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October 2006

Optic Nerve Structure in Healthy Subjects—Reply

Arch Ophthalmol. 2006;124(10):1508-1509. doi:10.1001/archopht.124.10.1508

In reply

In a recent study,1 we investigated structural and functional differences between the eyes of healthy blacks and whites, as blacks tend to be disproportionately affected by primary open-angle glaucoma.2 We found that blacks had a significantly thicker RNFL in the superior and inferior quadrants and larger optic discs than whites. Our interpretation of these findings is that large discs may contain more nerve fibers than small discs. This is consistent with histological evidence showing that healthy blacks have larger optic discs than whites3 and that in monkeys, the number of nerve fibers is positively correlated with the size of the optic nerve.4 Savini and colleagues, however, have suggested that our results could instead be explained by an artifact associated with the fixed diameter used for the circular scans of the OCT. These scans are centered on the optic disc and performed at a fixed diameter of 3.4 mm. The OCT circular scans are therefore performed closer to the optic disc margin in large discs as compared with small discs. Previous studies5-7 have shown that in healthy eyes, the RNFL is thickest near the optic disc margin and becomes gradually thinner as the distance from the optic disc margin increases. Given that the OCT scans were taken closer to the optic disc margin in blacks (with larger discs) than in whites (with smaller discs) and that the RNFL is thicker near the optic disc margin, Savini and colleagues suggest that our results may be an artifact of the fixed-diameter scans. While this explanation is intuitive, we argue against it and welcome the opportunity to discuss our rationale.

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