Interocular Asymmetry of OCT Retinal Nerve Fiber Layer Values in a Normative Population: The Framingham Heart Study

Purpose: To explore interocular asymmetry in retinal nerve fiber layer (RNFL) thickness using optical coherence tomography (OCT), assess factors that predict this asymmetry, and quantify the 95% central range for OCT-defined average cup-disc ratio (CDR). Methods: Participants from the Framingham Heart Study were included. Interocular differences in OCT parameters were calculated by subtracting left eye values from the right eye. To quantify the range for interocular differences, the percentile distributions were described, with normal ranges established as 2.5th to 97.5th percentiles. Multivariable linear regression models were employed to explore predictors of interocular differences in RNFL thickness. Results: In total, 522 participants were studied with a mean (standard deviation [SD]) age of 74.5 (6.9) years, and most were female (59.4%) and white (88.3%). The mean (SD) peripapillary RNFL thickness was 88.1 (9.7) in the right eye and 87.7 (9.7) in the left eye, with a nonstatistically significant difference (mean [SD], 0.4 [6.1]; P = 0.18). The 2.5th and 97.5th percentiles for the interocular difference in average RNFL were −12.7 μm and 12.7 μm, while that of average CDR was −0.19 and 0.21, respectively. In multivariable models, only differences in rim area (β = 8.06/mm²; P < 0.001) and differences in signal strength (β = 1.37/unit; P < 0.001) were significantly and positively associated with interocular differences in average RNFL thickness. Conclusions: The 95% limits for average RNFL and CDR were within 12.7 microns and 0.2 units, respectively, between eyes. Translational Relevance: Comparing OCT RNFL of both eyes may aid in detecting early cases of unilateral/asymmetric glaucoma or other optic nerve–related pathology when used in conjunction with other clinically or biologically relevant findings.