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Editors Selection IGR 11-2

Imaging: Stratus OCT and RNFL measurement in myopes

Comment by Joel Schuman on:

21738 Peripapillary retinal nerve fibre layer thickness in highly myopic Caucasians as measured by Stratus optical coherence tomography, Vernon SA; Rotchford AP; Negi A et al., British Journal of Ophthalmology, 2008; 92: 1076-1080

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Vernon et al. (1089) sound a note of caution in interpreting Stratus OCT results in highly myopic eyes. The retinal nerve fiber layer (RNFL) measures significantly thinner in high myopes than in eyes with less than six diopters of myopia, most prominently in the nasal clock hours. Interestingly, RNFL thickness was not associated with spherical equivalent. The authors present a valuable graph plotting the 256 points of the circumpapillary RNFL scanning circle showing the expected upper and lower 95% confidence limits for Stratus OCT, as well as the mean and upper and lower 95% confidence limits for their highly myopic study group (mean spherical equivalent -7.7 diopters, mean axial length 26.5 mm). These curves tell the story of why there is a difference in OCT measured RNFL thickness between people with more than six diopters of myopia and those in the Stratus OCT normative data. More specifically, they explain why it is not the spherical equivalent that influences the OCT measurement. In the figure described above, the reader can note that the shape of the RNFL thickness confidence limit curves are very similar whether in highly myopic eyes or the Stratus OCT normative data. There are two major differences. First, the mean for the highly myopic eyes fall at nearly the same level as the lower 95% confidence limit. Second, the peak thicknesses are displaced temporally in the highly myopic eyes relative to the normative data.

The further from the center of the optic nerve the RNFL thickness is measured, the thinner the measurement will be

The reason for these findings, although not mentioned in the article, is related to the size of the projected scanning circle. The further the OCT beam must travel to reach the retina, the larger the diameter of the scanning circle will become. This explains why the authors found that there was a thinner retina in high myopia than in the normative dataset ‐ the further from the center of the optic nerve the RNFL thickness is measured, the thinner the measurement will be. In addition, since this is related to distance traveled and not refractive error, this phenomenon explains why the relationship that the authors identified was between axial length and Stratus OCT measured RNFL thickness, not spherical equivalent and RNFL. The shape of the circumpapillary RNFL distribution curve is similar in the normative data and the high myopes because the high myopes were chosen to be healthy except for their high myopia, and do not actually have RNFL loss. Rather their RNFL is simply being measured at a greater distance from the center of the optic nerve head than was true for those in the normative dataset. Finally, the displaced peak thicknesses demonstrate that the scanning circle is larger relative to the normative data circle size. The larger circle results in an artifactual displacement of the peak thicknesses temporally, as demonstrated in this paper. The authors have identified an important caveat in the interpretation of Stratus OCT circumpapillary RNFL scans in highly myopic eyes.

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