Ophthalmoscopic-Perspectively Distorted Optic Disc Diameters and Real Disc Diameters

PURPOSE. To investigate the difference between two-dimensionally measured disc diameters (DDs) based on fundus images and three-dimensionally measured DDs based on enhanced depth imaging optical coherence tomography (EDI-OCT). METHODS. The DDs were three-dimensionally measured on EDI-OCT images of optic nerve heads and two-dimensionally measured on near-infrared reflectance fundus images in 90 normal eyes of 90 subjects. Disc rotation around sagittal axis was defined as the angle between maximal DD and vertical axis. Disc rotation around horizontal and vertical axes was defined as cosine of DD on fundus images divided by DD on EDI-OCT images. RESULTS. Mean optic disc rotation around vertical axis was 14.4 +/- 9.38, rotation around sagittal axis was 23.0 +/- 21.38, and rotation around horizontal axis was 4.7 +/- 6.68. Horizontal, vertical, minimal, and maximal DDs as measured three-dimensionally were significantly (P < 0.001) higher than those measured two-dimensionally. Difference between three-dimensional and two-dimensional measurement of horizontal DD and vertical DD was associated with axial length (r = 0.38; P < 0.001, and r = 0.23; P = 0.03, respectively). Difference between three-dimensional and two-dimensional measurements was larger (P < 0.001) for horizontal DD (69.6 +/- 68.1 mu m) than for vertical DD (14.8 +/- 25.6 mu m). Correspondingly, the ratio of maximal-to-minimal DD (ovality index) was significantly larger (P < 0.001) for two-dimensional (1.33 +/- 0.21) than for three-dimensional (1.27 +/- 0.16) measurements. CONCLUSIONS. Two-dimensional measurement of DDs leads to falsely low results, more for horizontal DD than for vertical DD. The discrepancy between low two-dimensional compared to three-dimensional disc measurements increases with longer axial length. The angle of disc rotation around vertical and horizontal axes can be measured with EDI-OCT imaging.