Repeatability of the glaucoma-scope measurements of optic nerve head topography

The Glaucoma-Scope is designed to measure the topography of the optic nerve head. Depth measurements are reported as a grid of cells where each value represents the average depth of an area 55 +/- 100 mum in size. The Glaucoma-Scope was tested on 10 normal and 18 glaucomatous subjects, >40 years of age. Two separate visits were simulated for each patient. To estimate measurement variability, depth values were analyzed in groups of 25 cells, comprising "areas of interest." A components of variance analysis was used to compare the variability of the difference between the depth of an area measured at the first and second visits. The analysis yields within visit standard deviation of the difference between depth measurements taken within the same visit (Sw), the standard deviation of the difference between depth measurements taken at separate visits (Sb), and the total standard deviation of the difference between depth measurements (Sd). For flat areas outside the optic nerve head, Sd = 11.68 mum. For flat areas within the optic nerve head, Sd = 17.91 mum. For areas at the bottom of the cup, which includes sloped areas, Sd = 32.01 mum. For sloped areas within the optic nerve head, Sd = 20.78 mum. For areas over a vessel, Sd = 34.76 mum. The mean standard deviation of a single pixel in the Glaucoma-Scope measurement is 15.42 mum. The Glaucoma-Scope measurements are highly reproducible for both healthy and glaucomatous subjects. Local variability in depth measurements should be considered when evaluating change in optic nerve head topography.