Reasons for rim area variability in scanning laser tomography

PURPOSE. To determine reasons for rim area variability in scanning laser tomography. METHODS. Regional rim area variability from testing in same and different visits and by same and different observers was characterized in 30 normal and 42 glaucomatous eyes. Variations in (1) optic nerve head (ONH) surface geometry (center of gravity: X, Y, Z), (2) image tilting (horizontally and vertically), and (3) position of the reference plane in relation to the ONH (REF) were analyzed by multiple regression analysis. Whether and how much these factors explain rim area variability was studied in cross-sectional and longitudinal data by using two different reference planes. RESULTS. Variability was higher in glaucoma and in testing by different observers in separate visits. Across a range of eyes, approximately 40% of variability in single-topography images and 60% of variability in mean-topography images was explained. In individual image series, a median 85% of variability was explained, exceeding 90% in at least 25% of eyes. The most frequent contributors to rim area variability were REF (in greater than or equal to95%) and Z (in greater than or equal to80%); they also usually explained more variability than other factors. The nature of variability differed between reference planes. CONCLUSIONS. A large proportion of rim area variability was explained by variation in the topographical features studied, especially REF and Z. Reference plane definition also influenced variability. Variation in the position of the reference plane in relation to the ONH can affect rim area measurements and should be considered when evaluating the progression of glaucoma.