Symmetry of optic nerve head parameters measured by the heidelberg retina tomograph 3 in healthy eyes: the Blue Mountains Eye study.

PURPOSE To assess the symmetry of optic nerve head parameters measured by the Heidelberg Retina Tomograph 3 (HRT 3) between fellow eyes in a normal elderly population. DESIGN Cross-sectional population-based study. METHODS Participants of the Blue Mountains Eye Study 10-year follow-up who did not have optic disc disease, including glaucoma, were included. Optic nerve head parameters measured by HRT 3 were compared between fellow eyes. The normal range of interocular asymmetry (larger disc minus smaller disc) was determined by the 2.5th and 97.5th percentiles. RESULTS A total of 1276 eligible participants had HRT scans of both eyes. HRT measurements in right eyes differed slightly in rim steepness and rim volume from those in left eyes (P < .05). The 2.5th and 97.5th percentile of interocular asymmetry limits were -0.41 and 0.45 for cup-to-disc ratio and -0.19 and 0.22 for cup-to-disc area ratio, respectively. The highest interocular correlation was found in disc area and cup area (r ranged from 0.74-0.76), whereas mean cup depth, cup volume, and retinal nerve fiber layer thickness had the poorest correlation (r ≤ 0.07). Greater optic disc area asymmetry was associated with a larger interocular difference in the other optic nerve head parameters. CONCLUSIONS There was minimal interocular difference and substantial interocular correlation in optic nerve head parameters measured by HRT 3. Interocular asymmetry greater than 0.2 for cup-to-disc area ratio was considered outside the normal range.