Determination of the neuroretinal rim area using the horizontal and vertical disc and cup diameters

This study evaluated whether the neuroretinal rim area can be calculated on the basis of the horizontal and vertical diameters of the optic disc and cup applying the formula of an ellipse. Color stereo optic disc photographs of 587 normal subjects and 1193 patients with glaucoma were morphometrically examined. In a first method, the areas of the optic disc and cup were planimetrically measured and the area of the neuroretinal rim was determined as optic disc area minus cup area. In a second method, the optic disc and cup were assumed to have an elliptical shape. Their areas were calculated by applying the formula of an ellipse, taking into account their horizontal and vertical diameters. As in the first method, the rim area was then determined as disc area minus cup area. The values of the neuroretinal rim area as measured using the first method differed by 0.05 +/- 0.05 mm2 from the values as calculated by the second method. The average error was 2.8 +/- 2.9% for the normal group and 8.2 +/- 20.8% for the glaucoma patients. It increased with decreasing neuroretinal rim area and increasing mean visual field defect. For clinical purposes, the neuroretinal rim area can be determined on the basis of the horizontal and vertical diameters of the optic disc and cup. The horizontal and vertical disc and cup diameters being determinable upon ophthalmoscopy, this finding is important for the direct measurement of optic disc morphology during any ophthalmoscopic examination.