Out of 488 highly myopic eyes, which had undergone swept-source OCT examination in the study period, the entire sclera was visualized in all 12 radial foveal scans in 278 eyes (57.0%) of 175 patients.
4 The mean age of these 175 patients was 60.9 ± 11.4 years (32–89 years) and the mean axial length of the 278 eyes was 30.7 ± 1.9 mm (26.5–36.6 mm). Among the 278 eyes, the episclera and/or Tenon's capsule were detected in 164 eyes (59.0%; or 33.6% of the original 488 eyes) of 117 patients (mean age: 59.4 ± 11.4 years; mean axial length: 30.9 ± 1.8 mm). The episclera appeared as a relatively uniform structure with a reflectivity that was slightly lower than the scleral reflectivity (
Fig. 2). The episclera was closely attached to the outer surface of the sclera in most areas; although in some regions, especially in the macular area, the episclera was clearly separated from the outer surface of the sclera (
Figs. 3,
4). The latter finding suggested that the episclera represented a different tissue, distinct from the sclera proper. The border between the sclera and episclera was relatively clear in most eyes due to differences in reflectivity between these tissues. Eyes with detectable episclera compared to eyes in which the episclera was not visible had a significantly longer axial length (
P = 0.003); thinner central retinal thickness (
P = 0.01); and thinner subfoveal scleral thickness (
P < 0.001). There were no significant differences in age between these groups (
P = 0.66). Because the subfoveal choroid was too thin to be measured in most eyes included in our study, choroidal thickness was not included in the list of outcome parameters.
Overlying the episclera, the tissue considered to be Tenon's capsule was detected in 11 of the 278 eyes (4.0%). Tenon's capsule appeared as loosely arranged tissue. In some eyes, a thin space was observed between the episclera and Tenon's capsule (similar to the slit shown in
Fig. 2A, although slightly wider than that shown in
Fig. 2C).
Blood vessels located in the sclera were surrounded by fibrous tissue within a rhomboid space (
Fig. 2C). A cluster of blood vessels was observed in the subfoveal area between the sclera and episclera (
Fig. 2E). In contrast to the intrascleral blood vessels, the blood vessels between the sclera and episclera did not appear to be ensheathed. Due to the presence of these blood vessels between the episclera and sclera, the outer border of the episclera was located outside of the imaging range of the OCT in these eyes. The region with the posterior episclera visible on the horizontal OCT images was located in these eyes temporal to the fovea. In three eyes, the outer surface of the sclera was folded inwardly and a cluster of blood vessels was located between the inwardly folded sclera and the episclera (
Fig. 4).
In the 164 eyes in which the episclera could be visualized in the OCT images, the mean scleral thickness in the foveal region was 197 ± 73 μm (range, 64–482 μm;
Table 1). In univariate analysis, scleral thickness decreased in conjunction with the longer axial length in the subfoveal region (
P = 0.008; β: −0.22) and at 1000 μm (
P = 0.02; β: −0.20) and at 2500 μm (
P = 0.01; β: −0.21) temporal to the fovea. The scleral thickness was significantly (
P < 0.0001) thicker in the foveal region than at the region that was located 1000 μm temporal to the fovea. The scleral thickness at this point was thicker (
P < 0.0001) than that found at 2500 μm temporal to the fovea (
Table 2). The same results were found when all of the measurements for all of the locations were included in the statistical analysis and subsequent ANOVA.
In the horizontal OCT sections of the 164 eyes with detectable episclera, the mean thickness of the sclera at 1000 μm temporal to the fovea was 164 ± 64 μm (range, 54–412 μm). The full thickness of the episclera was detectable in 77 eyes with a mean thickness of 80 ± 27 μm (range, 30–135 μm). The thickness of Tenon's capsule was measurable in 4 eyes at this location. However, the outer surface of Tenon's capsule could not be detected in any of the other eyes. The mean thickness of Tenon's capsule in these four eyes was 60 ± 32 μm (range, 19–93 μm). At a location of 2500 μm temporal to the fovea, the mean scleral thickness was 146 ± 59 μm (range, 58–561 μm) in the 164 eyes that underwent episcleral imaging. At this location, the thickness of the posterior episclera that was measured in 112 eyes was 82 ± 30 μm (range, 29–174 μm). The thickness of Tenon's capsule that was measurable in seven eyes at this location was 55 ± 13 μm (range, 30–70 μm).
Among the 77 eyes in which the full thickness of the episclera was observed, 25 eyes exhibited an extremely thin posterior sclera with a thickness of less than 100 μm at 1000 μm temporal to the fovea (mean, 87 ± 11 μm; range, 65–98 μm;
Fig. 5). In a subset of 15 eyes that had a detectable outer border of the episclera at this location, the mean episcleral thickness was 65 ± 15 μm (range, 39–94 μm). In the entire group of eyes in which the full thickness of the episclera was visible, the episcleral thickness measured at 1000 μm temporal to the fovea was significantly correlated (univariate analysis) with scleral thickness at the same location (
P = 0.03; regression coefficient
r: 0.25), but not with the axial length (
P = 0.12). In multivariate analysis that used episcleral thickness as the dependent variable, episcleral thickness was marginally associated with scleral thickness (
P = 0.06; β 0.24; B: 0.11; 95% CI: −0.01, 0.22), but not with the axial length (
P = 0.27). Episcleral thickness measured at 2500 μm temporal to the fovea was significantly correlated (univariate analysis) with axial length (
P = 0.02), but not with scleral thickness at the same location (
P = 0.55). Similar results were obtained for the multivariate analysis.