Symmetry between the right and left eyes of the normal retinal nerve fiber layer measured with optical coherence tomography (an AOS thesis).

PURPOSE To determine the limits of the normal amount of interocular symmetry in retinal nerve fiber layer (RNFL) thickness measurements obtained with third-generation time domain optical coherence tomography (OCT3). METHODS Both eyes of normal volunteers were scanned using the peripapillary standard and fast RNFL algorithms of OCT3. RESULTS A total of 108 volunteers were included in the analysis. The mean +/- standard deviation (SD) of age of the volunteers was 46.0 +/- 15.0 years (range 20-82). Forty-two participants (39%) were male and 66 (61%) were female. Mean RNFL thickness correlated extremely well, with intraclass correlation coefficients of 0.89 for both algorithms (95% confidence interval [CI], 0.84-0.93). The mean RNFL thickness of the right eye measured 1.3 mum thicker than the left on the standard scan (SD 4.7, 95% CI 0.4-2.2, P = .004) and 1.2 mum on the fast scan (SD 5.2, 95% CI 0.1-2.2, P = .026). The 95% tolerance limits on the difference between the mean RNFL thicknesses of right minus left eye was -10.8 and +8.9 mum with the standard scan algorithm and -10.6 and +11.7 mum with the fast scan algorithm. CONCLUSIONS Mean RNFL thickness between the 2 eyes of normal individuals should not differ by more than approximately 9 to 12 mum, depending on which scanning algorithm of OCT3 is used and which eye measures thicker. Differences beyond this level suggest statistically abnormal asymmetry, which may represent early glaucomatous optic neuropathy.