Abstract
PURPOSE: To evaluate the effect of age on optic nerve axon counts, spectral-domain optical coherence tomography (SDOCT) scan quality, and peripapillary retinal nerve fiber layer thickness (RNFLT) measurements in healthy monkey eyes. METHODS: In total, 83 healthy rhesus monkeys were included in this study (age range: 1.2-26.7 years). Peripapillary RNFLT was measured by SDOCT. An automated algorithm was used to count 100% of the axons and measure their cross-sectional area in postmortem optic nerve tissue samples (N = 46). Simulation experiments were done to determine the effects of optical changes on measurements of RNFLT. An objective, fully-automated method was used to measure the diameter of the major blood vessel profiles within each SDOCT B-scan. RESULTS: Peripapillary RNFLT was negatively correlated with age in cross-sectional analysis (P < 0.01). The best-fitting linear model was RNFLT((μm)) = -0.40 × age((years)) + 104.5 μm (R(2) = 0.1, P < 0.01). Age had very little influence on optic nerve axon count; the result of the best-fit linear model was axon count = -1364 × Age((years)) + 1,210,284 (R(2) < 0.01, P = 0.74). Older eyes lost the smallest diameter axons and/or axons had an increased diameter in the optic nerve of older animals. There was an inverse correlation between age and SDOCT scan quality (R = -0.65, P < 0.0001). Simulation experiments revealed that approximately 17% of the apparent cross-sectional rate of RNFLT loss is due to reduced scan quality associated with optical changes of the aging eye. Another 12% was due to thinning of the major blood vessels. CONCLUSIONS: RNFLT declines by 4 μm per decade in healthy rhesus monkey eyes. This rate is approximately three times faster than loss of optic nerve axons. Approximately one-half of this difference is explained by optical degradation of the aging eye reducing SDOCT scan quality and thinning of the major blood vessels. TRANSLATIONAL RELEVANCE: Current models used to predict retinal ganglion cell losses should be reconsidered.