In the present study as well as in preceding investigations, optic disc size was not significantly associated with the frequency of progression of glaucomatous visual field defects.
17 18 19 In accordance with preceding investigations, it suggests that neither a large nor a small optic disc size predisposes the eyes to the progression of glaucomatous visual field loss. Previously, the marked interindividual variability in optic disc area has been reason to raise the question of whether the disc size correlates with the susceptibility for glaucomatous optic nerve damage.
1 In some hospital-based studies, optic disc area was significantly larger in non–highly myopic patients with normal-pressure glaucoma than in non–highly myopic patients with high-pressure open-angle glaucoma.
31 In a parallel manner, the larger optic disc size in combination with the reportedly higher glaucoma susceptibility in the Afro-American population compared with whites has led to the hypothesis that eyes with large optic discs may be more prone to glaucomatous optic nerve fiber loss than eyes with small optic discs.
32 33 34 Deducing from purely mechanical factors, the pressure gradient across the lamina cribrosa produces a more pronounced displacement of the lamina cribrosa in large optic discs than in small optic nerve heads.
32 35 Inside the optic disc, the susceptibility for neuroretinal rim loss is higher in regions with a long distance to the central retinal vessel trunk than in sectors with a short distance.
36 These pro factors indicating a higher risk for glaucomatous damage in eyes with large discs are opposed by the con factors. Optic nerve fibers are more crowded in eyes with small optic nerve heads than in eyes with large discs.
37 A dense arrangement of the nerve fibers in small optic discs may suggest that the lamina cribrosa, mechanically deformed by the glaucomatous process, may more easily press the optic nerve fibers in small optic discs than in large optic nerve heads. Small optic discs have been reported to contain fewer optic nerve fibers than large optic nerve heads.
38 It suggests that eyes with small optic discs have a smaller anatomic reserve capacity. Nonarteritic anterior ischemic optic neuropathy and optic disc drusen occur more frequently in small optic nerve heads than in large optic discs.
1 39 For both entities, similar pathogenic mechanisms have been discussed as for glaucoma—that is, a perfusion problem, as is present in ischemic optic neuropathy, and a blockade of the orthograde axoplasmic flow, as in the case of optic disc drusen. Results in other studies have suggested that the higher glaucoma susceptibility in the inferior and superior disc regions compared with the temporal and nasal disc sectors is associated with a higher percentage of pore-to-disc area.
1 40 41 This ratio increases with decreasing optic disc size.
42 In summary, one might infer that the effects of the pro and con factors may compensate for each other. This notion agrees with findings in the present investigation as well as with those in other recent studies, such as the Baltimore Eye Survey, in which Quigley et al.
43 and Jonas et al.
44 concluded that the optic disc area may be a weak predictive factor for open-angle glaucoma.