The UWF-OCT images enabled us to examine the posterior vitreous in greater detail in children as well as young adults. Earlier studies reported that both conventional spectral domain OCT or SS OCT can obtain images of the vitreous but the scanned area was limited to the macular area. To overcome this limitation, the imaging fields of OCT devices have been extended.13 In addition, it has been difficult to obtain images of the vitreous in the area outside the macula especially in children because children cannot maintain a steady fixation which is required during the examinations. UWF-OCT can obtain vitreal images that are 23 (H) x 20 (V) mm in just 4 seconds which has been very helpful in examining children.
Among the eyes of children ages 6 to 12 years, partial PVDs were seen in 7% of the HM eyes but none in the non-HM eyes. Previous studies showed that PVDs developed earlier in HM than non-HM young adults.1–4 Our results showed that the PVD might start to develop much earlier in HM eyes than reported and even during childhood.
Of the 15 HM eyes with a partial PVD, 10 eyes had an asymmetrical partial PVD. The asymmetrical PVDs developed in the vertical direction in all of the 10 eyes, and 6 eyes had a superior partial PVD and the other 4 eyes had an inferior partial PVD. Tsukahara et al. investigated the posterior vitreous of 98 healthy normal adults and reported that a PVD began at the superior quadrant more frequently.7 However, it has not been determined how and why PVDs begin at the superior quadrant. We recently reported that fluid cisternae were commonly seen within the vitreous gel that extended towards the periphery in the superior quadrant in the three-dimensional OCT images obtained by AI-applied segmentation (Ohno-Matsui et al. in submission). Thus, the decrease of the density of the superior vitreous gel might make the vitreous more fragile structurally and may induce earlier PVDs in these locations. Takahashi et al. examined 167 eyes of HM patients whose ages were > 50 years and reported asymmetrical PVDs were commonly seen in HM patients.8 Different from the results in the current study, Takahashi et al. showed that asymmetrical PVDs were seen commonly either at the inferior quadrant or at the area where the sclera protruded most posteriorly, usually the temporal quadrant, in HM eyes of older patients.8 These findings suggested that although asymmetrical PVDs may be common in HM patients in general, they appeared to be distributed differently according to age, and different factors may be involved in the pathogenesis of partial PVDs. Scleral deformities such as posterior staphylomas increase during adulthood in HM patients, and the progression of PVDs might differ between young and old HM patients.
Tissues bridging the posterior surface of vitreous and inner retinal surface were seen in 4 HM eyes. Mojana et al. reported similar bridging tissues connecting the posterior vitreous surface and the retinal vessels during the development of PVDs in adult eyes, however the relationship between HM and bridging tissue was not analyzed.14 At the site of the adhesions, the reflectivity of the vitreous cortex become brighter than the other area, and lamellar structures of vitreous cortex appeared to merge as a thick vitreous membrane (Fig. 5). Thick posterior vitreous membranes in young HM patients might induce light scattering and appear as floaters in these HM young patients as is often seen in eyes without a complete PVD.
Large hyporeflective cystic lesions were seen next to the vessels in the inner retinal layer at the sites of the paravascular vitreal adhesions (Figs. 4 and 5). These findings suggested that abnormal paravascular vitreal adhesions may cause traction on the retinal vessels as the partial PVD progressed which then forms the paravascular abnormalities. In addition, granular tissues were present between the detached vitreous and the inner retinal surface (Figs. 4 and 5). We suggest that such granular tissues might be the migration of deteriorated perivascular cells, such as glia cells, into the vitreous cortex during the progress of PVD. A migration of paravascular glial cells into the vitreous of HM children or young adults may cause pathological and proliferative changes in vitreous.
Our results showed that 7% of HM children, 18% of HM adolescents, and 38% of HM young adults had partial PVD, and the differences between the age groups were significant in the HM patients. Among the paravacular retinal abnormalities, the incidence of vascular microfolds was also significantly correlated with the age of the patients, but the incidence of paravascular RS, paravascular retinal cysts, and paravascular lamellar holes were not. Vascular microfolds were reported to be present in vitrectomized and non-vitrectomized eyes of older HM adults in previous studies.15,16 Accordingly, we suggest that earlier PVD formation might lead to traction on the inner retinal surface and result in the development of microvascular folds in young HM patients. The results of this study showed that vascular microfolds occurred at a much younger age than reported.
The exact cause of the paravascular retinal abnormalities has not been determined. Recent histopathological study showed that retinal cell debris was adherent to surgically removed internal limiting membranes (ILMs) in HM patients but not in non-HM patients.17 Thus, pathological adhesions between the vitreous cortex and the ILM and inner retinal layer may play a role in the development of paravascular retinal abnormalities and vitreal changes.
The incidence of myopic RS has not been investigated in detail either at the macular area or paravascular area in young HM patients. The results of this study showed that myopic RS was found in 3 young HM eyes (1 child and 2 young adults), and the myopic RSs were paravascular RS and not macular RS. In addition, paravascular retinal cysts and vascular microfolds were seen more commonly in eyes with paravascular RS than those without. Our recent investigation using UWF-OCT also showed the relationship between myopic macular RS, paravascular vitreal adhesions and paravascular retinal cysts in adult HM patients.18 Accordingly, it is possible that HM eyes with paravascular retinal cysts may be predisposed to paravascular RS and that the paravascular RS in young HM patients expands toward the macular area during aging and myopia progression. We conclude that the UWF-OCT is powerful tool for detection and following of paravascular RS in young HM patients by being able to examine a wide area of the fundus simultaneously.
How and what kinds of paravascular changes occur between the vitreous and retina in young HM eyes are shown in Fig. 7. PVDs develop in accordance with irregular vitreoretinal adhesion at the site of retinal vessels. Paravascular retinal tissues are pulled anteriorly and the dislocated perivascular cells are forced into lamellar vitreal cortex layers to merge into a single thick vitreous membrane. On the other hand, broad paravascular vitreal adhesions occasionally induce paravascular RS. Paravascular RS extend toward macular area and eventually develops macular RS.
This study has several limitations. First, our study did not have a population-based recruitment of its participants. Thus, it is not clear whether the results can be directly transferred to other HM patients in the general population. Second, this was not a longitudinal study, thus, it is not clear whether the paravascular RS actually progresses to macular RS with time. There were not many eyes with bridging tissues and paravascular RS, and we do not have strong evidence about high myopia and paravascular abnormalities in young HM patients. The progression of the vitreous cortex thickening and paravascular RS are assumed based on our findings. Third, comparisons of UWF OCT findings between HM and non-HM eyes have not been performed in adolescents and young adults. Then, earlier PVDs in HM adolescents and young adults were not examined in this study. However, we have shown earlier PVDs in patients who are > 50-years-old age using UWF OCT. Thus, we believe HM eyes develop a PVD earlier than non-HM eyes throughout adulthood.
In conclusion, posterior vitreous detachments occur in HM children but not in non-HM children, and the incidence increases with aging. Early partial PVD may play a role in the thickening of the vitreous cortex. An intense vitreoretinal traction with bridging tissues connecting the posterior vitreous and retinal vessels may cause various paravascular abnormalities accompanied by the migration of damaged tissues of the paravascular changes. These may initiate the pathological and proliferative vitreous changes in HM eyes. Paravascular RS is already present at an early age which may progress to macular RS with aging.