In a study of myopic, hypermetropic and emmetropic eyes, peripapillary retinal nerve fiber layer thickness (RNFL) decreased with increase in axial length as measured by spectral-domain optical coherence tomography (SD-OCT). RNFL thickness measurements varied with refractive status and axial length of the eye. However, this study also found that this influence appeared to be due to the ocular magnification effects associated with refractive error.
Researchers in Pakistan investigated whether any correlation exists between RNFL measurements and the axial length/refractive error in their country’s population. They found that RNFL thickness measurements varied with refractive status and axial length of the eye. Of the 93 patients, 35 were myopic and 29 each were hypermetropic and emmetropic. Mean axial length was 23.40±1.25mm. Mean RNFL thickness after correction of magnification effect was 100.59±9.25µm. Correlation analyses showed that the average retinal nerve fiber layer thickness and mean thickness of major superior and nasal quadrants had negative correlation with axial length. However, correction of the magnification effect by applying Littmann's formula eliminated the relationship between the two.
All modern OCT devices have built-in software to correct the effect of refractive error and axial length. The authors believe this should be taken into account to enhance the diagnostic accuracy of SD-OCT while measuring the RNFL thickness.
|Kausar A, Ali K, Akhtar N, Afzel F. Effect of refractive errors/axial length on peripapillary retinal nerve fibre layer thickness (RNFL) measured by Topcon SD-OCT. J Pak Med Assoc. 2018;68(7):1054-9.|