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Editors Selection IGR 16-4

Optical Coherence Tomography: Variability of RNFL thickness by Stratus OCT

Comment by Don Budenz on:

24022 Effect of signal strength and improper alignment on the variability of stratus optical coherence tomography retinal nerve fiber layer thickness measurements, Vizzeri G; Bowd C; Medeiros FA et al., American Journal of Ophthalmology, 2009; 148: 249-255

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The study by Vizzeri et al. (1105) demonstrates that OCT retinal nerve fiber layer measurement signal strength and alignment are sources of variability in retinal nerve fiber layer measurements and that these variables must be taken into account when trying to use this technology for following glaucoma patients for progression. The conclusions are based on retrospective analysis of healthy subjects who were scanned at least twice on the same day (94) or yearly (30). The data supports the conclusions that signal strength and scan circle placement may affect RNFL measurements. The authors show that increases in signal strength result in thicker RNFL measurements. They also show that average RNFL thickness changes if the scan circle is decentered horizontally a significant amount. Vertical decentration had little effect on average RNFL thickness variability but significant effect on clock hour and quadrant variability. The effects of signal strength and horizontal decentration were significant even though the range of measurements was small (signal strength of seven to ten and normal average RNFL thicknesses, 95.1-102.6 microns). The narrow range of signal strength and RNFL thicknesses limits the generalizability of the results. We would expect that lower signal strengths and thinner RNFL measurements would be subject to the same problems, but this study does not address anything outside the ranges of their normal subjects with good scan quality. The authors attempt to generalize their results to following glaucoma patients for progression and make the excellent point that large differences in signal strength (like four units or more) may introduce variability beyond the normal test-retest variability of the instrument.

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