Myopia is one of the most common causes of visual impairment and blindness, so early detection and management of degenerative eye diseases is of utmost importance. The results of the current study demonstrated that average macular retinal thickness of the fovea did not vary with myopia, while total volume and the retinal thickness of the parafovea were thinner with increased myopia. There was good agreement between the two OCTs in myopic eyes in all macular subfields, and the Cirrus HD-OCT system provided thicker macular retinal thickness measurements than the Stratus TD-OCT system.
Previous studies reported that Cirrus HD-OCT measurements resulted in a significantly thicker retinal thickness than the Stratus TD-OCT [16–18]. We also found that Cirrus HD-OCT measurements resulted in thicker retinal thickness measurements than the Stratus TD-OCT in myopic eyes. Different technologies and segmentation algorithms of the two OCT systems may explain the thicker retinal thickness measurements observed in Cirrus HD-OCT. The most important difference is that the Cirrus segmentation identified the thickness of the retina from the retinal pigment epithelium (RPE) to the internal limiting membrane (ILM), while the Stratus segmentation identified retinal thickness from the inner segment/outer segment junction (IS/OS) to the ILM, thus Cirrus HD-OCT measurements resulted in an inherently thicker distance between the IS/OS and the RPE .
Our study showed that the two OCTs showed macular retinal thickness of the parafovea, especially in outer retinal subfields, that was significantly lower in the high myopia group compared with the low to moderate myopia group, except at the fovea (all P-values less than 0.001, except at the fovea). Other studies demonstrated that the average macular retinal thickness of the parafovea and total macular volume decreased with axial length (all P-values less than 0.001, except at the fovea). So with an increase of axis length, macular thickness becomes thinner, especially in the outer retinal subfields. Marcus et al. and Lim et al. also reported that the parafovea was thinner in myopic patients’ myopia . This observation may be the result of the anatomical features of the retinal macular foveolar region. The foveolar is the thinnest area of the retina, which consists of the five innermost cell layers of the retina, including the outer plexiform layer, the outer nuclear layer, the external limiting membrane, the photoreceptor layer, and the retinal pigment epithelium. Previous histopathologic studies reported that the thickness of neurosensory retina (RNL), especially the inner nuclear layer, were thinner in several animal models of myopia . The foveolar lacks the innermost retinal layers, and there is no inner nuclear, so the retinal thickness variation is not obvious between the high myopia group and moderate myopia groups. Therefore, the macular retinal thickness in the parafovea was thinner in the high myopia group than in the moderate myopia group, but no difference was found in the foveolar. Moreover, other pathological processes, such as choroid retinal degeneration, and choroid and RPE atrophy, could likewise play a role in changes of the retina.
The results of the current study characterized macular retinal thickness in young myopic patients. These characteristics can be used as an auxiliary diagnosis for making a distinction with other macular retinal related diseases. If the macular thickness in the foveolar is abnormally thin, pathological disease should be considered. However, if the changes occurred in the parafovea, other factors can first be eliminated, such as refractive factors, which could influence the measurements of macular thickness by the OCTs, especially among high myopia patients. In conclusion, when the macular thickness is abnormal, clinicians should consider the effect of the refractive factor combined with clinical manifestations to comprehensively evaluate its clinical significance.