DETERMINATION OF THE NEURORETINAL RIM AREA USING THE HORIZONTAL AND VERTICAL DISC AND CUP DIAMETERS

Background: This study evaluated whether the neuroretinal rim area can be calculated on the basis of the horizontal and vertical diameters o f the optic disc and cup applying the formula of an ellipse. Methods: Color stereo optic disc photographs of 587 normal subjects and 1193 pa tients with glaucoma were morphometrically examined. In a first method , the areas of the optic disc and cup were planimetrically measured an d the area of the neuroretinal rim was determined as optic disc area m inus 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 ve rtical diameters. As in the first method, the rim area was then determ ined as disc area minus cup area. Results: The values of the neuroreti nal rim area as measured using the first method differed by 0.05+/-0.0 5 mm(2) from the values as calculated by the second method. The averag e error was 2.8+/-2.9% for the normal group and 8.2+/-20.8% for the gl aucoma patients. It increased with decreasing neuroretinal rim area an d increasing mean visual field defect. Conclusions: For clinical purpo ses, the neuroretinal rim area can be determined on the basis of the h orizontal and vertical diameters of the optic disc and cup. The horizo ntal and vertical disc and cup diameters being determinable upon ophth almoscopy, this finding is important for the direct measurement of opt ic disc morphology during any ophthalmoscopic examination.