Purpose: The cup-to-disc ratio is a widely used clinical measure of optic nerve damage in the management of glaucoma patients and those suspected of having glaucoma. Knowledge of the optic disc size allows for a better assessment of the clinical significance of the cup-to-disc ratio. In this study, two optical methods-slit-lamp biomicroscopy and confocal scanning laser ophthalmoscopy-were used to measure the vertical disc diameter as an indirect measure of optic disc size. The aim was to provide a simple and clinically useful examination technique in the management of glaucoma.
Methods: The vertical optic disc diameter was measured with aspheric lenses (Volk 60, 78, and 90 D, and Nikon 60 and 90 D) using slit-lamp biomicroscopy in 25 emmetropic normal volunteers. The average of three readings was determined for each of the lenses and the measurements were correlated with the magnification-corrected diameter measured with the Heidelberg Retina Tomograph.
Results: The mean vertical disc diameter of the group, as measured with the Heidelberg Retina Tomograph, was 1.81 +/- 0.18 mm. Results obtained with the lower-power lenses correlated best with the Heidelberg Retina Tomograph measurements (for example, Volk 60 D: r = 0.80, p = 0.0001, 95% confidence interval 1.76-1.85 mm), whereas the 90-D correlation was the weakest (for example, Volk 90 D: r = 0.59. p = 0.002, 95% confidence interval 1.75-1.87 mm). With the Volk 60-D lens, the "best fit" equation for estimating disc diameter, in millimetres, is Y = 0.85X + 0.06, where Y is the Heidelberg Retina Tomograph value and X is the slit-lamp biomicroscopy measurement.
Conclusion: Using these formulae, the clinician is able to estimate the optic disc diameter with sufficient accuracy to allow clinical decisions to be made in the evaluation of patients with glaucoma, without recourse to expensive technology.