This study found minimal interocular differences in optic nerve head (ONH) parameters in a normal elderly population measured with Heidelberg Retina Tomograph 3 (HRT 3). It also found that the traditional vertical cup-to-disc ratio can differ between fellow eyes by as much as 0.4 in normal elderly people.
While assessing cup-to-disc area ratio is difficult without quantitative imaging, this study should prompt clinicians to pay more attention to the cup-to-disc area ratio on HRT 3 scans when evaluating glaucoma suspects.
The authors evaluated the symmetry of HRT 3-measured ONH parameters in 1,276 healthy elderly subjects who reached the 10-year follow-up of the Blue Mountains Eye Study.
There were substantial correlations for most HRT 3 parameters between right and left eyes, with the most symmetrical parameters being optic disc area (r = 0.74), optic cup area (r = 0.76) and cup-to-disc area ratio (r = 0.74). Mean cup depth, cup volume and retinal nerve fiber layer (RNFL) thickness had the poorest correlation (r ≤ 0.07).
Asymmetry in optic disc size appeared to be the main source of interocular difference. Subjects with symmetrically large optic discs were likely to have greater asymmetry in the neuroretinal rim area than those with small optic discs.
Cup-to-disc area ratio had a stronger interocular correlation than vertical cup-to-disc ratio (0.74 vs. 0.67), with the cut-off limit for normalcy being 0.2 for cup-to-disc area ratio versus 0.4 for vertical cup-to-disc area ratio. The authors write that cup-to-disc area ratio asymmetry may be a better sign of glaucoma than vertical cup-to-disc ratio asymmetry, as the former parameter has the highest symmetry in healthy eyes and also takes into account the whole ONH morphology rather than the ratio along the vertical axis only.
They note that although no significant interocular difference in RNFL thickness was found, the correlation between eyes was very poor (r = 0.03). In addition, the relative interocular RNFL asymmetry at the 2.5th (-0.15 mm) and 97.5th (0.13 mm) percentiles compared with the overall mean (0.23 mm) was nearly 60 percent, much higher than that reported measured by Cirrus OCT (9 μm /93 μm = 10 percent). These findings suggest that the symmetry of normal RNFL thickness measured at the neuroretinal rim is much less than that measured in the peripapillary area.
Since HRT RNFL thickness is taken at the neuroretinal rim, differences in disc size and hence the location of the neuroretinal rim relative to the center of the disc may be associated with differences in RNFL thickness. The results of this study support this hypothesis, the authors write, because the interocular correlation of RNFL thickness increased significantly in subjects who had symmetrical optic discs. OCT may therefore be a better tool to detect the true asymmetry of RNFL thickness than HRT 3.