VERTICAL OPTIC DISK DIAMETER - DISCREPANCY BETWEEN PLANIMETRIC AND SLO MEASUREMENTS

Purpose. To compare measurements of the vertical diameter of the optic disk using the Heidelberg Retinal Tomograph (HRT) and planimetric met hods. Methods. On the HRT, three measurements of the vertical disk dia meter were made on 32 eyes from a composite image using the circle dra w facility and were compared with three measurements of the central im age of the 32 tomographic cuts. These were repeated by a second observ er. Measurements were compared with those obtained from photographs us ing the three corrections described by Bengtsson and Krakau. Results. For the HRT, there was minimal intraobserver variation, coefficient of variation (1.515% to 1.882% for the two observers). Interobserver var iation was also small (CoV 1.969%). The authors found the closest agre ement with the HRT measurements using regression analysis was obtained using correction 3 (r = 0.8258). HRT measurements were significantly smaller (P < 0.0001) by a constant amount of 0.13 mm across the range of optic disk size. There was a significant difference between the HRT measurement of the composite image and the central cut (P = 0.0003 fo r observer 1). Conclusions. HRT measurements are significantly smaller than those from photographs and, therefore, are not interchangeable.