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Editors Selection IGR 9-4

Refractive Errors: RNFL thickness in myopia

Comment by Chris Leung on:

47801 Characteristics of peripapillary retinal nerve fiber layer thickness in eyes with myopic optic disc tilt and rotation, Hwang YH; Yoo C; Kim YY, Journal of Glaucoma, 2011;

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Interpreting optical coherence tomography (OCT) circumpapillary retinal nerve fiber layer (RNFL) measurement in myopic eyes has been complicated by the high prevalence of optic disc tilt and parapapillary atrophy in eyes with a long eyeball. Using a spectral-domain OCT, Hwang et al. (1922) measured the degree of horizontal and vertical optic disc tilt in the OCT images and the degree of optic disc rotation in the fundus photographs, and investigated their association with the superior and inferior RNFL peak locations detected with reference to the 3.46 mm diameter circle scan.

In myopic eyes with optic disc tilting, relative RNFL thinning at the superior and inferior quadrants may not necessarily signify pathological loss

An important finding of the study is that an increase in optic disc tilting is associated with temporal shifting of the superior and inferior RNFL bundles, resulting in an increase in the temporal RNFL thickness. In other words, in myopic eyes with optic disc tilting, relative RNFL thinning at the superior and inferior quadrants may not necessarily signify pathological loss.

One confounding factor in analyzing the superior and inferior RNFL peak locations with a circle scan is ocular magnification. By default, the Cirrus HD-OCT (Carl Zeiss Meditec, Dublin, CA) optic disc cube scan contains an area of 6 x 6 mm² at the optic disc region and the measurement circle has a diameter of 3.46 mm. In myopic eyes, the scan area would be larger than 6x6 mm² and so is the measurement circle because of ocular magnification.

Analyzing all pixel measurements derived from the cube scan would be more informative than a circle scan to unfold the anatomical variations of RNFL distribution in myopia

An alternative approach to measure the peak locations of the RNFL bundles is to analyze the distribution profile of the RNFL bundles in the RNFL thickness map provided in the analysis printout. Analyzing all pixel measurements derived from the cube scan would be more informative than a circle scan to unfold the anatomical variations of RNFL distribution in myopia.

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