Effect of scan repetition on the reproducibility of optic nerve head topographic measurements with the Top SS

Purpose: To determine the effect of scan repetition during a single visit for improved reproducibility of optic nerve head topographic measurements. Methods: One randomly selected eye of 20 healthy subjects with no systemic or ocular disease was evaluated. Half of the subjects were under 30 years of age and the other half were older than 60 years. Five independent image series, each consisting of 10 newly recorded consecutive images, were obtained for each subject over a period of 10 weeks with a confocal scanning laser ophthalmoscope, the Top SS. Subjects were divided into two groups according to disc area being either smaller or larger than 2 mm2. Reproducibility of the measurements in three parameters (volume below, half depth area and the average depth) were calculated for each set of scans. The coefficient of variation (standard deviation divided by the mean) was taken as the measure of reproducibility. Repeated measures of analysis of variation and independent sample t-test were used for statistical analysis. Statistical significance was set as P < 0.05. Results: Both age groups were found to be equally distributed with respect to sex. There were no significant intergroup differences in keratometry, pupil diameter or refractive error. Increasing the scan repetition per visit up to five improved the reproducibility of the measurements. Further increases in scans per visit reduced the reproducibility. Subjects with larger discs and older age were found to have lower coefficients of variation for topographic measurements. Conclusions: Normal subjects should be examined at least three to four times per visit to have significantly improved reliability of measurements of optic nerve head topography. Factors associated with decreased reproducibility included older age, smaller discs, and more than five scans per visit. Sex, refraction and pupil diameter were shown to have no effect on the reproducibility.