A knowledge of the optic disc size may be of value when assessing the glaucoma suspect. The vertical diameter of the optic disc was measured using a Zeiss four mirror gonioscope and a 900 Haag-Streit slit-lamp in one eye of 39 patients, 32 with refractive errors within 3 dioptres of emmetropia. The disc was measured by projecting a slit beam of known height onto the image of the disc. A magnification factor for the contact lens was calculated from first principles and disc height recalculated. These measurements were compared with those obtained by photographic methods using the corrections suggested by Bengtsson and Krakau. In the analysis on the 32 eyes within 3 dioptres of emmetropia the best correlation with clinical measurements was obtained with correction 3 using spectacle refraction and keratometry (r = 0.8614). The contact lens measurement was within plus or minus 0.1 mm of the photographic measurement in 67% of cases and plus or minus 0.2 mm in all cases. This simple method is advocated for the routine assessment of optic disc size.
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