Measurement of disc parameters.

Before the measurement, refraction, corneal curvature, and axial length were entered to the software to adjust for the ocular magnification effect. A) Ovality Index: Longest (1) and shortest (2) diameters of the optic disc were identified and manually traced. The length of the line was automatically calculated with the data management software (Imagenet R4). The ovality index was calculated by dividing the shortest disc diameter by the longest diameter. B) Disc Area: The edge of the optic disc was manually traced. The area surrounded by the line (= disc area) was automatically calculated. C) β-zone parapapillary atrophy area: First the line was manually drawn to enclose both optic disc and parapapillary atrophy. The area of the optic disc plus β-zone parapapillary atrophy was automatically calculated. Then the area of β-zone parapapillary atrophy was calculated by subtracting the disc area.