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S.C. Anderson, P.S. Wagner, C. Doyle, K. Woodward, M. Wall, R.H. Kardon; Long–Term Variability of the Retinal Nerve Fiber Layer Thickness (RNFL) in Glaucoma Measured by Optical Coherence Tomography (OCT) . Invest. Ophthalmol. Vis. Sci. 2005;46(13):637.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: to define the confidence limit of RNFL thickness repeatedly measured by OCT of glaucoma patients over time and to determine sources of measurement variability. Methods: 6 patients with glaucoma and moderate visual field loss (average mean deviation= –9.0db; range= –1.77db to –15.66db) had their RNFL thickness measured by OCT Stratus (fast RNFL circular scan set=3 individual sequential scans) on 5 different test days within a 3 month time period. Average patient RNFL thickness was 63 microns; range= 47 to 88 microns. On each test day, the visual field was measured (HVF 24–2 SITA) and 3 sets of 3 OCT scans were obtained (total# scans=5 test days x 9 scans/day=45 scans). Results: When RNFL thickness variation was analyzed by clock–hour segments, the long–term variability (2SD) was 13.8 microns for abnormal clock–hours (range=3 to 24microns) and 14.8 microns for normal clock hours (range=6 to 25microns) and this difference was not significant. There was a significant correlation between RNFL thickness and long–term variability for abnormal clock–hours (slope=0.27, R2=0.41, p=.0007) and also for normal clock hours (slope=0.09, R2=0.20, p=.01). This indicated that thinner segments showed less measurement variability, but there was also less variability for abnormal segments having the same thickness as normal segments. Clock hour segments in the superior and inferior bundle regions had the highest long–term measurement variability. Signal strength of the scans positively correlated with average RNFL thickness and negatively correlated with long–term variability (SD of RNFL thickness). Conclusions: RNFL thickness in glaucoma patients shows between session measurement variability that was as high as 25 microns (2SD) in some clock–hours segments and as low as 3 microns in others, and was related to RNFL thickness and clock–hour location. The long–term variability of the RNFL thickness appears to be primarily influenced by eye position with respect to intended scan location, accuracy of scan placement, and signal strength of the OCT scan.
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