Regional test-retest variability of confocal scanning laser tomography

To determine the regional differences in the test-retest variability of topographic measurements of the optic nerve head obtained with a confocal scanning laser retina tomograph.

Three 10-degree topographic images centered on the optic disk of 20 individuals (20 eyes), five normal subjects, seven glaucoma suspects, and eight glaucoma patients, were acquired and averaged. For each eye, the standard deviation of the mean height at each pixel was calculated from the three images and displayed as a gray scale map, which represents regional test-retest variability. The standard deviation of the mean height of the eight surrounding pixels of each pixel was calculated and plotted as a gray scale map, which represents regional steepness.

The variability maps and steepness maps showed close correspondence in all eyes. Regions of high steepness had high variability (r = .31; P < .001). The average variability in relatively flat areas (sixth to 15th percentiles for steepness) was 19 microns, whereas steeper areas (81st to 95th percentiles for steepness) had an average variability of 37 microns. The overall mean variability was 28 +/- 7 microns.

The regional variability of topographic measurements made with the confocal scanning laser tomograph correlates with the steepness of the corresponding region and is highest at the edge of the optic disk cup and along vessels. Because of their high test-retest variability, these steep areas may not be the best locations to follow up patients for progressive glaucomatous damage.