Interocular Asymmetry in Retinal Nerve Fiber Layer Thickness Measured With CirrusTM HD-OCT in Normal Eyes

Abstract

Purpose: : To determine normal tolerance limits for interocular difference in RNFL thickness measured with Cirrus HD-OCT in normal eyes.

Methods: : 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 2.5^th and 97.5^th 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.

Results: : 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 2.5^th and 97.5^th 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.

Conclusions: : 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.

Keywords: imaging/image analysis: clinical