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D. L. Budenz, J.-C. Mwanza, M. Durbin, Cirrus OCT Normative Database Study Group; Interocular Asymmetry in Retinal Nerve Fiber Layer Thickness Measured With CirrusTM HD-OCT in Normal Eyes. Invest. Ophthalmol. Vis. Sci. 2010;51(13):214. doi: https://doi.org/.
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To determine normal tolerance limits for interocular difference in RNFL thickness measured with Cirrus HD-OCT in normal eyes.
Two hundred and eighty-four normal subjects were included in the study. Both eyes of each participant were scanned with Cirrus HD-OCT using the Optic Disc 200x200 protocol. Interocular differences were calculated for average, quadrants and clock-hours RNFL thickness. Normal ranges for interocular differences were established as the 2.5th and 97.5th percentiles. The correlation between interocular difference in average RNFL thickness and demographic/ocular characteristics was assessed with the Pearson correlation coefficient. Multiple regression analysis was used to determine the best predictor of interocular difference in average RNFL thickness.
The average RNFL in OD was 0.5 µm thicker than in OS; the difference was marginally significant (p = 0.049). All quadrants showed statistically significant interocular differences in RNFL thickness (p < 0.05). The temporal, nasal and inferior quadrants had thicker RNFL in OD whereas OS showed thicker RNFL in the superior quadrant. Except for clock-hours 4, 5 and 6, all other clock-hours showed statistically significant interocular differences in RNFL thickness. The RNFL was thicker in 7 of the clock-hours in OD and 2 (clock-hours 12 and 1) in OS. The 2.5th and 97.5th percentiles interocular difference tolerance limits for average RNFL thickness were -7.9 µm and 8.1 µm, respectively. Average RNFL thickness correlated well in both eyes (r = 0.89, p < 001). The difference in average RNFL thickness correlated with differences in axial length (r = -0.229, p < 0.01), disc area (r = 0.213, p < 0.01), cup-to-disc ratio (r = 0.145, p = 0.014), and vertical cup-to-disc ratio (r = 0.152, p = 0.01). However, only differences in axial length (p < 0.01) and disc area (p < 0.01) predicted the interocular variability of average RNFL thickness.
An interocular difference in average RNFL thickness exceeding 8 µm when measured with Cirrus HD-OCT in normal eyes may be considered as abnormal asymmetry, and may indicate early glaucomatous damage.
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