Probability maps of sequential glaucoma-scope images help identify significant change

The purpose of this study was to identify areas of the optic disc showing high variability of repeated depth measurements, and to minimize the effect of baseline variability in interpretation of possible change over time using the Glaucoma-Scope.

Seventy-four eyes from 70 subjects were analyzed with the Glaucoma-Scope. Three images were obtained on each of two separate sessions during the same day. At each location, the mean depth of the three images for each session was calculated to create a "baseline image." A contour map of standard deviation (SD) values at each topographic location was created for each subject reflecting local variability at different parts of the disc. The contour map and disc photograph were compared to determine what photographic features predicted high variability. A modified two-sample t-test was used at each topographic location to obtain p-values for the likelihood that a difference in mean depth between sessions was attributable to measurement variability alone.

Contour plots of SD for most subject eyes showed high variability in steeply sloped areas of the disc and along large blood vessels, with low variability near the cup center. The use of probability plots for significance of depth changes between test sessions automatically accounted for increased pointwise variability. The proportion of topographic locations showing statistically significant change but attributable to chance variation when no true change has occurred approximated the predicted proportion based on our modified t-test model.

A contour map of standard deviations of depth based on Glaucoma-Scope baseline images can identify areas of the disc with high variability. Statistical methods such as probability maps that account for local variability in the baseline image may be helpful in distinguishing true change from artefactual change over time.