In this study, we determined RT of the macula using an SD-OCT volume scan in eyes with varying degrees of myopia. Although the 2- and 3D curvature of the RPE layer has previously been reported in emmetropic or highly myopic eyes,
3,4,28–30 this is the first study to investigate 3D RT in eyes with varying degrees of myopia. In a previous study, which used horizontal scans, the inward convexity may be less clearly detected.
31 In this study using a pseudocolor coded topographic image map of the RPE from 3D-OCT scans, we were able to easily identify B-type RT in eyes with all degrees of myopia. The rainbow pseudocolors in the topographic layer image represent height from the coronal plane of the eye, with blue indicating low height and red indicating high height. This could be used as an objective tool to depict the 3D configuration of the RPE. Using this method, we were able to examine the shape of the photoreceptor array in situ. The topographic images for RPEs on Topcon 3D-OCT are recognized as segmentation images of the top of the RPE layer, which corresponds to the outer segment tips of the photoreceptors.
32 Thus, RT images in this study may provide realistic measurements of the retinal photoreceptor array. However, the topographic images of the RPE layers are provided not only by the 3D-OCT but also by other SD-OCTs. Since the photoreceptor layers are usually arrayed with RPE layers, topographic maps for the RPE layers generated by other SD-OCTs may be useful alternatives for observing the topographic characteristics.
We found that the distribution of RT type differs according to the degree of AL. In eyes with a low degree of AL, 50.5% exhibited R-type topography. However, as the degree of AL increased, fewer exhibited R-type and a greater proportion were found to be W or B-type. In eyes with a high degree of AL, 37.5% were P-type. These results support the previous suggestion that the posterior curvature of the eye changes with progression of myopia.
33 Ohno-Matsui and associates
3 previously hypothesized that progression of eye deformities in myopic eyes occurs due to changes in the curvature of the inner scleral surface and Bruch's membrane. They also suggested that the posterior sclera may expand when a posterior staphyloma develops later in life. Other studies have shown that the bulge in eyes with a dome-shaped macula may prevent schisis or detachment, and also act as a compensatory mechanism during ocular development.
34 We defined B-type as having a dome-shaped macula or band-shaped ridge.
28,29 B-type topography was observed in eyes with all degrees of myopia in our study. This suggests that while B-type topography may be present in myopic eyes of varying degrees, it is potentially enhanced with progression of myopia. This may also mean that B-type RT is not simply a consequence of staphyloma, but may also be an early indicator of the development of staphyloma. In this study, we newly defined W-type RT, which is similar to B-type without the horizontal ridge, which continues between the fovea and optic disc. Both types can exhibit outward concavities of the RPE. However, B-type may be the more advanced form of W-type, since W-type was more frequently observed in eyes with low- and medium-grade myopia. In our study of eyes with varying degrees of AL, R-type topography was predominantly seen in cases of low grade myopia, W-type in moderate myopia, and B- or P-type in high myopia. While our results suggest that changes in RT type occur with progression of myopia, additional prospective studies are needed with long term observation to confirm our findings.
We characterized RT using several factors such as the area of greatest depression, slope, and central zone topography. On 3D segmentation maps, 81 (48.5%) of 167 eyes exhibited an area of greatest depression in the inferior area, and only 12 (7.2%) of 167 eyes showed this within the visual axis. These results differ from those of a previous study that used 2D line scan in eyes with high myopia, in which 21.7% of eyes had an area of greatest depression below the visual axis.
2 We also found that 29 (17.4%) of 167 eyes had an area of greatest depression in the superior portion of the macula, compared with 0 cases reported in the previous study.
2 These differences may be due to varying degrees of myopia or the use of different methods to determine the area of greatest depression. In the current study, the most depressed area was determined using 3D maps of the RPE layer. The distribution varied according to AL. In 22.9% of eyes with an AL less than 26 mm, the area of greatest depression was in the superior portion of the macula. However, this was only true in 4.2% to 10.5% of eyes with an AL greater than 26 mm. The proportion of eyes with the most depressed area in the inferior macula was greater in eyes with a longer AL. This may be because the lower one-half of the eye is structurally weaker since the optic fissure develops on the ventral side of the optic cup during formation of the eye.
35 In a previous study that used 2D scan images, the curvature of the RPE was classified into two types: straight contour that sloped toward the optic disc or roughly symmetrical curvature centered on the fovea.
3 In our study, we classified the slope into three types, each of which were representative of the curvature. Nasal slope was the predominant type observed in all degrees of myopia. Temporal and middle slope were more frequently observed in eyes with moderate to high myopia. The macular center is the region in which cone cells are highly concentrated, and is known to be important for fine vision. Photoreceptors are regularly arrayed in the visual axis to maximize the ability to receive photons passing through the eye.
10,11 In this study, the symmetry around the foveal center was considered in the classification of central zone configuration. In myopic eyes, the topography of the central zone was more frequently slanted. However, in eyes with B-type RT, it was more frequently flat. This suggests that B-type RT sustains the array of photoreceptors in the visual axis.
In this study, P-type RT had lower BCVA than the other three types. However, the BCVA of the W- and B-types, which had similar topographic characteristics to those of R-type, was not different from R-type. Difference between refractive astigmatism and corneal astigmatism was different among RT types. We found that difference between refractive astigmatism and corneal astigmatism was different among RT types. Retinal topography may be related with the optical characteristics of the eye. The optical characteristics of the eye can be determined according to the features of its components, including the cornea, pupil, crystalline lens, and retina.
36 Each photoreceptor functions as a “light pipe,” or a fiber optic, which is directed to the second nodal point of the eye.
11,36 This orientation optimally receives light and forms an image.
36,37 Since myopic eyes were often found to exhibit irregular topography, there may be concurrent suboptimal orientation of photoreceptors contributing to visual deterioration. In addition to corneal, pupillary, and lens factors, ocular aberration measurements could be influenced by retinal factors and the array of photoreceptors, which is dependent on the 3D configuration of the choroid and the sclera.
38,39 However, it is difficult to measure retinal factors that influence ocular aberration. In this retrospective study, we did not measured higher-order aberrations. However, we present methods of measuring RT using commercially available SD-OCT. This technique could be used to monitor retinal factors influencing ocular aberration and vision. Additional prospective studies that use an aberrometer and SD-OCT could help to further establish the impact of retinal aberration on ocular aberration in eyes with retinal disease.
Previously, staphylomas were classified into several subtypes.
1 However, in this study, we were unable to classify staphylomas using macular OCT. Only a localized portion of the macula was the focus of this study. Although this study covered a limited area, the results were obtained based on images that included the macular area using 3D volume scans with compact B-scans. While we used images that covered a 6 mm × 6 mm area of the macula that is likely important for fine vision, the myopic change in RT occurs in the entirety of the eye. Thus, use of a smaller defined area may limit our understanding of these changes. This study was limited by the small sample size included in each group as well as by the retrospective design. Additionally, we included myopic eyes, but not eyes with other macular diseases. The classification of RT types as seen on pseudocolor topographic maps into discrete groups is an arbitrary designation, since the range of topographic types in a population of myopic retinas is part of a continuum. In this study, we determined the topographic type using an imaging program, and obtained a high degree of reproducibility. Although we did not find the difference in the slope between images from 3D-volume scanning protocols and line scanning protocols, an alignment between an eye and OCT at the time of image acquisition may influence the result regarding the slope and central zone topography. Finally, while this was a cross-sectional study, longitudinal studies using a more advanced scan protocol that covers a wider area may be useful in overcoming these limitations.
In conclusion, RT representing the outer retinal boundary varied with the degree of AL. The difference of RT characteristics may be originated from the difference between healthy eyes and highly myopic eyes. And the differences may be related with optical biometry of myopic eyes. Retinal tomography using SD-OCT might be useful for monitoring morphologic changes of the retina with respect to progression of myopia.