The Cervical Mechanical Bottleneck of the Lymphatic–Glymphatic Clearance Chain: Conditional Coupling Between the Investing Cervical Fascia and the Carotid Sheath and the Intrasheath Lymph Node Hypothesis

1. Introduction

1.1. Central Clearance Impairment and Lymphatic/Glymphatic Pathways

Impaired clearance of cerebral metabolic waste is a core pathological process in Alzheimer’s disease (AD), Parkinson’s disease (PD), and other neurodegenerative diseases. The retention and aggregation of β-amyloid (Aβ), tau, and α-synuclein constitute a common downstream pathway in these diseases. Over the past decade, the discovery of the glymphatic system and meningeal lymphatic vessels has reshaped our understanding of central clearance: cerebrospinal fluid (CSF) and interstitial fluid (ISF) exchange along perivascular spaces, carrying metabolic waste through meningeal lymphatic vessels to deep cervical lymph nodes (dcLNs) and ultimately into the systemic circulation [1,2]. The glymphatic system has been defined as a “lymphatic analogue” of the central nervous system, and its status as a fundamental neurobiological component has been substantiated by systematic reviews [3]. Independent studies have demonstrated that dural lymphatic vessels drain CSF and brain ISF to dcLNs through cranial base foramina. In a lymphatic hypoplasia model, clearance of cerebral macromolecules was significantly impaired, indicating that this pathway is necessary for clearance function [4]. Integrative reviews have described the glymphatic system and the meningeal lymphatic system as functionally connected clearance pathways [5], and vasomotor activity can dynamically modulate lymphatic-glymphatic coupling efficiency [6]. Human intrathecal gadolinium-enhanced magnetic resonance imaging (MRI) has shown that CSF tracers can reach dcLNs via meningeal lymphatic routes, providing in vivo visualization evidence for the brain-to-cervical lymphatic transport direction [7]. Recent quantitative MRI studies in humans have shown that the exchange rate between CSF and ISF can be quantified and that CSF inflow is not correlated with total craniospinal outflow, suggesting that ISF may recirculate into CSF rather than following a single outflow pathway [8]. This provides a human physiological basis for understanding the multipath nature of clearance pathways.

1.2. The Mechanical Space of the Cervical Outlet and the Marginalized Problem

An anatomically obvious fact has long been marginalized in functional inference: the final outlet of this clearance chain in the neck lies deep within a crowded mechanical space formed jointly by fascia, blood vessels, and nerves. The common and internal carotid arteries, internal jugular vein, vagus nerve, and deep cervical lymphatic vessels and lymph nodes run within and adjacent to the carotid sheath, and the cervical sympathetic trunk lies immediately posterior to it. These structures are not suspended in loose connective tissue. Rather, they are collectively enveloped and constrained by the three layers of the deep cervical fascia—the investing fascia, pretracheal fascia, and prevertebral fascia. When the mechanical properties of this fascial constraint system change, the effects are not limited to the local symptom of neck stiffness. They may instead alter the mechanical boundary conditions shared by the six functional domains within the sheath.

Importantly, human histological studies suggest that the common sheath does not constantly fuse with the investing fascia or the pretracheal fascia; instead, it often forms a thick plate with the visceral fascia, and sheath-wall layering and thickness vary among individuals and regions [9]. The rate of pathological involvement of the carotid sheath itself in neck dissection is extremely low [10], suggesting that the mechanical and pathological substrate of the outlet should be localized to lymphatic structures within the carotid sheath rather than to the sheath wall. At the same time, the mechanical space of the cervical outlet is not composed of fascia alone. Ligamentous cervical instability can directly encroach on the sheath contents through creep of the posterior cervical ligamentous complex and anterior displacement of C1, particularly the internal jugular vein and the vagus nerve, constituting an upstream pathway independent of investing fascia fibrosis [11]. Craniocervical instability (CCI) and atlantoaxial instability (AAI) are also associated with autonomic dysfunction, and their mechanisms include vagal involvement, internal jugular vein compression, and altered cerebral venous outflow [12]. Therefore, the mechanical transmission discussed here should be understood as a composite process that may be realized through multiple pathways—including direct fibrous continuity, visceral fascia, or interstitial tissue pressure—rather than as uniform transmission through a single continuous medium. Bone-ligament structural displacement should be incorporated as a parallel, distinguishable, and competing upstream mechanism [11,12]. In addition, the cross-sectional association between anterior shift of sagittal spinal alignment and cognitive decline in the general older population provides a reference baseline for the proposition that postural exposure itself has population-level cognitive relevance, although it measures global spinal alignment rather than forward head posture specifically [13].

1.3. The Hypothesis and the Argumentative Path

This article attempts to integrate two previously independent research traditions—fascial mechanics and central lymphatic/glymphatic clearance—into a testable framework. The basic proposition is that fibrosis of the investing fascia, through composite pathways including the deep cervical fascial network and interstitial tissue pressure, alters the conditional mechanical environment of lymphatic structures within the carotid sheath. This renders the intrasheath lymph node a condition-dependent functional bottleneck in the serial clearance chain at the cervical level. Its downstream outcome should be expressed as altered clearance function (reduction or pathological redistribution) rather than a decline in a single clearance measure. Functional stenosis, as defined here, specifically refers to functional limitation of lymphatic structures within the carotid sheath under a conditional mechanical environment. It does not presuppose homogeneous sheath-wall narrowing or constant direct fibrous continuity; its anatomical substrate and relative pathway contribution require measurement in humans. This proposition does not exclude bone-ligament structural displacement and global postural load as parallel competing upstream factors [11,12,13], but it maintains that fascial mechanics constitutes a candidate contributory pathway that can be independently measured and independently targeted. The following sections address the anatomical basis, the core propositions, the special role of lymphatic-glymphatic coupling, the distribution and strength of evidence, and empirical challenges and validation pathways.

2. Anatomical Basis: Conditional Mechanical Coupling and the Intrasheath Lymph Node

2.1. Investing Fascia and Carotid Sheath: Nonuniform, Nonconstant Continuity

The anatomical starting point of this hypothesis is not a newly discovered structure. It is a long-existing yet functionally underappreciated feature of the deep cervical fascial system: the mechanical continuity between the investing fascia and the carotid sheath, and the spatial heterogeneity of this continuity. The investing fascia is traditionally depicted in textbooks as a complete cylinder enveloping the neck. However, three-dimensional anatomic studies have raised a key revision: between the sternocleidomastoid and trapezius muscles, the investing fascia does not exist as a fibrous layer. This interval is filled with adipose tissue indistinguishable from subcutaneous tissue, so the carotid sheath is relatively directly exposed to subcutaneous tissue through this gap [14]. This anatomical fact has two mechanical implications. First, the investing fascia is not a uniform tension-bearing shell, and its tension distribution exhibits marked spatial heterogeneity. Second, the carotid sheath lacks direct mechanical shielding by the investing fascia in the posterolateral cervical region, so changes in subcutaneous tissue tension or postural load can be transmitted relatively directly to the sheath wall through this weak window.

At the same time, the investing fascia and the carotid sheath are not completely free of each other. Direct fibrous continuity exists in some regions, constituting a composite relationship of partially loose and partially directly continuous tissue. Human histological studies of the carotid sheath have revised this picture. The common sheath does not constantly fuse with the investing fascia or the pretracheal fascia; it often fuses with the visceral fascia to form a thick plate-like fusion. The common sheath and the prevertebral fascia are sometimes interrupted or poorly defined, and sheath-wall thickness and layering show marked interindividual and regional variation [9]. The mechanical implication of this revision is that the effect of investing fascia fibrosis on the mechanical environment within the carotid sheath may not be achieved in humans through direct transmission along a continuous medium. Instead, it may be transmitted through indirect pathways involving the visceral fascia, interstitial tissue pressure, or postural load. This composite relationship—partially loose, partially directly continuous, and partially mediated by intervening structures—is the structural prerequisite that allows mechanical abnormalities to be concentrated along specific pathways rather than uniformly dissipated. It also requires the hypothesis to reframe direct continuity as conditional, pathway-dependent mechanical coupling. Bone-ligament structural displacement can directly alter the spatial position and compression state of the sheath contents [11], which requires anatomical analysis to record structural parameters such as cervical curvature and anterior displacement of C1 in order to distinguish fascial transmission from bone-ligament encroachment. Sagittal spinal alignment parameters, as a feasible proxy for postural exposure, can also be incorporated into this structural assessment framework [13].

2.2. Fascial Architecture of the Carotid Sheath and Differential Mechanical Responses

The posterior wall of the carotid sheath is anatomically directly adjacent to the cervical sympathetic trunk running behind it, constituting a short mechanical pathway from the superficial fascia to the deep autonomic neural structures. The deep cervical lymphatic vessels and lymph nodes within and around the carotid sheath make this region both a lymphatic outlet and a mechanical space shared by neurovascular structures. Human histological studies show that the carotid sheath is not an anatomical artifact but a structure with a defined histological architecture. The adventitia of the artery and vein may fuse to form a distinct septum, and the adventitial layer closely apposed to the vessels is usually intact circumferentially, whereas the fusion relationship between the common sheath and surrounding fascia is variable [9]. The sheath wall itself is also not a homogeneous structure. Animal histological studies show that the artery, vein, and vagus nerve within the sheath each possess a three-layer architecture consisting of an inner fascial layer, an intermediate zone, and an outer fascial layer. The inner fascial layer of the artery appears first and thickens markedly, whereas the inner fascial layers of the vein and nerve are relatively thin. This differential thickening pattern corresponds to the level of intrinsic pulsatile hemodynamic stress experienced by each structure, while the fusion of the outer fascial layers of adjacent structures reflects the influence of extrinsic mechanical interaction [15]. This suggests that the mechanical response within the sheath is not uniformly distributed. The arterial wall may be the primary load-bearing structure in mechanical transmission, whereas the vein and nerve may be more susceptible to indirect effects of an altered external mechanical environment because of their thinner fascia. Human histological evidence [9] and animal histological evidence [15] jointly suggest that the mechanical response within the carotid sheath is constrained both by the overall architecture of the sheath wall and by the spectrum of adventitial and common sheath variation of the vessels.

Thus, the carotid sheath is not a mechanically neutral container. It is a mechanical hub whose boundary conditions are regulated by the state of the superficial fascia. However, the shared character of this hub should be based on the spectrum of variation in human sheath-wall layering and fusion relationships [9], rather than assumed to be a homogeneous, closed mechanical compartment. Anterior displacement of bone-ligament structures can independently alter the spatial boundary of this hub [11], and CCI and AAI may affect autonomic function through compression of the vagus nerve and internal jugular vein [12]. Together, these observations suggest that the boundary conditions of the mechanical hub have multiple origins. When the investing fascia retains its elasticity, it acts as a buffering layer that disperses and dissipates the shear forces generated by routine head and neck movements. Once it becomes fibrotic, this buffering function is lost, and residual tension and abnormal shear forces concentrate in deep structures along the fascial continuum or through the visceral fascia and interstitial tissue. This alters the mechanical environment of the sheath wall and tissues surrounding the carotid sheath. Because the fascial thickness and mechanical response of the various structures within the sheath differ [15], and because the fusion relationship between the human common sheath and surrounding fascia is not constant [9], this abnormal stress is not uniformly distributed within the sheath. It may preferentially act on the arterial wall, where fascial thickening is more marked, or may indirectly affect the vein and nerve through fusion of the outer fascial layers. Shear-wave elastography (SWE) has shown in healthy adults that nuchal fascia stiffness is significantly higher in the forward head position than in the neutral position [16], providing in vivo human proof-of-principle evidence for postural load leading to alteration of the cervical fascial mechanical state. However, its measurement site, time course, and reliability boundaries still require cautious extrapolation.

2.3. Lymph Nodes Within the Carotid Sheath as the “Bottleneck Carrier”

Multiple lines of evidence jointly narrow the anatomical localization of the shared bottleneck from the sheath wall to the lymph nodes within the carotid sheath. Intrasheath lymph nodes are a subset of deep cervical lymph nodes (dcLNs) and are defined here by their location within the carotid sheath. Hayashi [9] showed that the human common sheath does not constantly fuse with the investing fascia or the pretracheal fascia and that sheath-wall layering and thickness exhibit marked interindividual variation. This indicates that direct continuous transmission through the sheath wall does not hold in humans and that mechanical coupling can only be conditional. Hussain et al. [10] showed that the rate of pathological involvement of the carotid sheath itself in neck dissection was only 0.8%, indicating that the anatomical substrate of outlet destruction is not the sheath-wall structure. Sun et al. [17] showed that the deep cervical lymph nodes were the primary site of obstruction while the lumen of upstream lymphatic vessels was largely patent, indicating that the bottleneck is located at the lymph node level.

The fourth line of evidence comes from direct anatomical localization from human cadaveric dissection. In bilateral dissection of the heads of 4 cadavers, Clapham et al. showed that, of 51 deep cervical lymph nodes, 12 were histologically confirmed to be located within the carotid sheath, directly adjacent to the internal carotid artery, with mean dimensions of approximately 13.5 × 4.8 mm [18]. This advances the concept of the intrasheath lymph node from a conceptual inference supported by the spectrum of histological variation [9] and the localization of pathological obstruction [17] to a human anatomical fact with specific proportions and dimensions. However, this evidence is from a small-sample cadaveric study. It reflects static anatomy and does not assess lymphatic drainage function or mechanical state within the sheath; its value is limited to anatomical localization and cannot replace in vivo functional measurement.

Taken together, these four lines of evidence require this article to describe the shared mechanical bottleneck as intrasheath lymph nodes plus conditional fascial coupling rather than homogeneous sheath-wall stenosis. This revision does not weaken the core proposition of the hypothesis—that the outlet is functionally constrained. It advances the proposition from the intuitive picture of sheath-wall structural compression to the testable statement of functional impairment of lymphatic structures within the carotid sheath under a conditional mechanical environment. Bone-ligament structural displacement as a competing upstream factor [11,12] requires that cervical structural parameters and spinal alignment parameters [13] be recorded simultaneously in this statement in order to distinguish fascial coupling from bony encroachment.

3. Core Propositions

3.1. Proposition One: Investing Fascia Fibrosis Is One of the Peripheral Mechanical Initiating Factors

Chronic forward head posture, aging-related connective tissue remodeling, and low-grade chronic inflammation are considered synergistic factors driving investing fascia fibrosis. However, forward head posture can also drive creep of the posterior cervical ligamentous complex and anterior displacement of C1, thereby directly encroaching on the sheath contents through a bone-ligament pathway [11]. CCI and AAI are also associated with autonomic dysfunction [12]. Therefore, investing fascia fibrosis should be expressed as one of the peripheral mechanical initiating factors rather than the sole initiating factor; its specific contribution must be tested in studies that simultaneously measure bone-ligament structural parameters [11,12]. Against the background of this competing upstream pathway, the population-level relevance of postural exposure itself warrants separate recording. In a cross-sectional study based on health examinations of the general older population, Nishimura et al. found that anterior displacement of the spinal sagittal vertical axis was independently associated with cognitive decline after adjustment for age [13]. This study measured global sagittal spinal alignment rather than forward head posture specifically, and its cross-sectional design cannot establish temporal precedence. Its function is not to prove the causal chain from posture to fascia to clearance. It provides a reference baseline, without measuring the fascia, for an association between postural exposure and cognitive outcomes in the population. The significance of this reference baseline is that if, in the future, covariation between fascial elasticity and clearance function is observed within the same individual, spinal alignment parameters must be adjusted concurrently before a fascia-specific contribution can be claimed. Otherwise, the effects of global postural load may be misattributed to fascial mechanics.

The histological consequences of fibrosis are increased collagen cross-linking, matrix stiffening, and reduced elastic recoil. Its mechanical consequence is the transformation of the fascia from an elastic buffering medium into a rigid stress-transmitting medium. SWE has enabled tissue-specific quantification of stiffness of fascia, muscle, and subcutaneous tissue in the human neck and has shown that fascial stiffness can increase from approximately 22–25 kPa to approximately 44–46 kPa under fibrotic conditions [19]. This provides a methodological anchor for measurability of the severity of investing fascia fibrosis, but the source of fibrosis in that study was radiotherapy and the measurement site was the fascia in the sternocleidomastoid region. These findings cannot yet be directly equated with chronic fibrosis of the investing fascia. In a reliability study of healthy adults, nuchal fascia shear modulus was significantly higher in the forward head position than in the neutral position; intra-rater reliability was intraclass correlation coefficient (ICC) 0.63–0.89, and inter-rater reliability was ICC 0.54–0.82 [16]. This suggests that the step from forward head posture to alteration of cervical fascial mechanical state has a measurable acute physiological basis in humans and that intra-rater reliability reaches the moderate-to-excellent level required for longitudinal follow-up. However, this study had a small sample size, measured the nuchal fascia rather than the investing fascia surrounding the carotid sheath, and its acute postural effects cannot be equated with the structural functional resistance present after chronic fibrosis. Its inter-rater reliability was only moderate, meaning that multicenter longitudinal studies using nuchal fascia stiffness as an exposure variable must first address inter-operator standardization. Another reliability study of cervical and lumbar fascial stiffness and thickness using SWE reported acceptable test-retest reliability in healthy individuals and in patients with chronic neck and low back pain [20]. That study measured posterior cervical fascia rather than the investing fascia surrounding the carotid sheath, but its data from a clinical population provide an additional reliability reference for longitudinal tracking of fascial stiffness in patients with cervical symptoms.

Downstream of this mechanical initiating step, a candidate cellular transduction layer is Piezo1. Piezo1 is a confirmed mechanically activated ion channel in lymphatic endothelial cells (LECs) that senses oscillatory shear stress and integrates it into the genetic program governing lymphatic valve development and maintenance. Lymphatic-specific knockout of Piezo1 inhibits valve formation, loss in adulthood leads to valve degeneration, and pharmacological agonists can directly trigger valve gene expression without exogenous shear exposure [21]. This evidence establishes the principle of Piezo1 as a force sensor in lymphatic endothelium, but its context is valve development and maintenance, not lymph node fibrosis or lymphatic structures within the carotid sheath. It has not addressed mechanical constraint in the cervical region. Therefore, this molecular layer is regarded as a candidate mediator rather than a confirmed chain component; the level of translation between its valve context and the intrasheath lymph node still requires independent testing.

In addition to Piezo1, transient receptor potential vanilloid 4 (TRPV4) provides another candidate molecular interface that is closer to the functional regulation of mature lymphatic vessels. TRPV4 is highly enriched in collecting lymphatic endothelial cells and LYVE1+ tissue-resident macrophages. Pharmacological activation of TRPV4 leads to multiphasic contractile dysregulation in collecting lymphatic vessels: initial vasospasm followed by vasodilation and suppressed contraction. In breast cancer-related lymphedema tissue, infiltration of macrophages co-expressing LYVE1 and TRPV4 is increased [22]. The same study notes that TRPV4 signaling dysregulation is associated with inflammation and tissue fibrosis, both of which are core pathophysiological features of lymphatic system diseases. Compared with the valve development and maintenance context of Piezo1 [21], the TRPV4 evidence is conceptually closer to functional dysregulation of mature lymphatic vessels under chronic mechanical constraint and the fibrosis-inflammation positive feedback. It therefore has greater conceptual proximity to this hypothesis. However, its evidence comes from inflammatory secondary lymphedema models and clinical samples rather than from cervical lymphatic structures within the sheath. The translational hierarchy between this evidence and investing fascia mechanics still requires independent testing. Together, the two constitute candidate molecular interfaces for the cellular mechanotransduction layer of this hypothesis, but neither can replace simultaneous human measurement across the core chain.

Aging has a dual role in this proposition. It is both a driver of investing fascia fibrosis and an independent systemic axis of vulnerability of the lymphatic clearance pathway. These two roles are parallel and may be mutually amplifying. A systematic review integrating 96 studies indicated that aging can lead to meningeal lymphatic remodeling and fibrotic atrophy of dcLNs, thereby weakening CSF drainage and disrupting neuroimmune homeostasis [23]. This provides direct systemic evidence for aging-driven clearance impairment beyond fascial mechanics. Aging also directly impairs the contractile pumping function of the lymphatic vessels themselves, reducing the intrinsic driving force [24]. In vivo sampling studies show that the concentration of phosphorylated tau 181 (pTau181) in dcLNs declines with age, suggesting that clearance function is age-dependent [25]. Animal studies show that aging can cause structural atrophy of the olfactory meningeal lymphatic pathway, reducing CSF flow to the dcLNs through this parallel entry route [26]. Studies in amyloid precursor protein/presenilin 1 (APP/PS1) mice show that meningeal lymphatic dysfunction is already present at 6 months of age, before substantial Aβ plaque deposition, suggesting a critical time window for early intervention [27].

3.2. Proposition Two: Abnormal Mechanical Forces Are Transmitted to the Carotid Sheath Through Composite Pathways

Shear stress generated by the fibrotic investing fascia during routine micromovements—head and neck rotation, nodding, and arterial pulsation—is no longer effectively dispersed. It is transmitted along the three-layer collagen network of the deep cervical fascia to the sheath wall and tissues surrounding the carotid sheath. The structural basis of this transmission is the fascial continuum, whose spatial heterogeneity—including the absence of a fibrous layer of the investing fascia between the sternocleidomastoid and trapezius muscles [14]—determines where stress concentrates. However, human histological evidence suggests that the common sheath does not constantly fuse directly with the investing fascia or pretracheal fascia; the common sheath often fuses with the visceral fascia, and sheath-wall layering is variable [9]. Therefore, transmission in Proposition Two should be expressed as transmission through composite pathways involving direct fibrous continuity, the visceral fascia, or interstitial tissue pressure. Its specific pathways and relative contributions require measurement in humans. At the same time, bone-ligament structural displacement can directly alter compression of the sheath contents [11], and its contribution must be distinguished from fascial transmission. Global spinal sagittal alignment, as a cumulative indicator of postural load, should also be incorporated as a competing exposure [13].

Direct in vivo mechanical measurements—for example, in vivo tracing of the coupling between changes in investing fascia tension and sheath-wall strain—have not been reported. Therefore, Proposition Two should currently be understood as an anatomically supported mechanical inference rather than a confirmed in vivo transmission pathway. The revision of direct continuity by human histological evidence [9] reinforces this cautious position. Although SWE can detect acute increases in nuchal fascia stiffness in the forward head position [16], it has not been measured simultaneously with sheath-wall strain or clearance function markers. It therefore cannot replace direct in vivo validation of mechanical transmission within the carotid sheath. At the functional level, non-invasive mechanical intervention targeting superficial cervical lymphatic vessels can double CSF outflow [28], suggesting that the causal direction from external mechanical action on cervical structures to altered clearance function is valid in principle. However, the intervention target in that study [28] was the superficial lymphatic vessels, and these findings cannot be directly extrapolated to fascial-sheath transmission in the deep carotid sheath.

3.3. Proposition Three: Conditional Co-Susceptibility, Rather Than Equivalent Involvement, of the Six Functional Domains

3.3.1. Anatomical Basis of Adjacency of the Six Functional Domains

Arterial perfusion, venous return, deep cervical lymphatic drainage, lymphatic-glymphatic coupling, sympathetic tone, and the vagal anti-inflammatory pathway are all spatially adjacent to the carotid sheath or directly traverse it. Therefore, changes in the mechanical environment within the sheath can, in principle, simultaneously alter the boundary conditions of these six functional domains. However, the six domains are not weighted equally or assumed to be equally affected. The term coupling of the six functional domains, as used here, refers to conditional co-susceptibility and potential mutual amplification under shared mechanical boundaries. It does not presuppose equal effect sizes, identical sequences of involvement, or that a single mechanism is sufficient to explain all findings. The strength of evidence is presented in layers of core node, core supporting node, and peripheral node. Human histological evidence of sheath-wall layering and fusion variation [9] and animal histological evidence of differential fascial thickening [15] provide structural possibilities for co-susceptibility, but they also suggest that the mechanical responses of the six systems within the sheath are not equivalent. Bone-ligament structural displacement can directly compress the internal jugular vein and vagus nerve [11]; CCI and AAI are also associated with autonomic dysfunction [12]. This provides another upstream explanation for multisystem coexistence that does not depend on fascia.

3.3.2. Three-Tiered Stratification of Evidence for the Six Functional Domains

To avoid co-susceptibility being misread as equivalent involvement, this article stratifies the six functional domains into three tiers according to evidence maturity. The core node comprises two functional domains: deep cervical lymphatic drainage and lymphatic-glymphatic coupling. Coupling is treated here as a functional domain because it is the process that links the two clearance segments. Both have human evidence supporting transport direction and pathological localization [7,17,25,29], and their interventional evidence is discussed in Section 4.4.2 and Section 4.4.3. The core supporting nodes comprise three functional domains: venous return, which has human quantitative evidence on head position and intracranial pressure [30] and animal evidence from venous ligation experiments [31]; and sympathetic tone and the vagal anti-inflammatory pathway, both of which belong to the autonomic dimension but are counted separately and have been observed to coexist in humans [32] and have an association with CCI [12], although causality has not been established. The peripheral node comprises one functional domain: arterial perfusion, which mainly provides plausibility for a posture-vascular association and cannot by itself support fascial-sheath transmission. The immunomodulatory function of dcLNs serves as an additional functional dimension and is not counted among the six functional domains [33]. This stratification does not alter the anatomical proposition of shared boundary conditions, but it clarifies that the domains differ in evidence maturity. Subsequent mediation studies should prioritize measurement of the core node and core supporting nodes and consider arterial perfusion a secondary endpoint.

3.3.3. Multisystem Coexistence in Clinical Populations

Cross-sectional observations from outpatient neck centers show a high prevalence of internal jugular vein stenosis, reduced vagus nerve cross-sectional area, and increased optic nerve sheath diameter, which frequently coexist. This suggests that mechanical involvement of structures within the sheath is not a rare event in specific clinical populations [32]. This coexistence pattern can be explained by fascial-sheath transmission or by bone-ligament structural displacement [11,12], and therefore cannot by itself serve as evidence for a fascial causal chain. In a retrospective case series of 227 consecutive patients, Hauser et al. reported a multisystem coexistence pattern of internal jugular vein stenosis, vagus nerve involvement, and altered cerebral venous outflow [11]. This extends multisystem coexistence from a single-center observation to a consistent signal across two independent case series, but both are retrospective or cross-sectional designs and still cannot establish causality or distinguish the relative contributions of fascial transmission and bone-ligament encroachment. dcLNs are not only the endpoint of lymphatic drainage but also an important immune site where dendritic cells, T cells, and B cells interact with centrally derived signals. Impaired drainage may simultaneously lead to reduced waste clearance and immune dysregulation [33].

3.4. Proposition Four: Candidate Positive Feedback and the Venous-Intracranial Pressure-Glymphatic Subcircuit

One subcircuit that has been clearly described is internal jugular vein compression leading to impaired intracranial venous return, elevated intracranial pressure, and further decline in glymphatic driving force. Human head position-intracranial pressure studies show that deviation of the head from the neutral position can produce changes in intracranial pressure on the order of several millimeters of mercury, and that such changes are related to the geometric configuration of the cervical vascular system [30]. In cross-sectional observations of an outpatient neck center population, elevated optic nerve sheath diameter coexisted with internal jugular vein stenosis and reduced vagus nerve cross-sectional area [32], providing clinical population-level indirect evidence for the subcircuit from internal jugular vein compression to elevated intracranial pressure. However, this observation is cross-sectional and cannot establish causality. The upstream internal jugular vein compression in this subcircuit can be caused by fascial-sheath transmission or directly by bone-ligament structural displacement [11,12]; therefore, both sources must be considered together when attributing causality. A recent mouse study directly manipulated venous return via jugular vein ligation and found that, although postoperative CSF drainage to dcLNs was accelerated, meningeal lymphatic vessels became depleted and CSF circulation in the brain parenchyma actually decreased. After recovery of the meningeal lymphatic vessels, glymphatic exchange in the brain also recovered [31]. This suggests that the downstream consequences of impaired venous return are not limited to elevated intracranial pressure but may also include structural remodeling of the meningeal lymphatic vessels themselves.

Another subcircuit involves the bidirectional relationship between sympathetic activation and fascial fibrosis: increased sympathetic tone can promote fibroblast collagen synthesis, whereas fascial fibrosis further amplifies sympathetic output via mechanical stimulation. The net effect of chronic outlet constraint is also not unidirectional. Age-stratified studies show that young rats can develop moderate intracranial pressure elevation and extracranial compensatory drainage pathways, whereas middle-aged rats do not develop corresponding structural compensation, although brain solute clearance is enhanced in both age groups and is accompanied by low-grade inflammation and upregulation of neurodegeneration-related pathways [34]. This suggests that the net effect depends on age-dependent compensatory capacity. This evidence comes from a rat model and cannot be directly extrapolated to human physiology.

3.5. Proposition Five: Lymphatic/Glymphatic Coupling Is a Key Intermediate Link

The downstream consequences of investing fascia-sheath transmission are numerous, but the one with direct pathological significance for the central nervous system is the mechanical bottleneck formation in the clearance chain at the cervical level. The glymphatic system mediates the exchange between ISF and CSF within the brain parenchyma, driving metabolic waste to the meningeal lymphatic vessels. The meningeal lymphatic vessels, as downstream collecting channels, drain the waste-carrying fluid to the dcLNs [1,2,4]. The serial topology implies that increased resistance at any segment of the pathway is transmitted upstream as a decline in clearance efficiency [5]. However, quantitative human MRI studies suggest that CSF inflow is not correlated with total craniospinal outflow, and that ISF may recirculate into CSF rather than following a single outflow pathway [8]. This suggests that serial in the serial clearance chain should be understood as functional interdependence rather than an anatomically unique linear pathway. Accordingly, the effect of the cervical outlet bottleneck is more likely to reflect redistribution of clearance pathways than a simple decline in flow. In addition, the immunomodulatory function of dcLNs [33] and the meningeal lymphatic-microglial axis [35] suggest that the downstream consequences of this bottleneck may not be limited to altered clearance efficiency but may also involve changes in the immune microenvironment. This dimension is discussed further in Section 4.5.

4. Lymphatic/Glymphatic Coupling: Serial Clearance Chain and Shared Bottleneck

4.1. Serial Clearance Chain and Parallel Entry Routes

Upstream glymphatic inflow and exchange efficiency affect the solute load received by downstream lymphatic drainage. In turn, the patency of the downstream lymphatic outlet determines the effective clearance rate of the entire serial chain. The pathway by which meningeal lymphatics drain to the dcLNs is not unique. Animal studies have revealed a parallel pathway involving arachnoid fenestrations around the olfactory bulb, olfactory dural lymphatic vessels, cribriform plate foramina, and nasal lymphatics. This pathway undergoes structural atrophy with aging, whereas intranasal delivery of vascular endothelial growth factor C (VEGF-C) can restore lymphatic structure and clearance capacity [26]. Cross-species anatomical studies show that the principal circuit of meningeal lymphatic drainage aligns with the dural venous sinuses and exhibits a conserved three-dimensional architecture in mice and humans [36]. Human dynamic contrast-enhanced MRI cross-validated against ex vivo histology shows that the ventral dural region surrounding the middle meningeal artery exhibits delayed, non-vascular outflow dynamics of CSF and ISF accompanied by lymphoid-like structures [37]. The nasopharyngeal lymphatic plexus has been shown to be a major hub for CSF drainage through meningeal lymphatic vessels to the dcLNs [38]. Studies of the cribriform plate foramina demonstrate at the functional level that CSF can drain via the cribriform plate pathway [39]. Human in vivo MRI studies have directly compared the kinetics of intrathecal tracers reaching the nasal mucosa and the parasagittal dura. The nasal mucosa pathway peaks rapidly at 2–8 hours, whereas the parasagittal dura pathway rises slowly, peaks at 24–48 hours, and continues to rise thereafter, suggesting that the two upstream pathways are functionally and kinetically separable [40].

These findings have several implications for this hypothesis. Because the dcLNs serve as the terminal outlet, their functional load depends on the relative patency of multiple upstream meningeal lymphatic pathways and the nasopharyngeal lymphatic plexus. The relative weight of the mechanical bottleneck at the carotid sheath may be amplified when the functional reserve of the olfactory pathway, nasopharyngeal lymphatic plexus, or ventral outflow region declines. The human in vivo kinetic evidence [40] provides a functional basis for the concept that the relative weight of the cervical bottleneck is modulated by the functional reserve of upstream pathways. Human quantitative CSF-ISF exchange studies [8], in turn, suggest at the physiological level that clearance pathways themselves have a multipath and recirculating nature. The effect of the cervical bottleneck should therefore be tested as pathway redistribution rather than as a simple reduction in flow.

4.2. Dynamic Regulation and Mechanical Plasticity

Experimental studies show that cervical lymphatic drainage can be regulated by upstream glymphatic exchange efficiency, whereas vasomotor conditions such as hypercapnia can independently inhibit lymphatic-glymphatic coupling, indicating that coupling efficiency is physiologically reversible [6]. Non-invasive mechanical intervention studies show that in aged mice, a force-controlled device applied through intact skin to stimulate superficial cervical lymphatic vessels can compress superficial lymphatic vessels and double CSF outflow [28]. This finding provides proof of principle that the causal direction from external mechanical intervention on cervical lymphatics to altered central clearance efficiency exists. However, its intervention target is the superficial cervical lymphatic vessels, which is at a different anatomical level from the deep cervical lymphatic outlet focused on by this hypothesis. Superficial cervical lymph nodes (scLNs) are highly sensitive to acute CSF volume changes, whereas dcLNs respond less strongly, suggesting that superficial and deep cervical lymphatic pathways are functionally separable [41]. More critically, dcLNs and scLNs differ in the source of their driving forces. dcLN drainage relies more on extrinsic forces such as intracranial pressure oscillations, whereas scLNs are driven by an intrinsic lymphatic pump and are recruited for partial compensation when dcLNs are obstructed. This compensation is insufficient to fully restore clearance efficiency [41]. Therefore, the principles of superficial mechanical intervention cannot automatically be extrapolated to the deep carotid sheath region.

Quantitative hydrodynamic studies show that the intrinsic contractile pumping of the cervical lymphatic vessel wall is one of the main driving forces for CSF drainage. In aged mice, lymphatic vessel contraction frequency and flow velocity decline, and topical application of prostaglandin F2α restores contractile function and enhances central clearance capacity [24]. A fully coupled fluid-structure interaction (FSI) model parameterized from in vivo imaging data shows that the pumping and valve function of cervical lymphatic vessels exhibit different transient dynamics and pumping performance in multi-vessel chains under favorable and unfavorable pressure gradients [42]. This establishes the key premise that coupling is mechanically and physiologically plastic. Changes in the mechanical environment of the carotid sheath region may impair clearance not only through a static mechanical compression-obstruction model but also by dynamically modulating coupling efficiency via changes in local hemodynamics, lymphatic vessel mechanical state, and intrinsic contractile function [6,24,28,41,42,43].

In addition to the dynamic regulation and mechanical plasticity described above, the extrinsic driving forces of deep cervical lymphatic drainage should also be understood to include periodic components, not only static pressure constraints. Papadopoulos et al. have demonstrated that dcLN drainage is largely driven by extrinsic forces, whereas scLNs depend on an intrinsic lymphatic pump [41]. An FSI model shows that active pumping and valve function of lymphatic vessels show different transient dynamics under favorable and unfavorable pressure gradients [42]. Human real-time MRI studies show that CSF flow is detectable mainly during inspiration, is suppressed by breath-holding, and receives a smaller contribution from cardiac pulsation, suggesting that inspiration is the major regulator of human CSF flow [44]. This suggests that measurement of the mechanical state within the sheath should record not only static mean pressure but also pulsatile and oscillatory components [41,42,44]. However, Dreha-Kulaczewski et al. [44] measured CSF flow rather than cervical lymphatic drainage rate, and the quantitative relationship between respiratory parameters and the cervical outlet bottleneck still requires independent study. Therefore, this article recommends recording pulsatile and oscillatory components only at the measurement level [41,42,44] and does not include them as an independent covariate.

4.3. The Carotid Sheath as a Conditional Shared Bottleneck

The position of the carotid sheath in this serial chain is where the mechanical bottleneck of the downstream outlet segment lies. The deep cervical lymphatic vessels and lymph nodes are located within and around the sheath, and the mechanical environment of their walls and surrounding tissues is directly influenced by sheath-wall tension and tissue pressure within the sheath. When investing fascia fibrosis, transmitted through the fascial network, causes sheath-wall thickening and stiffening of tissues surrounding the carotid sheath, the mechanical constraint on the lymphatic outlet increases. The differential fascial layering of the sheath wall itself—the inner fascial layer of the artery is markedly thickened, whereas the inner fascial layers of the vein and nerve are relatively thin—suggests that the effect of altered mechanical environment within the sheath on the lymphatic outlet may be mediated indirectly by pericarotid tissues and the lymph node capsule [15]. The variation in the fusion relationship between the human common sheath and surrounding fascia requires describing the shared bottleneck as a conditional mechanical hub [9]. Pathological evidence from patients with AD localizes the bottleneck to the lymph node level. dcLNs exhibit fibrotic remodeling, with structural disruption, fibrosis, and macrophage accumulation, whereas the lumen of upstream afferent lymphatic vessels is largely patent, suggesting that the dcLNs themselves may be the primary site of pathological obstruction in the cervical lymphatic outflow pathway [17]. This finding anchors the mechanical inference of functional stenosis in this article to observable pathological alterations. Moreover, the site of obstruction is located within and around the carotid sheath, anatomically coinciding with the conditional shared mechanical hub localized in this article.

Compression of the internal jugular vein within the sheath can further weaken the glymphatic driving force by elevating intracranial pressure. Human head-position studies show that changes in head position can significantly alter intracranial pressure and its pulsatile component [30]. Internal jugular vein compression can be caused by fascial-sheath transmission or directly by anterior displacement of bone-ligament structures [11]. Animal jugular vein ligation experiments show that the direct consequences of impaired venous return include not only altered intracranial pressure but also depletion of meningeal lymphatic vessels and reduced glymphatic exchange in the brain parenchyma [31]. The lymphatic and glymphatic segments are simultaneously locked at the same mechanical hub, forming a coupling impairment. However, the relative weight of this bottleneck is not fixed but varies with the functional reserve of upstream parallel pathways [26,36,37,38]; human in vivo kinetic evidence [40] suggests that upstream parallel entry routes are separable on the transport timescale.

4.4. Human Evidence: Multidirectional Signals for Transport Direction, Reconstruction, and Destruction

4.4.1. Multimethod Evidence Chain for Transport Direction

Ultrasound-guided fine-needle aspiration was used to directly sample dcLNs in humans, and neurodegenerative biomarkers including Aβ40, Aβ42, pTau181, glial fibrillary acidic protein (GFAP), and neurofilament light chain (NfL) were detected. Except for NfL, the concentrations of all markers in dcLNs were significantly higher than plasma concentrations, and the concentration of pTau181 in dcLNs declined with age [25]. Human intrathecal gadolinium-enhanced MRI has demonstrated in vivo that CSF tracers reach dcLNs via meningeal lymphatic routes [7]. Human cadaveric dissection combined with dural indocyanine green (ICG) lymphangiography and immunohistochemical analysis directly mapped, at the anatomical level, the posterior drainage pathway from the dura mater to level II dcLNs. In one cadaver with confirmed dementia, this approach showed selective enrichment of Aβ in level II dcLNs [29]. In vivo visualization by Eide et al. [7], endpoint enrichment by Al-Diwani et al. [25], endpoint pathology by Sun et al. [17], and human anatomical pathway mapping by Nguyen et al. [29] together constitute a multi-method evidence chain for transport direction: in vivo imaging, in vivo sampling, endpoint pathology, and human anatomy. Together, these four lines of evidence support the proposition that brain-derived substances do reach the dcLNs and that dcLNs undergo age-related pathological remodeling. However, none can prove that cervical mechanical constraint regulates this transport rate.

4.4.2. Outlet Reconstruction: Multidirectional Signals of dcLVA and Pressure Physiology Challenges

Deep cervical lymphaticovenous anastomosis (dcLVA) aims to bypass resistance at the cervical lymphatic outlet. A single-arm exploratory study reported that Mini-Mental State Examination (MMSE) scores improved in some patients with dementia within one week after surgery, and central nervous system-derived tau protein was detected in cervical lymph nodes [45]. After dcLVA, blood flow velocity and cross-sectional area in the internal jugular vein and carotid artery increased significantly, and the improvement in carotid artery blood flow was moderately correlated with cognitive scores [46]. A translational imaging study in 8 patients with AD showed that cortical amyloid burden decreased and indices of myelin integrity increased after dcLVA [47]. A prospective single-arm study of 139 patients with severe AD showed that MMSE scores increased modestly within 6 months after surgery, CSF Aβ42, Aβ40, and pTau181 levels decreased, and corresponding plasma marker levels increased [48]. Recently, a functional dcLVA model in APP/PS1 mice provided direct animal-level evidence that dcLVA can enhance lymphatic drainage from the subarachnoid space to the dcLNs [49].

The inference from outlet reconstruction to clearance improvement described above faces a fundamental challenge from pressure physiology. An opinion article noted that the cervical lymphatic system is essentially a low-pressure system. Available physiological measurements show that cervical lymphatic pressure is usually below 5 cm H2O, whereas jugular venous pressure often exceeds this value. Therefore, the lymphatic-to-venous drainage direction that dcLVA attempts to establish may inherently oppose the pressure gradient under physiological conditions. Impaired brain waste clearance in AD may primarily reflect a functional disturbance of glymphatic-meningeal lymphatic transport rather than mechanical obstruction of peripheral lymphatic vessels [50]. Taking the four single-arm studies [45,46,47,48] together with the animal model [49] and the pressure physiology challenge [50], the dcLVA evidence should be described as association signals between outlet reconstruction and downstream improvement rather than causal proof that outlet reconstruction is effective. An FSI model suggests that active lymphatic pumping and valve function may maintain forward flow under specific pressure gradients, providing a mechanistic rationale for dcLVA benefit possibly extending beyond bypassing mechanical resistance. However, this model cannot substitute for human pressure measurements and controlled trials.

4.4.3. Outlet Destruction: Neck Dissection Associations, Tau Pathology, Systematic Review Evidence, and Mechanistic Supporting Evidence

A retrospective analysis showed that bilateral supraomohyoid or modified radical neck dissection was associated with a higher incidence of dementia, and that the risk was higher with bilateral than with unilateral dissection [51]. Human tissue analysis after long-term cervical lymphadenectomy showed that cervical lymphatic blockade not only physically impedes drainage but is also accompanied by activation of the ERK1/2 pathway, impaired autophagy, and suppression of presentation of brain antigens to dcLNs, together with the development of tau pathology [52]. A systematic review including 23 studies and 28,054 patients with head and neck cancer noted that the association between neck dissection and cognitive decline has been repeatedly observed in multiple independent studies. However, studies specifically addressing dcLN dissection and cognitive outcomes account for only 3.0%, and this association remains understudied [53]. A recent study combining a retrospective clinical review with an animal model showed that neck dissection is associated with an increased risk of postoperative cognitive impairment. In its human portion, a retrospective analysis of the medical records of 1,035 patients who underwent neck dissection and did not receive adjuvant chemoradiotherapy reported that approximately 18% of patients younger than 65 years developed cognitive impairment within two years after surgery [54]. The animal component of that study showed that acute interruption of lymphatic drainage can induce metabolic stress in the CSF and synaptic protein oxidation, but these harmful changes did not occur when lymphatic drainage declined gradually [54]. This suggests that acute complete blockade and the chronic partial mechanical constraint described in this article differ fundamentally in timescale and extent of blockade.

Notably, the anatomical consequences of neck dissection require more precise definition. A study of 121 patients and 145 neck sides showed that in neck dissections without clinical evidence of extranodal extension, the rate of pathological involvement of the carotid sheath itself was only 0.8%, and carotid sheath preservation did not increase short-term regional recurrence [10]. This suggests that the anatomical substrate of the pathway from outlet destruction to cognitive risk is more likely loss of clearance function by lymphatic vessels and lymph nodes within and surrounding the carotid sheath rather than direct injury to the sheath-wall structure. Human evidence of cognitive impairment associated with cervical lymph node dissection [51,52,54] coexists with early signals of cognitive improvement after lymphaticovenous anastomosis (LVA) [45,46,47,48], although the two point in opposite directions. Prospective trials are key to confirming causal effects. Systematic reviews have positioned deep cervical lymphatic surgery as a treatment strategy for brain lymphedema, while noting that this field remains dominated by case series and single-arm studies [55].

4.4.4. Experimental Blockade: Acute Versus Chronic Blockade and Superficial Versus Deep Compensation

Deep cervical lymph node ligation in APP/PS1 mice can aggravate Aβ accumulation, neuroinflammation, and cognitive deficits [56]. An in vivo gadolinium-enhanced MRI study in rats with bilateral dcLN ligation showed that outlet blockade can directly reduce CSF tracer outflow toward the nasal cavity and increase intracranial tracer retention [57]. Jugular vein ligation experiments provide another class of outlet manipulation evidence from the venous side. After venous return was obstructed, meningeal lymphatic vessels were depleted and brain parenchymal glymphatic exchange declined, and glymphatic exchange recovered after the meningeal lymphatic vessels recovered [31]. However, the net physiological effect of chronic outlet constraint may not be a simple decline in clearance. In the chronic dcLN blockade study reviewed by Hablitz and Nedergaard, impaired dcLN drainage in young rats was associated with moderate elevation of intracranial pressure and the appearance of extracranial compensatory drainage pathways. Both age groups showed enhanced brain solute clearance despite reduced glymphatic inflow, accompanied by low-grade inflammation and upregulation of neurodegeneration-related pathways [58]. Here, enhanced brain solute clearance should be understood as a net manifestation of pathological redistribution. Total clearance may not decline, but the pathways, pressure, and inflammatory burden change. This does not contradict the core proposition of decline or pathological redistribution. Age-stratified studies show that young rats can develop extracranial compensatory drainage pathways, whereas middle-aged rats do not develop corresponding structural compensation [34]. The difference in driving forces between dcLNs and scLNs provides a mechanistic basis for redistribution. After dcLN blockade, the nasal-to-scLN pathway can be recruited, but this recruitment is insufficient to fully compensate for delayed clearance [41]. Therefore, the bottleneck effect predicted by this hypothesis should be tested as a net balance of clearance pathway redistribution, intracranial pressure, and inflammatory burden, rather than as a single measure of clearance volume. Quantitative human CSF-ISF exchange studies provide a human physiological basis for redistribution [8]. Human in vivo MRI evidence that upstream pathways are kinetically separable [40] provides functional evidence from upstream entry routes for pathway redistribution.

4.4.5. Human Imaging Evidence Linking Cervical Symptoms to Glymphatic Function: A Circumstantial Association

In patients with chronic neck and shoulder pain, the diffusion tensor imaging along the perivascular space (DTI-ALPS) index in the left hemisphere was significantly lower than that in healthy controls, and both pain intensity and disease duration were significantly negatively correlated with this index [59]. However, this study assessed a clinical pain phenotype rather than a fascial mechanical state and did not establish a mediating pathway from fascia to sheath to clearance. It can therefore serve only as circumstantial evidence that cervical abnormalities are associated with clearance function.

4.4.6. Symmetry and Limitations of the Multidirectional Evidence

Human association and pathological anchoring evidence for transport direction [7,17,25,29] can support that brain-derived substances reach dcLNs and that age dependence exists, but it cannot prove that mechanical exposure regulates the transport rate. Human intervention signals for outlet reconstruction [45,46,47,48] and mechanistic boundaries [42,49,50] can support association signals between outlet reconstruction and downstream improvement but cannot exclude single-arm bias or prove the pressure-gradient premise. Human association and pathological anchoring evidence for outlet destruction [10,51,52,53,54] can support outlet destruction being associated with cognitive risk and the anatomical substrate being lymphatic structures within the carotid sheath. However, retrospective designs cannot establish causality, and findings from acute near-complete blockade cannot be extrapolated to chronic partial constraint. Evidence from experimental blockade [31,34,56,57,58] can support the proposition that outlet constraint has functional consequences, undergoes partial compensation in the chronic setting, and shows age-dependent compensatory capacity, but it cannot be directly extrapolated to chronic partial fascial constraint in humans. Evidence on parallel pathways [26,36,37,38,40,41] can support the proposition that upstream entry routes are multilayered, superficial and deep pathways are functionally separable, and parallel entry routes are kinetically separable, but it cannot replace direct measurement of cervical mechanics. Evidence on competing upstream structures [11,12,13] can support the proposition that bone-ligament structural displacement and global postural load are competing upstream factors, but causality has not been established. Evidence on downstream neuroimmune mechanisms [35] can support the proposition that the downstream consequences of meningeal lymphatic dysfunction are not limited to reduced clearance efficiency, but it cannot be directly equated with effects under chronic partial fascial constraint in humans.

Six conclusions follow. First, human intervention signals have appeared only in the direction of outlet reconstruction and are subject to challenges from pressure physiology [50]. Second, the outlet destruction direction has human association and pathological anchoring evidence, but the anatomical attribution has been refined to lymphatic structures within the carotid sheath, and the mechanistic evidence comes mainly from acute blockade [54]. Third, the acute and chronic conclusions in the experimental blockade direction are inconsistent. This requires the outcome to be expanded from clearance volume to clearance pathway redistribution, intracranial pressure, and inflammatory burden, and compensatory capacity is age-dependent [34,58]. Fourth, the neuroimmune consequences of meningeal lymphatic dysfunction provide an independent mechanistic dimension for inflammatory burden [35]. Fifth, quantitative human CSF-ISF studies provide a human physiological anchor for redistribution [8]. Sixth, human in vivo MRI showing that upstream parallel entry routes are kinetically separable [40] provides human functional evidence from upstream entry routes for variable relative weight and pathway redistribution, but it cannot replace direct measurement of the cervical outlet.

4.5. Disease-Level Integration: The Parkinson’s Disease Template and the Aging Axis

Research on glymphatic dysfunction in PD further integrates multiple segments of the clearance chain into one pathophysiological pathway. Aging-related glymphatic decline, reduced meningeal lymphatic drainage, altered imaging biomarkers, and impaired α-synuclein clearance are interrelated. Strategies to restore glymphatic function, such as dcLVA, have been discussed as potential therapeutic strategies [60]. The immunomodulatory function of dcLNs adds another layer of explanation to this disease template. Impaired drainage not only reduces protein clearance but may also indirectly contribute to the positive feedback loop of neuroinflammation and protein aggregation by disrupting the immune output of dcLNs [33]. Meningeal lymphatic dysfunction can disrupt cortical excitation-inhibition balance and impair memory through a microglia-dependent interleukin-6 (IL-6) pathway, whereas restoration of meningeal lymphatic function can reverse these changes [35]. This adds a neuroimmune dimension—the meningeal lymphatic-microglial-synaptic axis—to the downstream consequences of the cervical outlet bottleneck. Systematic review evidence indicates that aging itself constitutes a systemic axis of vulnerability of the lymphatic clearance pathway [23]. Aging also directly impairs the contractile pump function of lymphatic vessels [24]. In vivo sampling studies show that clearance function is age-dependent [25]. Animal studies show that aging causes structural atrophy of the olfactory meningeal lymphatic pathway [26]. Natural history studies in APP/PS1 mice suggest that lymphatic functional decline may precede pathological deposition [27]. Age-stratified studies of chronic dcLN blockade show that compensatory capacity declines with age [34]. Therefore, the cervical mechanical bottleneck proposed in this hypothesis is not the only pathway by which aging affects clearance. It is one of several mechanisms that are parallel to, and may mutually amplify, aging-related lymphatic structural decline, physiological decline, reduced compensatory capacity, and loss of parallel pathway function.

5. Distribution and Strength of Evidence: A Node-Differentiated Picture

5.1. Core Node: Lymphatic/Glymphatic

The anatomical discovery and functional necessity of meningeal lymphatic vessels have been independently validated [1,4]. The conceptual framework and driving mechanisms of the glymphatic system have been systematically substantiated in systematic reviews [2,3]. The conceptual framework of lymphatic-glymphatic coupling is gradually becoming clearer [5], and experimental evidence exists for dynamic regulation of coupling by vasomotor activity [6]. External mechanical intervention [28] and pharmacological restoration of intrinsic contractile pump function [24] demonstrate from two dimensions that cervical lymphatic function is plastic, and FSI models provide a mechanical explanation for lymphatic pump function [42]. The functional separation of superficial and deep cervical lymphatic pathways and the differences in their driving mechanisms [41] require further refinement of the anatomical level of mechanical plasticity. Cervical lymphatic mechanics has been distilled as a modifiable dimension [43].

Aging-related lymphatic decline has been confirmed at the systematic review level [23]. dcLNs as the site of pathological obstruction have been directly localized pathologically [17]. The anatomical localization of intrasheath lymph nodes has been quantitatively supported by human cadaveric dissection [18]. The three-layer fascial architecture of the sheath wall itself and its differential mechanical response provide a structural basis for the mechanical hub [15]. The revision by human histological evidence of the fusion relationship between the common sheath and surrounding fascia [9] requires that the hub be described as a conditional mechanical interface subject to anatomical variation.

The olfactory meningeal lymphatic pathway [26], functional evidence of cribriform plate drainage [39], the dorsal meningeal lymphatic principal circuit [36], the human ventral middle meningeal artery-associated outflow region [37], and the nasopharyngeal lymphatic plexus hub [38] collectively suggest that the relative weight of the cervical bottleneck is modulated by the functional reserve of multiple upstream parallel pathways. Human in vivo MRI shows that the nasal mucosa and parasagittal dura pathways are kinetically separable [40]. Human intrathecal tracer MRI [7] and dcLN biopsy [25] anchor the brain-to-cervical lymphatic transport direction at the levels of pathway visualization and endpoint enrichment, respectively. The immunomodulatory function of dcLNs provides a functional dimension for lymph nodes beyond a passive waste-drainage role [33].

SWE has achieved tissue-specific quantification of fascial stiffness in the human neck [19] and has shown in healthy adults that nuchal fascia stiffness is significantly increased in the forward head position, with intra-rater reliability ICC 0.63–0.89 and inter-rater reliability ICC 0.54–0.82 [16]. Another SWE reliability study of cervical and lumbar fascia reported acceptable test-retest reliability in healthy individuals and in patients with chronic neck and low back pain [20]. Bone-ligament structural displacement [11] and the association of CCI and AAI with autonomic dysfunction [12] suggest that the upstream exposure of the mechanical hub is not a single fascial variable. The population association between spinal alignment and cognition [13] provides a reference baseline independent of fascia for the clinical relevance of postural exposure.

Meningeal lymphatic remodeling induced by jugular vein ligation [31] provides direct animal causal manipulation evidence for the venous subcircuit. The dcLVA animal model [49] provides an experimental platform for the intervention mechanism of outlet reconstruction. The meningeal lymphatic-microglial-IL-6 axis [35] provides a mechanistic dimension for downstream neuroimmune burden. Quantitative human CSF-ISF exchange [8] provides a human physiological anchor for clearance pathway redistribution, and the natural history of APP/PS1 mice [27] provides an experimental anchor for the early time window of lymphatic functional decline. Age-stratified compensation studies [34], FSI models [42], and human upstream pathway separation evidence [40] respectively reinforce age dependence, lymphatic pump mechanical mechanisms, and parallel entry functional separation.

Piezo1 as a mechanosensor in lymphatic endothelium provides a cornerstone for the cellular mechanotransduction layer in the context of valve development [21]. Functional evidence for TRPV4 in mature lymphatic vessel contraction and inflammatory remodeling provides a second candidate interface with greater conceptual proximity to chronic mechanical constraint for the same layer [22]. The two have different contexts, and their translational hierarchy relative to intrasheath lymph nodes requires independent testing. Real-time MRI evidence that respiration is the major driver of CSF flow provides a human anchor for periodic extrinsic forces [44], but it measured CSF flow rather than cervical lymphatic drainage rate.

5.2. Core Node: Venous and Autonomic Domains

The venous node has human evidence for mechanical sensitivity. Head rotation can cause ipsilateral internal jugular vein torsion and compression, and changes in head position can further quantitatively alter intracranial pressure and its pulsatility [30]. Internal jugular vein compression can also be caused directly by anterior displacement of bone-ligament structures [11] and is associated with autonomic dysfunction in patients with CCI or AAI [12]. The animal jugular vein ligation experiments of El Kamouh et al. [31] complete the mechanistic link from impaired venous return to meningeal lymphatic remodeling and decreased glymphatic exchange in the brain parenchyma. Regarding the autonomic node, multisystem coexistence observations in the carotid sheath region show that reduced vagus nerve cross-sectional area and internal jugular vein stenosis frequently coexist in specific clinical populations [32]. A narrative review of CCI/AAI and autonomic dysfunction shows that vagal involvement, internal jugular vein compression, and altered cerebral venous outflow are mechanisms repeatedly proposed, but the evidence is mainly retrospective and heterogeneous [12]. The ligamentous cervical instability framework directly attributes internal jugular vein and vagus nerve compression to anterior displacement of bone-ligament structures [11]. Therefore, the coexistence evidence for the autonomic node has multiple origins.

5.3. Peripheral Node: Arterial

The association between forward head posture and posterior circulation ischemic stroke has been repeatedly observed in case-control studies. Negative correlations between cervical muscle stiffness and internal carotid and vertebral artery blood flow velocities have been quantified in patients with cervicogenic headache. However, the direct effect of forward head posture on the vertebral artery can by itself explain the stroke risk, and fascial-sheath transmission is not the only pathway. Therefore, evidence at the arterial node can serve only as upstream plausibility evidence that abnormal mechanical exposure may produce vascular consequences. The spinal alignment-cognition association reported by Nishimura et al. [13] can likewise serve only as indirect support for the clinical relevance of postural exposure.

5.4. Core Empirical Gap and Logic for Distinguishing Competing Explanations

The core empirical gap can be distilled into a falsifiable proposition: within the same individual, does a decline in investing fascia elasticity precede or accompany changes in the mechanical state of intrasheath lymph nodes and mediate altered clearance function (reduction or pathological redistribution), and does it remain statistically significant after adjustment for cervical bone-ligament structural parameters and spinal alignment parameters? The retrospective cohort study by Khan et al. [54] reinforces the adjacent proposition that surgical outlet destruction is associated with cognitive risk, rather than the core proposition. El Kamouh et al. [31] reinforce the animal causal manipulation of the venous subcircuit. Liao et al. [16] reinforce the human acute proof of principle for posture leading to nuchal fascia stiffness, and Liu et al. [49] reinforce the mechanistic possibility that outlet reconstruction can enhance lymphatic drainage. Kim et al. [35] reinforce downstream neuroimmune consequences. Wåhlin et al. [8] reinforce the human physiological basis for the multipath nature of clearance pathways. Gursky et al. [34] reinforce the age-dependent compensation of chronic outlet constraint. Kim and Tithof [42] reinforce the mechanical mechanism of the lymphatic pump, and Urbach et al. [40] reinforce the functional kinetic separation of upstream parallel entry routes. Choi et al. [21] reinforce the valve-development context of Piezo1 in lymphatic endothelium. Schulz et al. [22] reinforce the candidate molecular interface of TRPV4 in mature lymphatic vessel contraction and inflammatory remodeling. Dreha-Kulaczewski et al. [44] reinforce the human evidence that respiration is the major driver of CSF flow, and Clapham et al. [18] reinforce the cadaveric anatomical localization of intrasheath lymph nodes. These lines of evidence each anchor different nodes of the chain but cannot replace simultaneous human measurement across the core chain.

A longitudinal study oriented toward mediation analysis requires simultaneous measurement of four classes of variables within the same individual. The exposure variable is investing fascia elasticity (SWE [19], with the postural manipulation protocol used by Liao et al. [16] for detecting acute postural effects and Zandi et al. [20] providing a reliability reference in a clinical population). Competing parallel exposures are cervical structural parameters (cervical curvature, anterior displacement of C1, and others) [11], sagittal spinal alignment parameters [13], and autonomic function indices [12]. The mediator variable is a surrogate for carotid sheath morphology and pressure, including pulsatile and oscillatory components [41]. The outcome variable is a glymphatic and lymphatic clearance function marker (DTI-ALPS [61,62], dcLN ultrasound phenotype [63], and others). Surrogate measures of mechanics within the sheath should record both static and pulsatile and oscillatory components [41,42] to distinguish static constraint from the effects of periodic extrinsic forces. Human evidence for periodic drivers such as respiration [44] has indicated that respiration is a major regulator of CSF flow, but it did not measure cervical lymphatic drainage rate and is therefore not listed as a parallel covariate. If future conditions permit, Piezo1- or TRPV4-related markers [21,22] may serve as candidate molecular mediators for mechanical exposure leading to cellular transduction and structural remodeling. The two have different contexts, and their translational hierarchy relative to intrasheath lymph nodes requires independent testing.

In statistical analysis, fascial elasticity should serve as the focal exposure, with cervical structural parameters and spinal alignment parameters treated as parallel exposures or covariates. This tests whether the mediating effect of fascial elasticity on clearance function is independent of bone-ligament structural displacement and global postural load. If the mediating effect of fascial elasticity attenuates significantly after adjustment for these parameters, this would suggest that the observed association is mainly explained by competing upstream factors. If it remains significant, this would support fascial mechanics as an independent contributory pathway. Multi-exposure mediation models should prespecify collinearity diagnostics. When collinearity is excessive, principal component analysis or latent variable methods within structural equation modeling should be used. Further stratification by cervical structural parameters and spinal alignment parameters is possible. In the subgroup with normal bone-ligament structures and normal spinal alignment, the mediation chain from fascial elasticity through mechanics within the sheath to clearance function should be tested. In the subgroup with abnormal bone-ligament structures or anterior spinal alignment displacement, it should be tested whether fascial elasticity provides additional explanatory value beyond competing upstream factors.

6. Clinical and Theoretical Implications

6.1. Disease Subtyping: The Possibility of a Cervical Mechanical Subgroup

In terms of disease subtyping, this hypothesis suggests that a subgroup of neurodegenerative diseases may exist in which cervical mechanical abnormality serves as a conditional upstream feature. The parallel upstream contributions of bone-ligament structural displacement, CCI, and AAI [11,12] suggest that this subgroup can be further divided into fascia-dominant and bone-ligament-dominant subtypes, with different intervention targets. Anterior displacement of spinal alignment [13] suggests that global postural load may also constitute an independent, non-fascia-specific risk dimension. The immunomodulatory function of dcLNs suggests that the potential benefit of outlet intervention may not be limited to improved drainage but may also involve modulation of the immune microenvironment [33]. The meningeal lymphatic-microglial-IL-6 axis [35] extends the potential benefit to neuroimmune regulation. A distinction must be made between functional stenosis, as defined above, and the surgical indication concept of brain lymphedema used in systematic reviews [55]. The latter positions deep cervical lymphatic surgery as a treatment strategy for fluid accumulation, whereas functional stenosis here specifically refers to functional limitation of lymphatic structures within the carotid sheath under a conditional mechanical environment and does not presuppose fluid accumulation or reversal of the pressure gradient.

6.2. Intervention Logic: Upstream Mechanical Modulation and Downstream Outlet Reconstruction

In terms of intervention logic, the hypothesis positions investing fascia release and fibrosis reversal as upstream mechanical modulation, and positions lymphatic outlet bypass procedures such as dcLVA as downstream outlet reconstruction. Bone-ligament structural displacement as a competing upstream factor [11] suggests that structural correction or stabilization may be another type of upstream intervention. Current preliminary evidence for dcLVA supports the feasibility and central nervous system relevance of downstream outlet intervention [45,46,47,48], but all such evidence is limited by single-arm design, small sample size, or short follow-up. The intervention logic of dcLVA faces a fundamental challenge from pressure physiology [50], and its specific contribution and physiological premise require independent validation in controlled designs. Experimental evidence that non-invasive mechanical intervention on superficial cervical lymphatic vessels doubles CSF drainage [28] provides proof of principle that external mechanical approaches are feasible. However, the functional separation of superficial and deep pathways and the differences in their driving mechanisms [41] indicate that this principle cannot simply be extrapolated to the deep carotid sheath region. Experiments involving pharmacological restoration of the intrinsic contractile pump function of lymphatic vessels [24] show that, even without altering external mechanical constraints, restoring the contractile capacity of lymphatic vessels themselves can enhance central clearance. The conceptual distillation of cervical lymphatic mechanics as a modifiable dimension [43] has given this intervention category terminological legitimacy within the field.

Cognitive impairment signals associated with cervical lymph node dissection provide counter-evidence that underscores the clinical importance of outlet integrity [51,52,53,54]. Conversely, the extremely low rate of pathological involvement of the sheath wall itself [10] suggests that the key anatomical substrate of outlet integrity is the lymphatic structures within the carotid sheath. The discovery of the olfactory meningeal lymphatic pathway suggests another category of upstream intervention strategy: reducing dependence on a single cervical outlet by restoring parallel entry routes [26]. The anatomical separability of the principal meningeal lymphatic circuit and the nasal pathway [36], the independent dynamics of the ventral outflow region [37], and the localization of the nasopharyngeal lymphatic plexus as a hub [38] provide a structural basis for parallel pathway intervention. Human in vivo MRI shows that the nasal mucosa and parasagittal dura pathways are kinetically separable [40], suggesting that the functional reserve of different parallel entry routes can be evaluated separately when designing interventions. A rat study of chronic dcLN blockade suggests that the net effect of outlet constraint may be partially offset by compensatory drainage pathways, accompanied by elevated intracranial pressure and low-grade inflammation [58]. Age-stratified studies suggest that compensatory capacity declines with age [34]. The asymmetry in compensatory capacity between dcLNs and scLNs [41] suggests that the net effect of deep outlet constraint should not be inferred from superficial compensatory capacity alone. Venous-side interventions (for example, improving internal jugular venous return) may indirectly improve brain glymphatic exchange by restoring the structural integrity of meningeal lymphatic vessels [31].

6.3. Assessment Strategy: Intermediate Phenotypes Linking Mechanics to Clearance

Neck ultrasound can assess internal jugular vein stenosis and vagus nerve cross-sectional area [32]. SWE can provide tissue-specific kPa values for fascial stiffness under fibrotic conditions [19] and has been shown to detect acute changes in nuchal fascia stiffness in the forward head position [16]. Another study has provided test-retest reliability data for SWE in a clinical population [20]. Glymphatic imaging and indicators of lymphatic drainage efficiency are maturing [60]. Cervical structural imaging [11], sagittal spinal alignment parameters [13], and autonomic function tests [12] should be included as parallel assessment dimensions. Two classes of studies provide useful precedents for longitudinal mediation designs, but neither constitutes causal evidence that fascial mechanics mediate clearance decline. First, a multimodal positron emission tomography (PET)/MRI study demonstrated in humans that glymphatic dysfunction, as reflected by the DTI-ALPS index, mediates cognitive decline through an Aβ-dependent pathway, providing a methodological template for glymphatic function as a measurable mediator endpoint [61]. Its limitation is its cross-sectional design. Second, a longitudinal study based on the Alzheimer’s Disease Neuroimaging Initiative (ADNI) cohort showed that a higher ALPS index was independently associated with a lower risk of mild cognitive impairment (MCI) and a lower risk of progression from MCI to AD, and that choroid plexus Aβ, some cortical tau, and executive function partially mediated these associations [62]. Its limitation is its observational cohort design. Neither study measured fascial mechanics or the mechanical state of the carotid sheath. They support the methodological proposition that glymphatic markers can serve as longitudinal mediator endpoints. A translational imaging study showed that amyloid burden and indices of myelin integrity can be quantitatively assessed in vivo after dcLVA [47]. The reduced DTI-ALPS index in patients with chronic neck and shoulder pain [59] suggests that this index can also be used for cross-sectional assessment in patients with cervical symptoms. The dcLN ultrasound phenotype has shown some discriminative ability in patients with AD [63], but the discriminative ability of this phenotype has been questioned on methodological grounds with respect to node-level inference and score construction [64,65]. This non-invasive phenotype, together with the pathological localization of Sun et al. [17], the biomarker transport evidence of Al-Diwani et al. [25], and the human tracer evidence of Eide et al. [7], jointly form a pathology-biomarker-non-invasive imaging-in vivo pathway hierarchy for assessing dcLNs.

Quantitative human CSF-ISF exchange studies [8] suggest that future longitudinal studies should measure pathway redistribution indicators in parallel with clearance volume indicators. Human in vivo MRI showing that upstream parallel entry routes are kinetically separable [40] suggests that upstream pathway functional reserve can serve as a candidate stratification variable for pathway redistribution. FSI models [42] suggest that lymphatic pump function can serve as a candidate mediator variable, although this remains a computational prediction only. Real-time MRI evidence that respiration is the major driver of CSF flow [44] suggests that respiratory parameters can serve as candidate covariates for periodic extrinsic forces. Because it did not measure cervical lymphatic drainage rate, however, the current recommendation is to record pulsatile and oscillatory components only, rather than to include them as independent covariates. Piezo1- or TRPV4-related markers [21,22] can serve as a candidate molecular mediator layer for mechanical exposure leading to cellular transduction and structural remodeling. The two have different contexts, and their translational hierarchy relative to intrasheath lymph nodes requires independent testing.

7. Limitations and Unresolved Questions

7.1. Integration Outpaces Empirical Evidence

The primary limitation of this hypothesis is that the scope of integration currently exceeds the available empirical evidence. Equating anatomical continuity with functionally significant resistance is an inference that requires mechanical measurement, rather than anatomical description, to adjudicate [14]. Human histological evidence indicates that the common sheath does not constantly fuse with the investing fascia or the pretracheal fascia and that sheath-wall layering and thickness show marked interindividual variation [9]. The first link in the hypothesis chain should be revised to conditional, pathway-dependent mechanical coupling, and its human in vivo evidence still requires further support. SWE can quantify cervical fascial stiffness [19] and has shown acute increases in nuchal fascia stiffness in the forward head position in healthy adults [16]. However, the structural-functional dissociation reported in a radiation fibrosis model—fascial stiffness nearly doubled while cervical range of motion was largely preserved [19]—together with the limitations of Liao et al. [16] regarding measurement site and reliability, suggests that increased stiffness alone is insufficient to prove the formation of functional resistance. The valve evidence for Piezo1 [21] and the inflammatory lymphedema evidence for TRPV4 [22] provide, respectively, two candidate mediator interfaces at the cellular layer. An unmeasured translational hierarchy still separates them from the pathway from investing fascia fibrosis to functional limitation of intrasheath lymph nodes. The anatomical localization by Clapham et al. [18] reinforces where the node is but does not answer how the node becomes functionally compromised under mechanical constraint.

7.2. Challenges in Operationalizing Pressure Within the Carotid Sheath

The carotid sheath is not a closed hydraulic cavity. Its pressure is more accurately a mechanical state jointly determined by tissue pressure within the sheath and sheath-wall tension, and standardized and reproducible direct measurements in humans are lacking [9]. Using internal jugular vein stenosis severity, sheath-wall thickness, or ultrasound elasticity parameters as surrogate measures introduces different validity concerns in each case. SWE measures tissue stiffness rather than tissue pressure within the sheath [16,19,20]. dcLN drainage is coupled to intracranial pressure oscillations, whereas scLNs are driven by an intrinsic lymphatic pump [41]. Therefore, measurement of the mechanical state within the sheath should capture not only static mean pressure but also pulsatile and oscillatory components. Human real-time MRI shows that inspiration is the major regulator of CSF flow [44], supporting the importance of periodic components. However, this evidence measures CSF flow and cannot replace direct measurement of tissue pressure within the sheath or cervical lymphatic drainage rate. FSI models suggest that flow within lymphatic vessels depends on the coupling of active contraction of the vessel wall, valve function, and transmural pressure difference [42]. Head position-intracranial pressure studies provide quantitative evidence for intracranial pressure as a downstream surrogate measure [30], but intracranial pressure is not equivalent to tissue pressure within the sheath. Human quantitative CSF-ISF studies suggest that clearance pathways themselves have a multipath and recirculating nature [8], and that any single pressure or flow indicator is insufficient to characterize the net effect of the cervical outlet bottleneck.

7.3. Multiple Possible Causal Directions

A bidirectional relationship may exist between fascial fibrosis and sympathetic activation or low-grade inflammation. Internal jugular vein compression has at least three sources: fascial-sheath mechanical transmission, anterior displacement of bone-ligament structures [11], and vagal involvement and altered cerebral venous outflow associated with CCI and AAI [12]. These three sources share the same neurovascular endpoint—internal jugular vein and vagus nerve compression—but differ in upstream mechanisms and intervention targets. Most existing studies have not measured all three sources simultaneously within the same design. Extrapolation of existing evidence is jointly constrained by boundaries of timescale, anatomical level, species and model systems, measurement, intervention premise, and anatomical attribution. With respect to timescale, anatomical level, and species, acute animal ligation [30,31,56,57] cannot be directly extrapolated to chronic partial mechanical constraint. The mechanical plasticity of superficial scLNs [28] cannot automatically be extrapolated to dcLNs and the carotid sheath [41]. A sheath-wall pathological involvement rate of 0.8% [10] is not equivalent to mechanical transmission efficiency. With respect to measurement, phenotype, and intervention premise, DTI-ALPS [61,62] is not equivalent to glymphatic flow. The dcLN ultrasound phenotype is not equivalent to a confirmed pathological diagnosis [63,64,65]. Signals from single-arm dcLVA studies [45,46,47,48] are not equivalent to validation of the pressure-gradient premise [50]. With respect to molecular translation and anatomical attribution, the valve evidence of Choi et al. [21] and the inflammatory lymphedema evidence of Schulz et al. [22] are not equivalent to mechanotransduction in intrasheath lymph nodes, and their translational hierarchy still requires independent testing. Dreha-Kulaczewski et al. [44] measured CSF flow, which is not equivalent to cervical lymphatic drainage rate. Clapham et al. [18] conducted a small-sample cadaveric dissection, which is not equivalent to in vivo function or mechanical state. Evidence on competing upstream structures [11,12,13] may be highly collinear and must be addressed a priori in the statistical design.

7.4. Methodological Limitations of In Vivo Assessment of the Human Glymphatic System

Surrogate indicators such as the DTI-ALPS index provide group-level association signals [61,62], but their specificity for glymphatic flow remains debated [60]. The ALPS reduction observed by Yang et al. [59] in patients with chronic neck and shoulder pain is likewise subject to this measurement limitation. The vasomotor regulatory mechanism of lymphatic-glymphatic coupling has been established in experimental models [6]. External mechanical intervention [28] and pharmacological restoration of intrinsic lymphatic pump function [24] have also been demonstrated in animal experiments, and FSI models provide a mechanical explanation for lymphatic pump function [42]. However, how these mechanisms operate under chronic cervical mechanical loading in humans remains unclear. Human intrathecal tracer MRI [7], cross-species meningeal lymphatic anatomical studies [36], studies of the human ventral outflow region [37], and nasopharyngeal lymphatic plexus hub studies [38] reinforce the existence of these pathways and their parallel anatomy. However, they cannot replace quantitative human studies of the pathway from cervical mechanical constraint to clearance efficiency. Neither quantitative human CSF-ISF exchange studies [8] nor studies demonstrating kinetic separation of upstream pathways [40] measured cervical outlet resistance or mechanics within the sheath. Human evidence that respiration drives CSF flow [44] and TRPV4 evidence for lymphatic vessel contraction [22] reinforce the periodic extrinsic force layer and the cellular transduction layer, respectively, but neither measured the cervical outlet bottleneck directly. The functional separation of superficial and deep cervical lymphatic pathways [41] suggests that the principles of superficial mechanical intervention cannot simply be extrapolated to the deep carotid sheath region. Systematic reviews indicate that evidence for deep cervical lymphatic surgery consists mainly of case series and single-arm studies, and that prospective trials are critically needed [55].

7.5. Limitations of Evidence on the Immunological Dimension, Aging Axis, Pathological Localization, and Imaging Assessment

Evidence in this field is constrained by three types of boundaries: review-level or small-sample evidence cannot support causal inference; single-arm intervention signals cannot replace controlled trials; and acute animal models cannot be extrapolated to chronic partial constraint in humans. By type of evidence, review-level or small-sample studies of the immunological dimension of dcLNs and aging-related lymphatic decline cannot support causal inference [23,33]. Direct human or animal experimental evidence is still lacking for the specific pathway from investing fascia mechanical exposure to altered immune output of dcLNs. Studies of transport direction and pathological localization are mostly small-sample, cross-sectional, or cadaveric studies [7,17,18,25], among others). Studies of intervention signals are mostly single-arm, small-sample, or studies with short follow-up [45,46,47], among others). Imaging assessment studies are mostly cross-sectional and have moderate discriminative ability [59,62,63], among others). Animal and histological studies constitute species and extrapolation boundaries [15,26,39], among others). Choi et al. [21] used mouse and in vitro LEC models, and the gap between their valve context and cervical lymphatic structures within the sheath represents the core boundary of this evidence within the framework of this article. Schulz et al. [22] used inflammatory lymphedema models and clinical samples, and a similar translational hierarchy exists between their context and cervical mechanical constraint within the sheath. Studies on competing upstream structures and integrative research are mainly retrospective, heterogeneous, or cross-sectional [11,12,13], among others).

8. Conclusions

The core contribution of this hypothesis lies not in providing a new causal assertion for any particular disease, but in reframing the question at a different level. It asks not why intracerebral clearance fails, but where and through which mechanical conditions the intracerebral clearance chain is systematically narrowed. The carotid sheath, as the conditional shared mechanical bottleneck of the lymphatic/glymphatic serial clearance chain at the cervical level, brings fascial state and central clearance function into the same testable framework.

Human histological findings revealing the spectrum of variation in sheath-wall layering and fusion relationships [9] require that this framework be expressed in terms of conditional, pathway-dependent mechanical coupling rather than homogeneous continuum transmission. The extremely low rate of pathological involvement of the carotid sheath itself in neck dissection [10] further refines the pathological carrier of the bottleneck to lymphatic structures within the carotid sheath. The direct localization of intrasheath lymph nodes by human cadaveric dissection [18] provides specific proportional and dimensional anchors for this anatomical carrier.

The parallel upstream contributions of bone-ligament structural displacement and CCI and AAI [11,12], together with the population association between spinal alignment and cognition [13], require that this framework retain competing explanations. The jugular vein ligation experiments of El Kamouh et al. [31] suggest that the consequences of the venous subcircuit may be mediated by meningeal lymphatic remodeling. Liao et al. [16] suggest that the acute effect of postural load on fascial stiffness is measurable in humans, but a substantial gap remains between acute postural effects and the functional consequences of chronic fibrosis.

The Piezo1 evidence of Choi et al. [21] and the TRPV4 evidence of Schulz et al. [22] provide candidate transduction mechanisms for the pathway from mechanics to lymphatic endothelium in the contexts of valve development and mature lymphatic vessel contraction and inflammatory remodeling, respectively. However, the gap between these mechanisms and intrasheath lymph nodes reflects the fact that the core chain has not yet been closed. The real-time MRI evidence of Dreha-Kulaczewski et al. [44] provides a human anchor for respiration as a major regulator of CSF flow, but it did not measure cervical lymphatic drainage rate and therefore cannot replace direct studies of the cervical outlet itself. The anatomical localization by Clapham et al. [18] reinforces where the bottleneck carrier is, but it does not answer how that carrier becomes functionally compromised under mechanical constraint.

The distribution of supporting evidence can be summarized as broad peripheral support but a core chain that remains unproven. The strongly supported evidence falls into four categories. Pathway and direction: pathway existence and functional necessity have been independently validated [1,4,7]; transport direction has been supported by a multi-method human evidence chain [7,17,25,29]; the conceptual status of the glymphatic system as a fundamental neurobiological component has been substantiated by review-level synthesis [2,3]; and the anatomical localization of intrasheath lymph nodes has been quantitatively supported by human cadaveric dissection [18]. Outlet manipulation: human clues exist in both the destruction and reconstruction directions [10,45,46,47,48,51,52,53,54], but the levels of evidence differ; venous-side outlet manipulation leading to meningeal lymphatic remodeling and subsequently to decreased brain parenchymal glymphatic exchange has been supported by animal causal manipulation [31]. Aging and parallel pathways: aging as a systemic axis of vulnerability has been confirmed by systematic reviews [23]; upstream parallel pathways have multiple anatomical dimensions [26,36,37,38,39], and human in vivo MRI shows that they are kinetically separable [40]; the population-level association between postural exposure and cognition is supported by a cross-sectional reference baseline [13]; the multisystem coexistence pattern has been repeatedly observed in two independent case series [11,32]; and the meningeal lymphatic-microglial-IL-6 axis provides a mechanistic dimension for downstream neuroimmune consequences [35]. Methodological anchors: tissue-specific quantification of fascial stiffness is supported by a methodological anchor [19]; the acute effects of postural load have been supported by human in vivo evidence and reliability estimates [16,20]; quantitative assessment of CSF-ISF exchange in humans provides a human physiological anchor for clearance pathway redistribution [8]; evidence that Piezo1 is a mechanosensor in lymphatic endothelium [21] and evidence for TRPV4 in mature lymphatic vessel contraction [22] provide two candidate molecular interfaces for the cellular transduction layer; and real-time MRI evidence that respiration is the major driver of CSF flow [44] provides a human anchor for the role of periodic extrinsic forces.

The moderately supported but clearly bounded evidence layer falls into four groups. Outlet manipulation boundary: signals from single-arm dcLVA studies [45,46,47,48] are subject to fundamental challenges from pressure physiology [50]; the net effect of chronic outlet constraint may include compensatory enhancement of clearance rather than simple decline [34,58], requiring revision of the linear inference from outlet constraint to clearance decline toward a net balance of clearance pathway redistribution, intracranial pressure, and inflammatory burden. Anatomical and hierarchical boundary: superficial and deep cervical lymphatic pathways are functionally separable and differ in driving mechanisms [41]; the rate of pathological involvement of the carotid sheath itself in neck dissection is only 0.8% [10]; the human common sheath does not constantly fuse with the investing fascia or pretracheal fascia [9]; the nasopharyngeal lymphatic plexus serves as the major hub for CSF entry into dcLNs [38]; and the anatomical localization of intrasheath lymph nodes has been supported by cadaveric dissection, but their functional and mechanical states still await in vivo measurement [18]. Compensation and net-effect boundary: this category includes evidence from in vivo gadolinium-enhanced MRI studies of acute dcLN ligation [57]; bone-ligament structural displacement as a competing upstream pathway [11] and its association with autonomic dysfunction in CCI and AAI [12]; the population association between spinal alignment and cognition [13]; human cadaveric dissection and ICG lymphangiography [29]; meningeal lymphatic remodeling induced by jugular vein ligation [31]; the natural history of APP/PS1 mice [27]; age-stratified compensation studies [34]; FSI models [42]; and Urbach et al. [40]. Assessment tool boundary: this category includes reliability evidence from Liao et al. [16] and Zandi et al. [20]; methodological debate regarding the dcLN ultrasound phenotype [64,65]; and the meningeal lymphatic-microglial axis [35].

The weakest evidence layer is the core link in the hypothesis chain—from investing fascia fibrosis to altered mechanical state within the carotid sheath to altered lymphatic and glymphatic clearance function—which still lacks synchronized measurement in humans. No study has yet simultaneously measured investing fascia elasticity, mechanical state of the carotid sheath, and clearance function markers in the same individual while also controlling for cervical bone-ligament structural parameters and spinal alignment parameters. Although SWE can quantify cervical fascial stiffness [16,19], the mediating relationship between stiffness and function has not been tested. Although DTI-ALPS can serve as a longitudinal mediator endpoint [61,62], no published human data have linked this index to fascial mechanics. The translational hierarchy between the candidate molecular evidence for Piezo1 [21] and TRPV4 [22] and intrasheath lymph nodes has not been measured. Although human evidence that respiration drives CSF flow [44] reinforces the periodic extrinsic force layer, it did not measure cervical lymphatic drainage rate and therefore cannot replace direct validation of the cervical outlet. Although Clapham et al. [18] reinforced the anatomical localization of intrasheath lymph nodes, their study was a small-sample cadaveric investigation and cannot replace in vivo functional and mechanical measurements.

Therefore, this hypothesis should be understood as follows: evidence at peripheral nodes is sufficient to support the plausibility assessment that the cervical outlet constitutes a functional constraint, but human causal evidence for the core mechanical chain remains lacking. The gap that future research should prioritize filling is a longitudinal mediation study that simultaneously measures fascial elasticity, cervical bone-ligament structural parameters, spinal alignment parameters, mechanics within the carotid sheath, and clearance function within the same individual, and that uses a multi-exposure mediation model to distinguish the fascia-specific contribution, the bone-ligament contribution, and the global postural load contribution. The sharpest testable proposition advanced by this framework is: within the same individual, does a decline in investing fascia elasticity precede or accompany changes in the mechanical state of the carotid sheath and mediate altered lymphatic and glymphatic clearance function (reduction or pathological redistribution), and does it remain statistically independent of cervical bone-ligament structural parameters and spinal alignment parameters?

Answering this question requires incorporating fascial elasticity imaging, surrogate measures of mechanics within the carotid sheath, cervical structural parameters, spinal alignment parameters, and clearance function markers into the same longitudinal design. SWE has provided the technical prerequisite for tissue-specific quantification of fascial stiffness [19]; the postural manipulation protocol used by Liao et al. [16] provides a referable operational approach for detecting the acute effect of posture on fascial stiffness, and Zandi et al. [20] provide additional reliability reference data in a clinical population. However, the mediating relationship between stiffness and function, as well as the compensatory net effect that may arise under chronic outlet constraint [34,58], both require the endpoint design to incorporate clearance efficiency, intracranial pressure, inflammatory markers, and neuroimmune markers [35] simultaneously, rather than relying solely on a single clearance measure as the outcome. Quantitative human CSF-ISF assessment [8] suggests that indicators of pathway redistribution should be measured in parallel with clearance volume indicators.

The functional separation of superficial and deep cervical lymphatic pathways [41] requires that exposure measurement and outcome measurement be strictly confined to the same anatomical level; the localization of the nasopharyngeal lymphatic plexus as a hub [38] requires that this intermediate segment be included in the assessment of upstream pathway reserve. In vivo imaging evidence from acute dcLN ligation [57] suggests that tracer kinetic parameters after outlet blockade can serve as a candidate intermediate phenotype bridging the anatomical outlet and clearance outcome; venous-side pressure and meningeal lymphatic vessel morphology [31] can serve as candidate mediator variables for the venous subcircuit; and bone-ligament structural parameters, spinal alignment parameters, and autonomic function measures [11,12,13] should be included as parallel exposures. Level II dcLNs can serve as an anatomical target for lymphatic drainage of brain-derived substances [29]; the cadaveric anatomical localization of intrasheath lymph nodes [18] can serve as a static anchor for the anatomical outlet, but it must be combined with in vivo functional measurements; and post-dcLVA lymphatic drainage kinetics [49] can serve as an intermediate endpoint for the intervention mechanism. Markers related to the meningeal lymphatic-microglial-IL-6 axis [35] can serve as candidate endpoints for downstream neuroimmune burden; the natural history of APP/PS1 mice [27] suggests that intervention timing should be treated as a stratification variable; and age-stratified compensation studies [34] suggest that age-related compensatory capacity should be included as a stratification variable for the net effect of outlet constraint. FSI models [42] suggest that if in vivo lymphatic mechanical measurements become available in the future, lymphatic pump function can be included as a candidate mediator variable; and Urbach et al. [40] suggest that dynamic separation of upstream parallel entry routes can serve as an upstream stratification variable in pathway redistribution assessment. Piezo1- or TRPV4-related markers [21,22] can serve as a candidate molecular mediator layer, but the two have different contexts, and their translational hierarchy relative to intrasheath lymph nodes requires independent testing. Respiratory parameters [44] can serve as candidate covariates for periodic extrinsic forces, but because cervical lymphatic drainage rate was not measured in that study, the current recommendation is to record only pulsatile and oscillatory components.

The key statistical criterion is whether, after adjustment for cervical structural parameters and spinal alignment parameters, the mediating effect of fascial elasticity on clearance function remains significant. If significant, this would support fascial mechanics as an independent contributor; if not significant, this would suggest that the observed association is mainly explained by competing upstream factors. Until then, this hypothesis should be understood as a structured, prioritized research agenda with clearly defined gaps, rather than an established causal model.