Quantification of Retinal Nerve Fiber Layer Thickness in Highly Myopic Eyes Using Optical Coherence Tomography

Purpose: To evaluate the relationship between peripapillary retinal nerve fiber layer (RNFL) thickness measured by optical coherence tomography (OCT) and the axial length/refractive error of the eye. Methods: Thirty-eight myopic eyes from 38 patients were studied retrospectively. Axial length was measured using A-scan ultrasonography and RNFL was measured using OCT. Subjects with identified ocular diseases were excluded. Patients were divided into three groups by axial length measurements. Group I: ＜26 mm, Group Ⅱ 26–27 mm, and Group Ⅲ: ＞27 mm. Average and segmental RNFL thicknesses were compared among the groups. Associations between RNFL measurements and axial length and spherical equivalents were evaluated using linear regression analysis. Taking into consideration the definition of high myopia, RNFL thickness and axial length＜or＞26.5 mm were also compared. Results: The average RNFL thickness for each of the three groups was 106.06±11.86 μm for Group Ⅰ, 104.81±11.60 μm for Group Ⅱ, and 92.81±14.19 μm for Group Ⅲ (p=0.018). The average RNFL thickness decreased with increasing axial length (r=-0.499, p=0.001) and negative refractive power (r=0.125, p=0.455). There was no statistically significant difference between axial lengths of greater than 26 mm and less than 26 mm. However, when the axial length was set to 27 mm, results showed a major distinction. The axial length of 26.5 mm was a meaningful threshold for differentiation (p= 0.025). Conclusions: As the level of myopia increased, the thickness of the RNFL decreased. Therefore, analysis of RNFL thickness in the evaluation of glaucoma should always be interpreted with reference to the refractive status. Axial length is a better reference when analyzing RNFL thickness measured by OCT. The axial length of 26.5 mm was a meaningful threshold that suggested decreasing RNFL thickness.