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 methods. METHODS On the HRT, three measurements of the vertical disk diameter were made on 32 eyes from a composite image using the circle draw facility and were compared with three measurements of the central image of the 32 tomographic cuts. These were repeated by a second observer. Measurements were compared with those obtained from photographs using 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 variation was also small (CoV 1.969%). The authors found the closest agreement 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 for observer 1). CONCLUSIONS HRT measurements are significantly smaller than those from photographs and, therefore, are not interchangeable.