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October 2006

Optic Nerve Structure in Healthy Subjects

Arch Ophthalmol. 2006;124(10):1507. doi:10.1001/archopht.124.10.1507-b

We read with interest the recent article by Racette et al1 on the differences in optic nerve structure between the healthy eyes of blacks and whites. They describe several significant findings, but we think 2 deserve particular attention: blacks have larger optic disc areas, as measured by the Heidelberg Retina Tomograph, and a thicker retinal nerve fiber layer (RNFL) in the superior and inferior quadrants, as measured by optical coherence tomography (OCT). According to the authors, the latter result, which may have important consequences from a clinical point of view, seems to contradict previous studies performed by scanning laser polarimetry that demonstrated a thinner RNFL in blacks.2,3 We suggest, as a possible explanation for this apparent discrepancy, that there is an artifact in the OCT measurement consequent to the relationship between the RNFL thickness and the optic nerve head (ONH) size.4 Standard OCT protocol measures the RNFL by means of a fixed-diameter (3.4 mm) circular scan; as a consequence, the larger the ONH, the closer is the scan to its rim. Given that the RNFL thickness decreases at increasing distances from the ONH rim, it is not surprising that larger discs are measured to have a thicker RNFL. Accordingly, a customized scan that permits the measurement of RNFL thickness at a fixed distance from the ONH rim5 would obviate this artifact of standard OCT measurement, and we suggest performing it to verify the results of Racette et al.

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