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Editors Selection IGR 9-4
Optical Coherence Tomography: Effect of myopia on RNFL thickness by OCT
Comment by Gadi Wollstein on:
24511 Myopia affects retinal nerve fiber layer measurements as determined by optical coherence tomography, Rauscher FM; Sekhon N; Feuer WJ et al., Journal of Glaucoma, 2009; 18: 501-505
The manuscript by Rauscher et al. (1102) investigated the effect of myopia on retinal nerve fiber layer (RNFL) thickness measurements obtained by time domain optical coherence tomography (OCT). Twenty-seven healthy subjects had comprehensive ocular examination, visual field and OCT scanning. The average axial length was 25.65 mm with 70% of the eyes with axial length > 25 mm. A significant and negative relationship was found between overall mean, superior and inferior quadrants RNFL thickness and axial length, stated quantitatively, the mean RNFL decreased by 7 μ/1 mm for an increase of 1 mm in axial length. For nasal and temporal quadrants, the relationship was not statistically significant. The temporal quadrant was unique in showing a positive relationship. The study joins several other studies investigating this same topic with conflicting results. The strength of the current study is the careful attention to confounders that might affect the results, such as tilted discs or peripapillary atrophy. However, the main limitation of the study is that the projected sampling location by the OCT was not accounted for. Therefore, in longer eyes the sampling location was further away from the optic nerve head center, therefore yielding artificially thinner measurements.
The full extent of increased axial length on RNFL thickness is still unclear
The authors speculate that the thinning of the RNFL might be an artifact stemming from an extended delay in the time of flight of the back reflected light in elongated eyes. This delay does not explain this finding because once the light reaches the retina the time through the retina should remain unchanged and translated into identical thickness as in emmetropic eye. In conclusion, the full extent of increased axial length on RNFL thickness is still unclear. However, the prevalence of high myopia in the general population is small and this clinical dilemma is not applicable for the vast majority of patients.
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