Correlation Between Axial Length and Peripapillary Retinal Nerve Fiber Layer Thickness Determined by Spectral Domain Optical Coherence Tomography: A Cross-sectional Study

Abstract

Introduction: Peripapillary retinal nerve fiber layer (pRNFL) thickness is an important indicator for the diagnosis and monitoring of glaucoma. Optical Coherence Tomography (OCT) allows for accurate assessment of pRNFL thickness, but previous studies have shown that axial length can affect pRNFL thickness. Hence, this study aimed to confirm this hypothesis. Aim: To determine the correlation between axial length and pRNFL thickness in healthy adults. Materials and Methods: This was a cross-sectional study conducted on 200 eyes of healthy adults aged 18-30 years. All subjects underwent a complete ophthalmic evaluation. Average pRNFL thickness and quadrant pRNFL thickness were recorded using Topcon Spectral Domain OCT (SD-OCT) in all subjects. Axial length measurements were performed using optical biometry with the Topcon IOL Master, and subjects were divided into three groups according to axial length: Group 1 (<23.5 mm), Group 2 (23.5-25.5 mm), and Group 3 (>25.5 mm). pRNFL thickness values were subjected to Littmann’s correction for ocular magnification. Data was analysed using a one-way ANOVA test, and the correlation between pRNFL thickness and axial length, before and after correction for ocular magnification, was determined using the Pearson correlation coefficient. Results: There was a significant negative correlation between uncorrected pRNFL thickness and axial length in the average pRNFL (r=-0.05, p<0.001), superior quadrant (r=-0.26, p<0.001), nasal quadrant (r=-0.44, p<0.001), and inferior quadrant (r=-0.48, p<0.001). Uncorrected temporal quadrant pRNFL thickness showed a positive correlation with axial length (r=0.17, p=0.015). After applying Littmann’s formula, the negative correlation between uncorrected pRNFL thickness and axial length disappeared in the average, superior quadrant, and inferior quadrant. Conclusion: A negative correlation was established between pRNFL thickness and axial length, but this correlation disappeared after applying correction for ocular magnification. Thus, to avoid misdiagnosis of glaucoma in individuals with varying axial lengths, the authors recommend using correction methods for the effects of ocular magnification induced by axial length when considering pRNFL thickness values obtained from OCT.