2.2.2). Measurement of sheath-wall mechanical properties should therefore be performed layer by layer and should record individual and segmental factors. The sheath wall should not be treated as a single entity. However, Zhao et al. [14] derive from rabbits, the developmental stage, and histological thickening rather than mechanical properties. Hayashi [15] is adult human morphology or histology rather than mechanical properties. Neither can be directly equated with the mechanical measurement of the adult human fibrotic sheath wall. Schachtel et al. [16] demonstrated that the fascial source and layer number of the sheath wall may differ across segments. This means that measurement of sheath-wall mechanical properties should record the sampling segment. The sheath-wall composition of one segment should not be extrapolated to the entire cervical segment. However, this evidence focuses on the skull-base region, has a small sample size, and concerns normal adult anatomy. Martínez-Sanz et al. [11] demonstrated a developmental anatomical association between the middle deep cervical fascia and lymphatic or venous structures. This is embryonic or fetal-stage, descriptive or associative evidence and cannot be directly used as evidence of adult fascial–lymphatic or venous mechanical coupling.

9.10. The Missing Sheath-Wall Measurement Step

The perivascular or fibrous-band compression evidence provided by Hou et al. [20], Dadsetan et al. [21], and Nguyen et al. [22] consists of human in vivo observations. However, its imaging or intraoperative measurement boundary conditions differ from the investing fascia–to–sheath wall pathway. All three are single-case feasibility reports. They cannot provide a measurement basis for the transmission coefficient or mechanical coupling efficiency of this pathway. Dapper et al. [70] provided human in vivo SWE quantification of post-radiotherapy cervical fascial fibrosis. However, both the etiology and the site are outside the investing fascia–sheath wall pathway. The synchronous involvement of fascia and muscle suggests that the stiffness change cannot be attributed to the fascia alone. These data cannot provide a measurement basis for the transmission coefficient or sheath-wall reliability of this pathway (see Section 7.2.1). He et al. [33] provided PIEZO1 positive feedback evidence from a skin fibrosis model. It cannot provide a measurement basis for the mechanical properties of the cervical investing fascia or the sheath-wall transmission coefficient (see Section 4.1.3). Caroccia et al. [32] provided Ang II–AT1R–YAP evidence from thoracolumbar fascia fibroblasts and biochemical stimulation. It cannot provide a measurement basis for the mechanical properties of the cervical investing fascia or the sheath-wall transmission coefficient. Struthoff et al. [39] provided perivascular nerve fiber distribution from normal human specimens. It does not address coupling efficiency under fibrotic conditions and cannot provide a measurement basis for the mechanical coupling efficiency from the sheath wall to sympathetic terminals.

More importantly, direct human measurement reliability studies of the mechanical properties of the carotid sheath wall itself remain absent from the existing literature system (see Section 4.2.4). Zandi et al. [67] validated the investing fascial side. Thomas et al. [27] validated the arterial wall side. Zhao et al. [14] provided histological evidence of layered rabbit sheath wall. Hayashi [15] provided histological evidence of layered adult human cervical sheath wall. Schachtel et al. [16] provided evidence of regional anatomical heterogeneity of the sheath wall. Dapper et al. [70] provided an SWE effect-size reference for post-radiotherapy cervical fascial fibrosis. He et al. [33] provided a mechanistic reference for the PIEZO1–Wnt2/Wnt11–CCL24 fibrotic loop in skin fibrosis. Pruijssen et al. [71] provided a technical boundary for SWE of thin-walled structures. However, the measurement-science step for the mechanical properties of the sheath wall as an intermediate entity has not yet been independently validated. This is Gap 2 and the most central methodological obstacle in the progression of this hypothesis from testable to validated.

The transitional framework of 8.4.2a changes the status of this obstacle without removing it. T1 provides a specimen-level transmission coefficient; T2 provides a posterior interval; T3 provides a bias-correction function and a minimal detectable change. None of the three is an in vivo reliability coefficient, and none is a direct in vivo measurement of the investing fascia-to-sheath wall coefficient. The most central methodological obstacle therefore remains Gap 2, but it is no longer a strict prerequisite for launching Gap 1 study design. The residual limitation introduced by the transitional framework is that a specimen-level coefficient cannot capture in vivo prestress, active vascular tone, or posture-dependent fascial tension. These three factors are the specific quantities that only in vivo reliability validation can supply.

9.11. Clinical Intervention Evidence and Phenotypic Heterogeneity

The level of clinical intervention evidence is limited. The meta-analysis of Lu et al. [72] provides the most systematic evidence summary to date. However, the overall quality of evidence is moderate to low or very low, and the disability improvement in the migraine subgroup was not significant. The randomized controlled trial of Uzun et al. [73] provides human interventional corroboration that cervical mechanical intervention alters cerebral blood flow. However, the sample size is small, the intervention target is not the investing fascia–sheath wall pathway, and the between-group difference in blood flow did not reach significance. It cannot therefore provide confirmatory evidence for the hypothesis. Matrongolo et al. [47], Du et al. [62], and He et al. [33] provide mouse-model evidence for PIEZO1 pharmacological activation, prostaglandin F2α restoration, and Piezo1 knockdown, respectively. All involve specific disease or aging or skin fibrosis backgrounds and pharmacological or genetic intervention approaches. They cannot be directly equated with the chronic fibrotic sheath-wall constraint scenario of interest to the hypothesis. They provide only a mechanistic background for exploratory intervention endpoints. Della Pietra et al. [53] further confirm that clinical research in the field of mechanosensitive receptors in migraine is scarce. PIEZO-related human evidence has not yet reached an evidence level that can support clinical intervention recommendations. Bagheri et al. [65] likewise confirm that causal evidence for the meningeal lymphatic or glymphatic pathway in migraine is scarce and that existing clinical studies are predominantly cross-sectional DCE-MRI. Ren et al. [63] and Wu et al. [64] provide human clinical relevance evidence for the lymphatic pathway. Both lack longitudinal follow-up and interventional design and cannot support causal or interventional conclusions.

The explanatory power for phenotypic heterogeneity is limited. The hypothesis can currently provide only a framework-level explanation of why the same upstream mechanical change manifests in different individuals as different combinations of headache, vertigo, dry eye, or anxiety. This explanation is based on differences in the weights of the five pathways, anterior or posterior circulation predominance, and sympathetic tone threshold status.

9.12. Terminology and Naming Risk

Mechanical vulnerability is used here as an operational term rather than a fully standardized nosological term. Alternative terms include mechanical fragility and differential vulnerability. The full text uses coupling consistently and does not use alternative terms interchangeably. The investing fascia should be annotated as the superficial layer of the deep cervical fascia at first occurrence.

10. Discussion and Conclusion

10.1. Structural Contribution: The Sheath as a Dynamic Mechanical Convergence Point

The investing fascia–carotid sheath coupling hypothesis integrates observations previously scattered across anatomy, biomechanics, neurovascular science, and lymphatic research into a single testable causal chain from peripheral fascial mechanics to cerebrovascular–neural function. Its first contribution is structural: it repositions the carotid sheath from a static fascial conduit to a dynamic mechanical convergence point. At that point, multiple cervical neurovascular structures coexist and can be acted upon, simultaneously or sequentially, by a change in sheath-wall mechanical state. This repositioning is weaker by design than the claim that the investing fascia constitutes the sheath. It is compatible with the non-fusion finding of Hayashi [15] and with the compositional controversy reviewed by Bond et al. [13]. This compatibility is the reason the hypothesis can be stated at all without resolving that controversy. The cost of this weaker positioning is that functional continuity from the investing fascia to the sheath wall must be carried as an unverified premise (2.2.2), not as an anatomical given.

10.2. Differential Contribution: Anterior and Posterior Asymmetry

The second contribution is differential. The hypothesis does not stop at the general proposition that the investing fascia affects the cerebral circulation. It asserts a systematic asymmetry between the anterior and posterior circulations in dynamic cerebral autoregulation and in arterial-wall mechanical response. It also localizes posterior vulnerability to the vertebral artery V2 segment as the peripheral input site. The empirical basis for this asymmetry is narrow but direct. Friend et al. [19] provide the dCA differentiation at the physiological level, with hypoxia-induced RoR reduction confined to the vertebral artery and compensatory dilation confined to the internal carotid artery. Thomas et al. [18] provide the mechanical differentiation at the wall-stiffness level, with vertebral stiffening and carotid softening during contralateral head rotation. The methodological feasibility of the second is supported by the reliability study of Thomas et al. [27]. These two paradigms are the load-bearing evidence for the asymmetry proposition, and the hypothesis depends on their replication and extension. The remaining VAH evidence, reviewed in Section 3.2.5, Section 3.3.2 and Section 8.3.3, and 9.7, is positioned as logical corroboration and structural reference only. It is not load-bearing evidence for the asymmetry proposition and is not used as load-bearing evidence anywhere in the argument.

Two qualifications on this contribution are stated here rather than repeated from earlier sections. First, the asymmetry is not a claim that the two circulations have different thresholds in a common metric. As specified in 3.1.3, the anterior circulation is characterized by a breakpoint in compensatory capacity, whereas the posterior circulation is characterized by a steeper slope in a passive transmission relation. These two characteristic nodes are not directly commensurable. The asymmetry must therefore be quantified either as a slope ratio at matched effective-radius perturbation or as a breakpoint-versus-slope comparison. Second, the posterior circulation is not a homogeneous unit. The VA–PCA distinction of Washio et al. [2] is carried through the hypothesis as an operational constraint. The anterior circulation is now stratified at three nodes (2.3.1, 3.1.3). This stratification ensures that the asymmetry is not an artifact of unequal stratification resolution.

10.3. Cross-Layer Conduction: Why Gap 1 and Gap 2 Are the Rate-Limiting Steps

The starting end of the causal chain rests on a small number of magnitude references. Each was measured on a pathway adjacent to, but not identical with, the investing fascia-to-sheath wall pathway. These references are the 6.06 N/cm² deep-fascia pressure threshold [1], the perivascular constraint magnitudes of Liu et al. [37], the 82%–89% external-cervical-pressure-to-carotid-sinus value [12], the approximately 30% ex vivo fascial force-transmission upper bound (8,9), and the anisotropic constitutive behavior of deep fascia [10]. None of these is a measured value of the transmission coefficient for this specific pathway. The rate-limiting step of the hypothesis is therefore not a missing biological mechanism but a missing measurement. That measurement has two components. Gap 1 is the transmission coefficient itself. Gap 2 is the reliability of sheath-wall mechanical measurement, on which any in vivo coefficient depends.

The argument of this article is that these two gaps, rather than the molecular or clinical steps, are what currently separate a testable hypothesis from a validated one. The molecular steps downstream of sheath-wall mechanics are supported by convergent evidence across cell types and species (10.4). The clinical steps downstream of the five pathways are supported by human association data (10.5). The measurement step between them is not. The transitional framework of 8.4.2a addresses this measurement gap. The framework does not close either gap. It converts the pair from a strict sequential prerequisite into a staged program. In that program, a specimen-level coefficient, a posterior interval, and a bias-correction function can be obtained first, and the in vivo reliability coefficient second. Three quantities can be supplied only by in vivo reliability: prestress, active vascular tone, and posture-dependent fascial tension. These are the residual content of Gap 2 after the transitional framework has done its work.

10.4. Molecular Conversion: What the Evidence Supports and What It Does Not

The molecular conversion step is the most densely evidenced part of the hypothesis and also the part where over-reading is easiest. Three distinctions are load-bearing. First, the neural-terminal default mediator is PIEZO2, on the basis of the human cervical fascia histology of Tereshenko et al. [40] and the PIEZO2–CGRP/substance P positive feedback of Liao et al. [35]. The capacity of PIEZO1 activation to trigger CGRP release, demonstrated by Mikhailov et al. [34], is retained as evidence of experimental capacity, not of physiological predominance. Second, the arterial-wall structural remodeling evidence (41–44) is a body of evidence on PIEZO1-dependent structural remodeling in vascular smooth muscle cells. It does not address the neural step from PIEZO2 to CGRP to nociceptive signaling, and its argumentative weight for the neural step is correspondingly lower. Third, the endothelial CGRP–PIEZO1 crosstalk of Gkouzioti et al. [36] is an endothelial-level, acute-pharmacological, in vitro finding. It is not the neural-terminal loop and is not treated as such.

With these distinctions held, the molecular step supports the following claim and no more. A PIEZO–CGRP apparatus is present in the human cervical fascia and carotid region. It is pathologically upregulable. It also has the demonstrated capacity to convert mechanical load into CGRP release and into PIEZO1-dependent structural remodeling. The step that remains inferential is the upstream one, namely that the mechanical tension generated by investing fascial fibrosis is sufficient to activate that apparatus in vivo. This is the same upstream step identified in 4.4.4, and it is not closed by any of the molecular evidence reviewed.

10.5. Five Downstream Pathways: An Asymmetry in Evidence Density, and What It Implies

The five pathways are not equally evidenced, and the hypothesis is stronger for saying so explicitly than for presenting them as a uniform set. The arterial and lymphatic/glymphatic pathways carry multilevel evidence from human tissue or in vivo function through to animal causality. The venous pathway carries clinical phenomenological evidence and developmental anatomical background but a lower mechanistic evidence density. The sympathetic pathway is threshold-dependent. The vagal pathway is exploratory. The sympathetic distal effects are threshold-, stimulation-type-, and site-dependent. The vagal upstream step from sheath-wall mechanics to PIEZO activation remains an inference.

This asymmetry has a consequence for the logic of the hypothesis that is worth stating in the discussion rather than in the pathway sections. The five-pathway framework is a biological claim about the routes through which a sheath-wall mechanical change can act. The five-tier evidence grading is a statement about how well each route is currently supported. These two statements are not equivalent, and the hypothesis does not require all five routes to be equally supported in order to be testable. It requires that the threshold-dependent four-pathway basal superposition and the threshold-dependent sympathetic recruitment be distinguishable in validation. That distinguishability, not the equal strength of the five routes, is the testable content of the five-pathway claim.

10.6. Measurement Methods: The Sheath Wall as the Missing Intermediate Node

Measurement tools exist on both sides of the sheath wall. On the fascial side, Zandi et al. [67] validated cervical fascial SWE reliability and provided a minimal detectable change. Their negative finding — no significant resting stiffness difference in chronic neck pain — calibrates the hypothesis against a single resting-state stiffness signal. On the arterial side, Thomas et al. [27] validated cervical arterial wall SWE reliability. Ghahremani et al. [68] provided a conventional-ultrasound biomechanical parameter set. Capron et al. [69] demonstrated the geometric dependence of carotid shear-wave velocity. The sheath wall itself, which is the mechanical transmission entity between the two, has no direct human reliability study. This is Gap 2, and it is the single most consequential methodological bottleneck in the progression of the hypothesis from testable to validated.

The transitional framework of 8.4.2a is the response to this gap. Its logic is that the sheath wall need not be measured in vivo with validated reliability before a transmission coefficient can be estimated. A specimen-level displacement-mapping measurement (T1), a posterior interval (T2), and a bias-correction function (T3) can together support a bounded coefficient and a launchable study design. The framework cannot supply the in vivo reliability coefficient itself. As stated in 8.4.2a and 9.10, this remains the residual content of Gap 2.

10.7. Relationship with Existing Theories

The trigeminovascular system and central sensitization theory explain the execution mechanisms of an attack. This hypothesis addresses the threshold regulation of attack onset. The PIEZO–CGRP mechanism provides a molecular interface between the two layers, and the hypothesis is positioned as a peripheral mechanical upstream complement rather than a replacement. The myofascial trigger point concept of cervicogenic headache rests on the same premise of cervical mechanical involvement. The specificity of this hypothesis lies in shifting the locus from muscle trigger points to the deep fascia, the sheath wall, and the vascular–neural interface. The perivascular-environment finding of Zhou et al. [23] provides biomechanical evidence, independent of this hypothesis, that perivascular constraint determines arterial strain. This is the general form of the mechanism that the hypothesis applies to the cervical segment. The fibrotic branches of Caroccia et al. [32] and He et al. [33] add candidate molecular nodes at the fibrotic initiation end. Their direct connection to the PIEZO–CGRP pathway remains to be verified in the target tissue. The target-vessel-side sympathetic quantification of Struthoff et al. [39] provides a human histological reference for the sheath-wall-to-adventitia spatial adjacency. Its connection to coupling efficiency under fibrotic conditions remains to be independently verified.

10.8. Research Agenda: A Dependency Structure, Not a Repetition of the Minimal Test Sets

The minimal test sets are specified in 8.4.3 and are not repeated here. What the discussion adds is the dependency structure among them, which determines the order in which they can be executed.

The first dependency is between Gap 1 and Gap 2. Gap 2 is logically prior in measurement terms, but the transitional framework of 8.4.2a makes the priority operational rather than absolute. T1 and T3 can be executed first, yielding a specimen-level coefficient and a bias-correction function. T2 supplies the posterior input, and the in vivo reliability coefficient follows. The first-tier priority is therefore not "close Gap 1, then Gap 2" but "execute T1–T3, then close Gap 1 and Gap 2 in parallel."

The second dependency is between the molecular step and the target tissue. Colocalization and functional validation of PIEZO1, PIEZO2, and CGRP in the investing fascia–sheath wall continuum can proceed independently of the measurement gaps, because it is a tissue-level question. It is second in priority not because it depends on the first, but because its interpretation depends on the first. A positive colocalization result is uninformative about the causal chain unless the mechanical load reaching the tissue is bounded.

The third dependency is between the asymmetry test and the stratification scheme. The prospective test of the differentiated dose–response can proceed on the four paradigms identified in 10.2, but its anterior denominator is unresolved unless the three-node stratification of 2.3.1 is implemented. The stratification is therefore a design prerequisite for the third priority, not an optional refinement.

The fourth and fifth dependencies concern the intrasheath lymphatic terminal and the sympathetic–sheath coupling. Both are exploratory modules whose evidence bases are summarized in 5.2 and 5.3. Each requires its own set of extrapolations before it can be promoted. For the lymphatic module, the three extrapolations are from superficial to intrasheath lymphatic vessels, from short-term to chronic mechanical constraint, and from normal to fibrotic tissue; these are specified in 5.2.2–5.2.4 and 9.4. For the sympathetic module, the three extrapolations are from anatomical tight adherence to mechanical coupling efficiency under fibrotic conditions, from coupling efficiency to the relative constriction magnitudes of the anterior and posterior circulations, and from those relative magnitudes to the VA–PCA difference within the posterior circulation after sympathetic tone crosses the threshold; these are specified in 9.6. The two sets are not the same and are not restated here.

10.9. Concluding Summary and Ultimate Test

The hypothesis is conditional on the three requirements of Section 6. First, cross-layer mechanical conduction must reach a sufficient magnitude. Second, the sheath-wall mechanical change must be sufficient to activate PIEZO1 or PIEZO2 in the investing fascia–sheath wall continuum. Third, mechanical stimulation of the sympathetic trunk must be sufficient to drive its tone across the hemodynamic threshold. Under these conditions, investing fascial fibrosis may lead to deep fascial network conduction, intrasheath and parasheath involvement, and PIEZO–CGRP molecular amplification, with a threshold-dependent five-pathway superposition as the downstream consequence. Below the sympathetic threshold, the superposition is basal and four-pathway; the sympathetic pathway joins only after its threshold is crossed.

The load-bearing evidence for the asymmetry proposition is the dCA differentiation of Friend et al. [19] and the wall-stiffness differentiation of Thomas et al. [18], with methodological feasibility supplied by Thomas et al. [27]. The load-bearing evidence for the molecular step is the PIEZO2–CGRP neural loop (35,40) and the arterial-wall PIEZO1 remodeling evidence (41–44). The PIEZO1–CGRP capacity evidence [34] and the endothelial crosstalk evidence [36] are assigned a correspondingly lower weight. The load-bearing evidence for the downstream pathways is the lymphatic/glymphatic chain (17,38,45–47,57–64) and the venous clinical series [56], with the sympathetic and vagal pathways carried as conditional modules.

The rate-limiting steps are Gap 1 and Gap 2, and the transitional framework of 8.4.2a is the mechanism by which they are made staged rather than strictly sequential. The residual content of Gap 2 after that framework — in vivo prestress, active vascular tone, and posture-dependent fascial tension — is named explicitly as the part of the problem that only in vivo reliability validation can solve.

The hypothesis should be judged not by whether it explains all functional syndromes, but by whether it can give a more precise and more readily falsifiable answer than the generalized statement that the neck is related to the cerebral circulation. That answer must specify the differentiated dose–response of the anterior and posterior circulations. It must do so in a metric in which the two circulations are commensurable, namely a slope ratio at matched effective-radius perturbation or a breakpoint-versus-slope comparison. It must not collapse both into a single "threshold" statement. It must also state which of its steps are load-bearing and which are corroborative, so that a negative result at a named step weakens the corresponding link rather than being absorbed into an unfalsifiable whole. On both counts, the present formulation is offered as a specified target for refutation, not as a settled account.