## 4 Discussion
In the current study, all of the CDR parameters, including horizontal, vertical, lineal CDR, and cup/disc area ratio were smaller in the more myopic eyes in anisometropic subjects. The Rim/disc area ratio was larger in the more myopic eyes. No significant difference in rim area was found between both eyes. A previous study by [PERSON] et al. suggested that vertical CDR asymmetry could be useful to identify patients with a higher risk to develop glaucoma [16]. They found that the population-weighted 50 th, 97.5 th, and 99.5 th percentiles of vertical CDR asymmetry were 0.05, 0.19, and 0.26, respectively in the US population. The current study included 18 healthy Chinese subjects with inter-eye differences in spherical equivalent (SE) of more than 1.50D (range: 1.50-3.88 D). The mean difference in vertical CDR was 0.13 \(\pm\) 0.18. Although the mean difference in vertical CDR fell within the 97.5 th percentile, the standard deviation was as large as 0.18. A considerable number of subjects (5 out of 18 subjects) were outside the normal range regarding vertical CDR. As CDR asymmetry is one of the parameters that raises suspicion of glaucoma, some otherwise healthy anisometropic patients with myopia might be considered glaucoma suspects because of asymmetric CDR. The current study found that all inter-eye differences in cup/disc area ratio, horizontal CDR, vertical CDR, and linear CDR were positively correlated with the inter-eye difference in AL and SE. Regarding the correlation between optic disc parameters and refractive error/AL, previous studies had inconsistent results. [PERSON] et al. and [PERSON] et al. found no correlation between optic disc size and refractive error within the range of \(-\)8D to +4D based on optic disc photographs [17; 18]. In contrast, [PERSON] et al. suggested a negative correlation between optic disc area and refractive error based on optic disc photographs [19]. [PERSON] et al. found that the optic disc area was correlated with AL/refractive error using HRT III [20]. In the same study, when only subjects within the range of \(-\)8D to +4D were analyzed, no correlation was found between optic disc area and refractive error/AL. [PERSON] et al. also found a weak correlation between disc area and refractive error based on HRT [13]. Thus, the correlation between optic disc parameters and refractive error might not have been clinically relevant. In the current study, the range of refractive error was from \(-\)8D to 0D (mean: \(-\)4.17 \(\pm\) 2.27D). The difference in disc area between the two eyes was not statistically significant. However, the inter-eye difference in the disc area increased as the inter-eye difference in refractive error/AL increased. Inter-eye differences in CDRs also increased as the inter-eye difference in refractive error/AL increased. We further evaluated the influences of AL, SE, and disc area on the measurement of cup/disc ratio parameters with the multiple linear regression analysis. A larger disc area was independently and significantly associated with a larger cup/disc area ratio and vertical CDR. No significant association was found between SE/AL and the cup/disc ratio parameters. The asymmetry of CDR might be caused by the asymmetry of the disc area between the eyes in myopic anisometropia. Since disc area was found to be correlated with cup/disc ratio parameters and AL, the inter-eye difference in disc area was used as a covariate in a regression model to evaluate the correlation between the inter-eye difference in AL and inter-eye differences in cup/disc ratio parameters. Among the cup/disc ratio parameters, only the inter-eye difference in vertical CDR was found to be positively correlated with the inter-eye difference in AL. The asymmetry of CDR could not be simply explained by the difference in refractive error between both eyes. As the inter-eye difference in refractive error increases, the asymmetry of CDR between two eyes may affect the diagnosis of glaucoma. Several limitations exist in the present study.