Peripapillary Intrachoroidal Cavitation

1. Introduction

The current epidemic of myopia will probably be followed by that of high myopia and its complications of which peripapillary intrachoroidal cavitation (PICC).

PICC is a well-circumscribed yellow-orange lobular lesion, located at the outer border of the myopic conus [1] (Figure 1A). Advances in optical coherence tomography (OCT) have outlined that PICC is a hyporeflective intrachoroidal thickening [2] below the preserved plane of the Bruch’s membrane [3] (Figure 1C). It appears as a neural-based triangular choroidal thickening in OCT sections crossing the optic nerve (ON) head [3,4] (Figure 1D).

A discontinuity of the choroidal border tissue is often associated with PICC [3] (Figure 1E and 1F).

Visual field defects (VF) are reported in up to 73.3% of PICCs [5]. These VF deficits are similar to those observed in glaucoma [5], constituting a cause of diagnostic uncertainty. Additionally, macular detachment with or without retinoschisis can complicate the prognosis of PICC [6,7,8,9,10,11]. Knowledge of the PICC allows to avoid misdiagnosis as a metastatic choroidal tumor which can lead to unnecessary and anxiety-provoking investigations [1].

PICC is related to other myopic complications namely posterior staphyloma and myopic tilted disc [4]. It is more common in eyes with higher maculopathy category [12].

While the pathogenesis of PICC has been hypothesized for years, recent findings in biomechanics of the ON have reopened the debate. Indeed, several methods have demonstrated high traction forces exerted by the ON sheaths on the peripapillary region of myopic eyes during eye movements [13,14,15]. These biomechanics findings renew interest in the pathogenesis of peripapillary changes in myopic eyes because these tensile forces have been suggested as promoters of PICC [16]. In this context, we performed a thorough review of PICC with an emphasis on pathophysiology with OCT documentation and potential future perspectives.

2. Epidemiology

PICC is found in 5-17% of highly myopic eyes [4,17]. Variability in study populations and study designs interferes with the prevalence range of the PICC [4,17,18].

Shimada et al. first identified the PICC in the fundus and then performed OCT imaging [17]. However, more recent studies revealed that only 46.7% to 53% of PICCs documented on OCT are detected on the fundus [4,19,20]. In addition, they looked for PICCs in the area below the disc and would have missed restricted PICCs in other peripapillary portions [17]. These two aspects may explain the low prevalence of PICC in this series [17].

Some studies set the minimum age for inclusion at 50 years [4] while others also recruited younger subjects [17,21,22]. Knowing that the prevalence of PICC takes off around the age of 30 [17,19], the inclusion of subjects aged at least 50 years [4] focuses on those most likely to have a PICC, thus explaining the higher prevalence of PICC in these cases.

In their study, Choudhury et al. the prevalence of PICC was 2.2% in the overall myopic group and 22% highly myopic eyes [18].

The average age at diagnosis of PICC is around 50 years [1,2,3,5,17,21,22,23,24], ranging between 19 and 79 years [19,25]. Shimada et al. showed that the prevalence of PICC takes off at age 30 and is maintained over 70 years [17].

There is no gender predilection [1,17,22] and PICC can be unilateral (53% subjects in one series) or bilateral [1,3,19,22,23].

PICC is more common in highly myopic eyes. The reported range for AL is 25 to 32 mm [1,20] and that for refractive error is -6 D to -23 D [1,21]. However, PICC is also found in non-highly myopic eyes and even in non-myopic eyes [19,21,22]. Patients with PICC and low myopia or non-myopia were significantly older than those with high myopia, highlighting the combined role of structural weakening and time [19].

The lesion is mainly located below the ON [1,2,3,5,12,17,19,21,22,24]. However, other peripapillary areas may also be involved [4,10,12,17,22,24], and the PICC may even surround the entire ON head [17,19]. Therefore, Shimada subdivided it into three grades based to its overall circumference around the papilla [17].

Uncomplicated cases of PICC do not induce loss of best corrected visual acuity, except for that related to other myopic complications [1,23]. However, visual field defects are reported in 66% [23] to 73.3% of PICCs [5] and macular detachment can complicate a PICC [6,7,8,9,10,11].

Although there is no known association of PICC with intraocular pressure or with any general condition [4], an anecdotic case of acquired PICC secondary to intercalary membrane detachment was reported in an eye with coloboma [26].

3. Clinical investigations and diagnosis of PICC

3.1. Fundoscopy and OCT

The yellow orange ophthalmoscopic appearance of PICC [1,3,4,17,19] (Figure 1A) is obvious only in 46.7% to 53% of OCT-detected PICCs [4,19,20]. Therefore, OCT is the recommended tool for the screening of PICC.

By using spectral domain OCT or swept source OCT, the diagnostic features of PICC are currently well established as mentioned above [2,3,21,22,27], thus obviating the need to use other more invasive modalities such as fluorescein or indocyanine green angiography whose characteristics are summarized below.

3.2. Fluorescein angiography

The sequence of fluorescein angiography shows early hypo-fluorescence followed by late hyper-fluorescence without dye pooling in the area of PICC [1,2,17,22,23,28,29]. This angiographic sequence may be explained by structural choroidal changes. The early phase (hypo-fluorescence) results from disorganization, thinning, and loss of normal choroidal architecture while the late phase (hyper-fluorescence) without dye pooling is related to the scleral impregnation by the dye, visible through the disorganized choroid [29].

3.3. Indocyanine green angiography

The area of PICC shows hypofluorescence over the entire sequence of indocyanine green angiography [2,3,17,23]. This indicates slow or absent choroidal flow.

3.4. OCT-Angiography

Analysis of highly myopic eyes showed that the vessel density of the radial peripapillary capillary network [30,31] (Figure 2B) and that of the ON head layers [30] were significantly reduced in eyes with than in those without PICC. Highly myopic and low myopic eyes also showed a reduced vessel density in comparison to non-myopic eyes [30].

In a study of 47 glaucomatous eyes with PICC, Kim et al., using En-face OCT images at the choroidal layer, describe two main vascular features. First, a well demarcated homogeneous area of vessel density reduction matching the location of PICC [24] (Figure 2C). Second, in 89.4% of cases, a choroidal microvasculature dropout (i.e., a focal sector with no visible choroidal and choriocapillaris network). These similar patterns were previously reported in two case-reports of non-glaucomatous PICC [32,33]. Using B scans of the OCT-A, Kim et al. showed that in the peripapillary area, the cavity inside the PICC maintains the choriocapillary signal against the posterior surface of the Bruch’s membrane whereas it shows no signal in the intrachoroidal cavity [24] (Figure 3B2 and D2).

3.5. Other modalities

Multimodal imaging [29,34], three-dimensional reconstruction imaging [35,36] and deep learning [36] have also been used to analyze PICC, providing insights into its understanding.

4. Association of PICC with other Myopia-related changes

Although the pathophysiologic mechanisms that underpin its OCT features remain to be understood, PICC is linked to several myopic changes. It is consistently located at the outer margin of the myopic conus [1,2,3,5,17]. It is correlated with myopic tilted disc and posterior staphyloma [4]. All cases of Freund’s series exhibited fundus complications of severe myopia [1].

4.1. Gamma peripapillary atrophy

PICC is always found at the outer margin of the conus [1,2,3,5,17,21]. Recent studies have shown that this region also called γ-peripapillary atrophy (γPPA) is acquired [37,38], and results from stresses applied locally during myopic elongation of the eye [13,15,38]. It extends from the opening of the scleral canal to the edge of Bruch’s membrane. The disappearance of Bruch’s membrane in this area is confirmed by histological studies [39] and visible on OCT [40].

4.2. Tilted disc

Dai et al., by intraindividual inter-eye comparison of patients having unilateral PICC, showed that the eye with PICC was more spindle-like because of a pronounced tilt [20].

The tilted disc is reported in 75% to 93.5% of cases of PICC, depending on the series [1,17]. A more recent study showed a prevalence of 100% [3].

A quantitative approach to estimate the magnitude of a disc tilt would be useful in assessing the true prevalence of tilted disc in the PICC [4,20,41].

4.3. Posterior staphyloma

Posterior staphyloma is reported in 40.2% to 100% of PICC [19,42]. This discrepancy in prevalence may result from the study design and the diagnostic tools used [21,43]. Recently, OCT has shown its sensitivity to detect inconspicuous cases while also revealing the outermost border in case of very wide types of posterior staphylomas. It is therefore recommended for diagnosing posterior staphyloma [43,44].

Since both tilted disc and posterior staphyloma are myopia-related changes [1,4,17], it is not surprising to find a higher prevalence of PICC in the myopic group.

4.4. Others

Myopia-related macular changes were found in 14% to 100% of eyes with PICC [1,19]. These include myopic macular degeneration, myopic choroidal neovascularization, foveoschisis, lacquer crack, patchy atrophy, retinal holes, macular puckering [1,17,19,22,23].

5. Structural changes in the vicinity of the PICC

5.1. Peripapillary atrophy

As mentioned above, each individual PICC is located at the outer margin of γPPA. The area of γPPA exhibits a posterior deformation of the sclera. Recent studies have suggested that the stress exerted in the peripapillary region during adduction may promote the emergence of γPPA. This stress is more marked at the location of γPPA [45].

5.2. Choroid

Intrachoroidal hyporeflective cysts and hyporeflective hollows, disclosed by OCT, witness that in addition to choroidal thickening, PICC exhibits other structural changes inside the choroid [3,19,21,22,27]. These hyporeflective intrachoroidal cysts (Figure 4B) were found in 19% to 39% of eyes with PICC [19,22]. Wei et al. described them as choroidal splitting or schisis adjacent to pocket-like spaces [22], while Yeh et al. reported intrachoroidal schisis characterized by intracavitary cleavage bands and fluidlike images [19].

Additionnaly, areas adjacent to intrachoroidal cysts show abnormal patterns of choroidal vessel [3]. Changes in the choroidal vascular signal are discussed in the OCT-A section above.

Considering the intrachoroidal hyper-reflective line located in front of the hypo-reflective space and separating this space from what seems to be the residual initial choroid, Spaide et al. suggested that PICC is a suprachoroidal detachment [3]. This hypothesis is supported by a more recent study in which a suprachoroidal detachment was identified exclusively in cases of PICC [16].

Finally, Ehongo et al. studied the configuration of the posterior curvature of the choroid from the peripapillary polar regions to the opening of Bruch’s membrane using OCT. They highlighted a peculiar sequence of choroidal deformity associated with PICC, suggesting the role of mechanical forces in its pathogenesis. Especially, they described the presence of a posterior wedge deformity of the choroidal wall on the γPPA side implying the existence of cross-forces at the level of the polar peripapillary regions [16].

5.3. Posterior scleral curvature

The sclera is deformed backwards in the PICC [3,17,21]. Spaide et al. have suggested that the choroidal thickening presented by PICC results from a posterior excursion of the sclera while the profile of the pigment epithelium remains preserved [3].

A defect in the deep scleral layer allowing the exit of the inferior temporal vein into the extrascleral space has been reported in a case in addition to an unusual anatomy of the parapapillary region [46].

5.4. Border tissue of the choroid

The border tissue is a fibro-astrocytic differentiation separating the nerve fibers from the surrounding structures in the neural canal: the choroid and the sclera. It extends from the sclera at the level of lamina cribrosa to the Bruch’s membrane [47,48,49]. It is divided in two parts: the border tissue of choroid (Elschnig) at the level of choroid and the border tissue of sclera (Jacoby) lining the sclera [47]. As the myopic conus extends during the myopic lengthening of the eye, the border tissue of choroid stretches [39]. A discontinuity in this structure has been disclosed using OCT [1,3,17,19,21,22,23] (Figure 1E-1F).

This discontinuity was first described as a cleft in the junction between the conus and the edge of the PICC in several studies since Freund’s paper [1,3,17,19,21,22,23] (Figure 4C). Its prevalence varies from 10% to 46.2% [1,22]. Through this full thickness defect that allows communication between the PICC and vitreous cavity, nerve fibers (Figure 3B1 and Figure 4C) and retinal vessels (Figure 4E) have been shown to herniate into the PICC [1,3,17,25].

Discontinuity of the border tissue of the choroid would be caused by mechanical stresses exceeding its resistance and leading to its rupture. Several mechanisms have been mentioned; Toranzo et al. discuss stress resulting from the posterior progression of peripapillary staphyloma [2]. Spaide et al. mention the stress related to the posterior deformation of the sclera within the thinned and fragile structures of the myopic conus [3]. Dai et al. suggest mechanical forces caused by papillary tilting [20]. Ehongo et al. discuss the mechanical stress induced by the traction of the ON meninges during adduction [16].

Recent versions of OCT allow to refine the diagnosis of this discontinuity by detecting it at the stage of a simple interruption of the hyper-reflective line that characterizes choroidal border tissue in up to 25% of PICCs [3] (Figure 5).

5.5. Vessels

There is often a marked bending of the inferotemporal retinal vein (Figure 4E) into the steep excavation exhibited by the PICC at its junction with the conus [3,17,19,23]. This vessel sometimes disappears on part of its course in some PICCs with deep and steep excavations [3,17,19].

OCT-A features of PICC have been discussed above (section clinical investigation): a reduced vessel density in En-face OCT-A at the choroidal level [24,32,33]. A reduction in the density of vessel of radial peripapillary capillary plexus is also present [30] as well as that of the ON head layer [30]. Finally, the intrachoroidal cavity shows no vessel signal in the B scans of OCT-A [24] (Figure 3).

An increase in visibility of peripapillary intrascleral vessels probably related to their dilation and scleral thinning has recently been disclosed in the vicinity of PICC [48]. Further studies are warranted to clarify this finding.

6. Clinical relevance of the PICC

Although the PICC has been considered a benign entity, it nevertheless deserves special attention. A recent series showed it to be more common in the eyes of higher categories of myopic maculopathy [12]. Additionally, visual field defects [5,23] and macular lesions (retinoschisis, macular detachment) can complicate the presence of a PICC [6,7,8,9,10,11].

6.1. Visual field defects and PICC

The main clinical significance of PICC is visual field defects which are reported [5,17,19,21,22,23,36,51] with a prevalence ranging from 37.5% to 73.3 % [5,22]. They mimic glaucomatous visual field defects, hence the concern about them [5,17].

A correlation has been found between the distribution of visual field defects and the location of the PICC in some cases [5,23,36]. It has thus been suggested that full-thickness defects, thinning, or disruption of nerve fibers at the PICC-conus junction may account for some of these visual field defects [23].

Recently, Okuma et al. showed a correlation between the location of PICC and that of the reduced thickness of macular ganglion cell complex in 66.7% of cases, using OCT [5]. They also showed a correlation between the distribution of visual field defects and PICC locations in 53.3% of cases, concluding that visual field defects in PICC are similar to those in early glaucoma [5].

As a correlation between the location of the PICC and the distribution of visual field defects is not found in all cases, this suggests that some of the visual field defects found in eyes with PICC could result from myopic distortions. Supporting this hypothesis, Shimada et al. found visual field defects in 23% of myopic eyes without PICC in their series [17].

Finally, a disc haemorrhage was found in a PICC without visual field defect or reduction of the peripapillary nerve fiber layer [52]. In another case, visual field defect could not be detected, even when testing the full thickness defect using micro-perimetry [25].

6.2. Macular abnormalities and PICC

From the first descriptions of PICC, a direct communication between the PICC and vitreous cavity at the junction between the conus and PICC has been reported [1,3,17] (Figure 6). Then it was revealed that macular retinoschisis and macular detachment can complicate the presence of a PICC [6,7,8,9,10,11,12,53].

The pathophysiological mechanisms underlying macular detachment complicating a PICC involve two communications with the PICC cavity [6,7,8,10]. First a connection to the vitreous through the full-thickness defect at the junction between the PICC and the conus, allowing vitreous fluid to access to the PICC cavity. Then a second connection between the subretinal or intraretinal space and the PICC cavity (Figure 6). A narrow path connecting the PICC to the schisis has been documented in some cases [6,7,8]. It is suggested that this channel results from a rupture in the atrophic and dysplastic herniated retinal tissue, allowing vitreous fluid to track subretinally [6,7]. Vitreous traction [11] or peripapillary epiretinal membrane [8] have been implicated in some cases.

Although one case of self-resolving recurrent macular detachment and retinoschisis in one eye with PICC [53] has been reported, many cases of macular detachment with or without retinoschisis have undergone successful vitrectomy [8,10,11].

7. Differential diagnosis – Natural history of uncomplicated cases of PICC

Nowadays, the diagnosis of PICC using the new versions of OCT (spectral domain-OCT and swept source-OCT) is well defined [3], thus allowing to avoid unnecessary and more invasive and anxiety-provoking investigations.

Additionally, for physicians unaware of the condition, the sequence of fluorescein angiography helps differentiate PICC from other potentially confounding entities namely pigment epithelial detachment, peripapillary choroidal neovascularization, metastatic lesion or central serous chorioretinopathy [2,17].

OCT-A is a complementary non-invasive diagnostic tool for PICC [24,32,33,34].

Uncomplicated cases of PICC are asymptomatic [1,17] and have a good prognostic. However, other vision-threatening myopic complications may accompany PICC as mentioned above [1,22].

The aforementioned visual field defects which are correlated with PICC [17] are clinically challenging because they are of the glaucomatous type.

In general, the PICC is stable. However, Freund et al. showed in their series a case of involution of the PICC in both eyes of the same patient over a follow-up of 15 years [1]. In these eyes, the PICC became smaller as γPPA widened towards it.

A case reported by Toranzo et al. showed enlargement over 10 years [2]. Forte et al., using En-face OCT, observed PICC enlargement in one of six eyes in their series over an 18-month period [23].

Lee et al. reported an enlargement of the PICC over one year, followed by its shrinkage over another one-year period [52].

Formally, the condition being acquired and related to myopic complications, it is not surprising that it shows changes, albeit slow, over time, as the myopia progresses.

8. Pathogenetic hypotheses of PICC

So far, the pathogenesis of PICC is not established. Congenital, fluidic and mechanical hypotheses have been proposed. Similarities between PICC and the morphologic features at the border of the optic disc coloboma have been discussed [54]. All these pathogenetic hypotheses have weaknesses or shortcomings and are summarized below.

8.1. Congenital hypothesis

Some authors have suggested that PICC could be an incomplete form of choroidal coloboma because it is mainly located in the inferior peripapillary region [1].

However, PICCs can extend widely around the papilla and some PICCs are restricted to the upper part of the disc which does not support this hypothesis [17]. Moreover, PICC is noticed around the age of 30 [17], suggesting that it is an acquired condition. Finally, its correlation with myopic tilted disc, γPPA and posterior staphyloma [4] suggests that it is another myopia-related condition.

8.2. Fluidic considerations

Since the lesion is more frequent below the ON, Freund et al. hypothesized that there may be gravitational displacement of subretinal fluid from the area of the ON [1]. This fluid would come either from the vitreous or from the optic canal [1].

Yeh et al. finding that subjects with non-myopic eyes and PICC are significantly older than those with myopic eyes and PICC, suggested that with aging, the transitional weakened tissue of the conus presents impaired resorption of fluids from the subretinal space, subarachnoid space, optic canal or vitreous cavity. The progressive and asymptomatic gravitation of these fluids promotes the formation of fluid pockets at the lower edge of the conus, hence the appearance of PICC [19].

However, this gravitational hypothesis does not explain why some PICCs are confined to the upper peripapillary part [4,17,22].

Many authors have suggested that the vitreous fluid enters the PICC through the retinal defect located at the PICC-conus transition zone [17,22,23]. Supporting this hypothesis, Spaide et al. observed that PICCs with the opening against the vitreous showed more prominent posterior bowing [3], while those without this opening showed a triangular thickening of the choroid with a neural.

Wei et al. suggested that after the rupture of the border tissue, the influx of vitreous fluid into the choroid creates a schisis or fluid pocket in the choroid [22]. They suggested that schisis and PICC are different stages of the same phenomenon [22].

8.3. Mechanical considerations

Wei et al. suggested that a complex of forces combining “posterior expansion force, vitreous tractional force and vitreous fluid dynamics determine the size and shape of the PICC” [22]. However, vitreous traction has never been demonstrated in any uncomplicated case of PICC [3].

Lee et al. reported a case of PICC in a non-glaucomatous myopic eye accompanied by a large adjacent disc haemorrhage. A schisis in the prelaminar ON tissue was also noticed. At the one-year follow-up visit, the schisis and PICC had widened while the disc haemorrhage was still observed. The disc haemorrhage disappeared at 2-year follow-up, the PICC showed shrinkage while the intraneural cyst was reduced. These changes suggest the presence of peripapillary mechanical forces [52].

Dai et al. showed by intraindividual inter-eye comparison that the eye with PICC was more tilted and more rotated, also suggesting the role of mechanical factors [20].

8.3.1. PICC as a complication of peripapillary staphyloma

Toranzo et al. suggested that the increased gap between Bruch’s membrane and the scleral planes resulting from thickening of the choroid as the posterior staphyloma progresses, stretches the border tissue which eventually ruptures. Secondarily, the choroid retracts from the ON margins, leading to a PICC [2].

Wei et al. added that adhesion of the retina and retinal pigment epithelium to the margin of the conus prevents the rupture from opening into the subretinal space [22].

8.3.2. PICC as a complication of myopic tilted disc and myopic conus

As already mentioned, in the presence of γPPA (myopic conus), the sclera may be bowed posteriorly, giving rise to many suggestions.

Many authors have assumed that the posterior scleral bulge of the conus was favored by its weakening due to the absence of overlying structures [3,21]. From there, the force acting to deform the sclera depends on the author.

Shimada et al. hypothesized that during the process of γPPA extension the conus and the surrounding peripapillary area bow posteriorly. Mechanically, this induces a stretching of the tissues which then causes splitting of the neighboring intrachoroidal structures, with appearance of cysts inside the choroid. Then, the intrachoroidal cysts enlarge, coalesce, and end up in a large hyporeflective intrachoroidal space [21]. But the flaw with this hypothesis is that the phenomenon of coalescence is not instantaneous. Therefore, hyporeflective intrachoroidal cysts, as precursors of PICC should also be found in the eyes of non-PICCs, which is not described in the literature. Further studies of longitudinal design should thus focus on these intrachoroidal cysts to clarify their impact on the pathophysiology of PICC.

Spaide et al. suggested that the driving force which is intraocular pressure against the wall of the eye induces the bulging of the weakened conus [3]. These authors explain that the stress/strain relationship ends with a more scleral deformation of the conus due to the absence or thinning of the tissues covering the sclera in this area. Subsequently, the posterior bowing of the sclera would itself favor the thickening of the neighboring choroid mainly in the inferior border of the ON. Finally, the widening of the choroid at the junction of the ON ends up breaking the border tissue, the latter promoting itself and secondarily the thickening of the choroid by allowing the entry of the vitreous fluid into the PICC [3].

Since the sclera of the conus deforms backwards, Forte et al. [23] hypothesized that the overlying structures are submitted to the same trend. Unable to follow the strong posterior excursion of the sclera, they detach from choroid, thus creating the zone of cavitation inside the choroid and choroidal thickening.

8.3.3. PICC as a complication of the optic nerve sheaths traction

Toranzo et al. hypothesized that in posterior staphyloma, the choroidal border tissue ruptures when stretched by the increasing gab between the plane of the sclera and that of Bruch’s membrane [2]. The driving force that increases the gap between the sclera and the Bruch’s membrane was unknown.

Beside their main hypothesis (intraocular pressure effect), Spaide et al. also opened the door to the possibility that in the tilted disc, unknown forces may be at play to bow the sclera backwards [3].

The question that remained open for years was: why in PICC does the sclera deform posteriorly while the overlying anterior structures remain undeformed?

Recent studies on the biomechanics of the peripapillary region have shown by several methods that the ON sheaths exert strong tensile forces on the ON head and the peripapillary region [13,14,15] during eye movements. From the similarities between the tilted disc and intermittent distortions of the ON and peripapillary structures related to eye movements, some authors have suggested that tilted disc results from remodeling and permanent fixation of these repetitive deformations [15,45,55,56]. Interestingly, a case of a preoperative round disc becoming oval after trabeculectomy has been reported [57]. The eye was highly myopic and had very high preoperative intraocular pressure. It seems that this elevated pressure counteracted the ovality of the disc. Therefore, the surgical reduction of ocular pressure had promoted the return to the oval shape of the disc by altering the balance of forces acting on the ON head, suggesting that the phenomenon of tilted disc can be reversed to some extent. This constitutes an avenue to explore, because PICC and tilted disc are related.

In this regard, very recently, Ehongo et al. confirmed that PICC is a suprachoroidal detachment (Figure 7A). They suggested that it is caused by the traction exerted by the ON sheaths on their scleral insertions [16]. They observed using serial OCT sections that the beginning of the thickening of the choroid and the posterior bowing of the sclera, coincided with the detection of the scleral insertion of the dura mater (Figure 7A-F). The slope of the posterior bowing of the sclera steepened in front of the subarachnoid space, with the choroidal thickening ending in a supra-choroidal detachment (Figure 7A). They thus suggested that the posterior excursion of the sclera results from the direct posterior traction exerted by the ON sheaths on the thinned and weakened myopic sclera (Figure 8). Due to this traction, the scleral flange (the part of the sclera between the margin of the ON and the dura mater) recedes while Bruch’s membrane, due to its structural rigidity maintains its plane. This results in thickening of the choroid [16]. Further studies are warranted to confirm this hypothesis.

Again, relying on serial OCT sections, they observed that the convexity of the posterior choroidal wall is followed at its edge by an anterior elevation suggesting that ON sheaths traction force has two components [16]. The first that acts directly by protruding the sclera backwards. The second tangential component which squeezes the choroid at the edge of the scleral convexity. This choroidal sequence characterizes a peripapillary staphyloma (Figure 8). They therefore suggest that peripapillary staphyloma and γPPA, result from repetitive traction of the ON sheaths on the peripapillary sclera during eye movements [16].

9. Conclusion and perspectives

The current epidemic of myopia is paving the way to that of high myopia and its complications, in particular posterior staphyloma, the presence of which classifies an eye in the group of pathological myopia.

Peripapillary staphyloma and tilted disc are related to PICC. They have all been suggested to be promoted by the traction of the dura on its scleral attachment during eye movement. The link between these three complications of high myopia must be investigated. Biomechanical and longitudinal studies are thus warranted to clarify mechanisms leading to them.

Tractions of the ON sheath on the peripapillary region are exerted on all the eyes. On elongated myopic eyes, for obvious geometric reasons, these traction forces are more marked, promoting the occurrence of peripapillary myopic complications. However, these complications also occur, although less frequently in non-highly myopic eyes. Unknowns therefore remain as to whether the eyes undergoing these changes are more sensitive to ON sheaths traction force or whether this traction force is stronger in the eyes presenting these entities.

In particular, the influence of time and age justifies additional investigations since subjects with PICC and non-high myopia were found to be older than those with PICC and high myopia.

Visual field defects in PICC pose diagnostic difficulties with glaucoma (the prevalence of which is also increasing). Understanding the pathogenesis underpinning the occurrence of PICC and other peripapillary myopic complications will allow the development of strategies to slow or reverse their onset and associated visual field defects.

A synopsis of publications on PICC is presented in the Appendix A.