OPHTHALMOSCOPIC MEASUREMENT OF THE OPTIC DISC

Purpose: The authors evaluated whether the optic disc dimensions can b e measured directly by ophthalmoscopy without the use of a fundus came ra or special ophthalmoscope. Materials and Methods: The horizontal an d vertical disc diameters were measured ophthalmoscopically in 158 eye s of 95 subjects using a standard Goldmann three-mirror contact lens a nd a commercial slit lamp with adjustable length of the beam. The refr active error ranged from -7.25 to 3.5 diopters. Based on these direct measurements, the authors calculated the disc area by applying a modif ied formula for an ellipse, where area = pi/4 X the horizontal diamete r X the vertical diameter. Additionally, the authors determined by pla nimetry the diameters and area of the disc on color photographs after correcting the ocular and photographic magnification. Results: The val ues of the disc diameters as determined by photography were X 1.27 lar ger than those measured with the Goldmann contact lens. Taking into ac count this constant correction factor of 1.27 for all eyes examined, t he disc diameters, as measured by the Goldmann lens, varied by 0.11 +/ - 0.06 mm or 5.8% +/- 3.5% from the values measured on the photographs . This error decreased slightly (P = 0.04) with increasing disc size. It was independent of the refractive error. The calculated disc area b ased on the ophthalmoscopically determined disc diameters differed by 9.4% +/- 6.6% from the disc area as measured by planimetry on the phot ographs. Conclusions: For clinical purposes, the optic disc and other structures of the posterior fundus can be measured by ophthalmoscopy u sing a Goldmann contact lens and a slit lamp with adjustable beam leng th. The optic disc area can be calculated by the formula: horizontal d isc diameter X vertical disc diameter X pi/4.