Effect of refractive errors/axial length on peripapillary retinal nerve fibre layer thickness (RNFL) measured by Topcon SD-OCT

Objectives: To evaluate the effect of refractive error and axial length on peripapillary retinal nerve fibre layer thickness in myopic, hypermetropic and emmetropic eyes by Topcon spectral domain optical coherence tomography. Methods: This cross-sectional study was conducted at the ophthalmology clinic of Shifa Foundation Falahee Clinic, Islamabad, Pakistan, from January to December 2016, and comprised eye patients including emmetropes, hypermetropes and myopes. All participants underwent detailed ophthalmic examination including axial lengths measurements, auto refraction andretinal nerve fibre layer thickness analysis. Retinal nerve fibre layer thickness was obtained from the peripapillary quadrants: temporal, superior, nasal and inferior and 12 sub-quadrants using Topcon spectral domain optical coherence tomography. Pearson correlation coefficients (r) were calculated to evaluate relationships between the retinal nerve fibre layer thickness and axial length before and after adjustment for ocular magnification by using Littmann's formula. Results: Of the total 93 eyes of as many patients, 46(49.5%) were right eyes and 47(51.5%) left eyes. There were 35(37.6%) myopes, 29(31.2%) hypermetropes and 29(31.2%) emmetropes. Mean age was 30.45 +/- 7.86years. Mean axial length was 23.40 +/- 1.25mm. Mean retinal nerve fibre layer thickness after correction of magnification effect was 100.59 +/- 9.25 mu m. Correlation analyses showed that the average retinal nerve fibre layer thickness and mean thickness of major superior and nasal quadrants and also in nasal, upper nasal, superonasal, inferior and inferonasal sub-quadrants had negative correlation with axial length. However, correction of the magnification effect by applying Littmann's formula eliminated the relationship between the two. Conclusions: Retinal nerve fibre layer thickness measurements were found to vary with refractive status and axial length of the eye. Ocular magnification significantly affected the retinal nerve fibre layer thickness, and it should be considered in diagnosing glaucoma.