Optic Disc Size
Overview
Careful and systematic evaluation of the optic nerve for glaucoma first begins with accurately determining the size of the optic disc. Correcting for optic disc size provides clinical context to cup-disc ratio, rim-disc ratio, and neuroretinal rim area measurements to help us diagnose glaucoma earlier and detect progression sooner.1
Pearls
Optic disc size correlates with the optic cup size and the neuroretinal rim area.2
The larger the optic disc size, the larger the cup and the neuroretinal rim.
A large cup in a large optic disc may be normal and a small or average cup in a small disc may suggest glaucomatous optic nerve damage.
The cup-disc ratio can range from 0.0 to 0.9 in the normal population with significant overlap between normal and glaucomatous eyes.3
Careful examination of the vertical cup-disc ratio corrected for disc size increases specificity and sensitivity.4
Number of retinal nerve fibers in normal eyes is variable and ranges from 750,000 to 1.5 million.5
Significant intrapopulation and interpopulation variability.
Prevalence of glaucoma increases with age.10
Gender: No statistically significant difference or consensus.8
Body length/weight: inconclusive in some studies, but others report increased disc area of 0.02 mm2 with each 10-cm increase in body length for normal body height.2
Refractive error: within -5 to +5 D, statistically independent9; other studies suggested that disc area linearly increased by 1.2% +/-0.15% for each diopter toward myopia.2
Higher hyperopia (>+5): smaller disc than emmetropic eyes.2
Higher myopia (>-8): larger, elongated disc than emmetropic eyes.11
Race: “one may infer that the disc size increases with ethnically determined pigmentation”2—i.e., African Americans > Asians > Hispanics > Caucasians.
