nal vein. Finally, measurements of the superi- or, inferior, nasal, and temporal disk rim widths were made. Horizontal and vertical disk diameters were compared and then the two dimensions were combined to generate an average disk diameter which was used for further calculations and comparisons. For 21 optic nerve heads, two to three slides of each nerve were available. In these cases, measurements were made from each slide and an average of the measurements was obtained. In order to minimize the magnification or minification of the photographic image caused by the eyes’ axial length, refractive power, and the camera optics, the actual disk and cup diameters were calculated by determining the ratio of the structure to that of a vein on the disk and multiplying by 125 ym, its actual size.’ This method assumes that the vein on the disk will be subject to the same optical enhancement or reduction as the disk. * Actual diameter structure = 125 pm X measured structure/measured disk vein Cup/disk ratios were calculated by dividing the cup diameter by that of the disk. To calculate the neural rim area, the horizontal and vertical disk and cup diameters were averaged and the areas for each were calculated using the following formula: Area of a circle = m(diameter/2)° The cup area (C) was subtracted from the disk area (D) to yield the rim area (R). The presence or absence of glaucomatous features such as disk asymmetry, vertical disk cupping, and notching of the neural rim were assessed. Correlations between disk diameter, cup diameter, rim area, and cup/disk ratio vs history of radiation exposure, visual acuity, and intra- ocular pressure were studied. VUL2a22 Results Acceptable photographs wereavailable for 54 eyes of 36 patients. There were 15 men (42%) and 21 women (58%). They ranged in age from 30 to 67 years, witha mean + S.D. age of 44.1 = 11.3 years for the men and 43.5 + 10.2 years for the women. Of the 36 patients, 19 (53%) hada history of radiation exposure. Only twopairs of subjects were known to be related. Thefirst pair, a mother and a son, had relatively large disk diameters of 1.8 mm and 2.1 mm, respectively, in their right eyes. The mother had a cup/disk ratio of 0.5, whereas her son with a larger disk had a cup/disk ratio of 0.7. The second pair of related individuals weresisters. Each had disk diameters of 1.65 mm and cup/ disk ratios of 0.4 in their left eyes. Information on the fellow eyes in both pairs of individuals was not available. Although there were similarities in disk size and cup/disk ratios between each of these related individuals, no firm con- clusions concerning heredity can be drawn given such a small sample size. The optic disk—Franceschetti and Bock! calculated the mean + S.D. optic disk diameter of a normal Swiss population to be 1.62 + 0.153 mm. In those eyes with oval disks, they averaged the vertical and horizontal measurements, as was done in this study. They rea- soned that since only 0.26% of the observations in a normal distribution would be expected to exceed three standard deviations from the mean, which corresponded to 2.08 mm in their study, megalopapilla could be defined as a disk diameter greater than 2.08 mm.’ Because photographs were taken mainly of large optic disks with some normal disks for comparison, the distribution in our study would naturally be skewed. Therefore, a disk diameter greater than or equal to 2.1 mm was adopted from Franceschetti and Bock’s study as the criterion for a large optic disk. The average disk diameter in this study population was 1.93 + 0.28 mm (Fig. 1). Of 54 eyes, 22 had optic disks with a diameter greater than or equal to 2.1 mm. Fifteen of 36 patients had large optic disks, the condition being bilateral in seven patients and unilateral in eight. Examples of large optic disks are shownin Figure 2. Cup/disk ratio—In the Framingham Eye Study, the average cup/disk ratio was 0.28 + 0.17 and two standard deviations above the tert Measurements of the horizontal and vertical disk diameters were made carefully, excluding peripapillary halos and crescents. Horizontal and vertical cup diameters were estimated by color contrast taking into account changes in vessel direction at the cup edge as an indication of cup contour change where possible. The diameter of the largest vein on the disk edge was measured before it joined the central reti- EN February, 1989 AMERICAN JOURNAL OF OPHTHALMOLOGY aLaM Peepe we ee 146