Vertical optic disk diameter: discrepancy between planimetric and SLO measurements

Abstract

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.