OPHTHALMOSCOPIC MEASUREMENT OF OPTIC DISC SIZE

Purpose To examine whether the optic disc size can be measured with co mmon ophthalmoscopic lenses. Patients and methods The horizontal and v ertical disc diameters in 125 eyes of 65 patients were measured ophtha lmoscopically using a commercial slit lamp with adjustable length of t he beam and a Volk 60 diopters lens or a Volk Superfield lens. The ref ractive error of the subjects ranged between -7.25 D and + 3.25 D (mea n +/- S.D.: -0.34 +/- 1.77). Based on these measurements we calculated the optic disc area by applying a modified formula for an ellipse, wh ere area = horizontal diameter x vertical diameter x pi/4. Additionall y, we measured planimetrically the horizontal and vertical diameters o f the optic disc on color stereo disc photographs after correcting the ocular and camera magnification according to Littmann's method (8). R esults The Values of the horizontal and vertical disc diameters evalua ted on the photographs were by factors of 1.0 and 1.5 larger than thos e values measured with the Volk 60 D lens, and the Volk Superfield len s, respectively. Taking into account these constant linear correction factors, the optic disc diameters as measured by the Volk 60 D lens an d the Volk Superfield lens varied by 0.11 +/- 0.09 mm (5.9 +/- 5.1%), and 0.11 +/- 0.09 mm (5.9 +/- 4.9%), respectively, from the values mea sured on the photographs. The error for the ophthalmoscopic measuremen t of the disc diameters decreased slightly with increasing disc size. With highly myopic eyes excluded, it was independent of the refractive error. Conclusion For clinical purposes, the optic disc and other str uctures of the posterior fundus can be determined by ophthalmoscopy us ing a slit lamp and commonly used ophthalmoscopical lenses.