Large Optic Disks Vol. 107, No. 2 have optic disk diameters that fell beyond two standard deviations above the mean of a normal white population. In most cases, physiologic cups were also large, resulting in pale disks because of the increased visibility of the lamina cribrosa. Each case was associated with a full, sharp rim of healthy pink neural tissue without evidence of glaucomatous damage and with good visual acuity. The 18-year follow-up in three patients showed no change suggestive of an acquired, progressive process. The definition of megalopapilla is based on the distribution of optic disk sizes in a normal white population, but may not be applicable to all racial groups. Neither the exact prevalence of megalopapilla nor the mean optic disk or cup diameters in the Marshall Island population could be calculated because fundus photographs were taken mainly of patients with large disks and high cup/disk ratios with several normal disks for comparison. The nonrandom nature of this study would skew the distribution of disk and cup sizes toward the larger end of the spectrum. However, the finding of 15 cases of megalopapilla, which is equal to the number previously reported, is certainly significant.* This is not without precedent. Ina study of uric acid levels in the Marshallese, it was concluded that the observed increases in serum uric acid levels were not restricted to a subset of persons with hyperuricemia. Instead, the distribution of uric acid levels throughout the entire population tested was gaussian, suggesting a mean value and normal range of uric acid that is approximately 1.0 mg/dl higher than that found in the United States. Since “hyperuricemia” is common throughout the Pacific, it is possible that megalopapilla also occurs with increased prevalence in the Marshall Islands. *” Another question is whether radiation exposure plays a role in these differences. In our study of megalopapilla, the small sample size precludes drawing conclusions based onstatistical significance; however, since both exposed C57 ms or. Pms.7s oo CPi and nonexposed groups have relatively the same proportion of patients with large disks, it is unlikely that radiation exposure wasa factor contributing to the formation of such. Additionally, the presence of a unilaterally large disk cannot easily be explained by whole-body radiation exposure. The effect of age and sex on disk and cup size must also be addressed. Although most authors agree that the cup size probably does not vary appreciably with sex, there is some con- 149 troversy as to whetherit increases with age."® Two studies have found no change in cup size with increasing age,!!? whereas the Framingham Eye Study,® Pickard, and Carpel and Engstrom™ all found a slight increase in cup size with age. However, because age did not cause the cup/disk ratios to exceed 0.6 in these last three studies, we could not attribute our 20 cases (out of 54) of cup/disk ratios greater than 0.6 to age alone, if at all. For these reasons, the effect of age and sex on cup and disk sizes was not considered to be significant for this study. Another interesting finding concerned the rim area. A study by Teal, Marin, and McCulloch® showed that although cup area increased with disk area, rim area remained constant, suggesting that in large disks, cups enlarged so as to keep the rim area and hence the amount of neural tissue the same from person to person. In contrast, our study showed that as disk and cup area increased, rim area increased as well. Although cup area increased with disk area enough to raise the cup/disk ratio, disk size seemed to be increased out of proportion to the physiologic cup, yielding a larger rim area than expected. Recent work"!§ confirms ourfindings that disk and rim areas vary directly. Whether this larger rim area results from an increase in neural tissue or an increase in extraneuronal supporting tissue is uncertain. In 1985 a study showed that blacks tend to have significantly larger cup/disk ratios than whites (0.35 mm for blacks and 0.24 mm for whites).'’ However, the investigators did not evaluate the disk size or rim area. It would be of interest to know whether physiologic cupping in blacks is associated with larger disks andif these disks contain a greater amount of neural or extraneuronal tissue. Conversely, disk size may not vary at all from that of the white population, thus leading to a situation where there is a smaller but adequate amountof neur- al or extraneuronal tissue. In either case, the amount of neural tissue becomes important when considering neuronal reserve under pathologic conditions such as glaucoma or compressive mass lesions that lead to optic atrophy. Racial differences in disk and cup sizes are consistent with both Armaly’s’ and Bengtsson’s® studies, which showed that cup and disk sizes are genetically determined. Although there were similarities in disk size and cup/disk ratios between two pairs of related individuals in this study, no firm conclusions