Comparison of Neuroretinal Rim Area Measurements Made by the Heidelberg Retina Tomograph I and the Heidelberg Retina Tomograph II

PURPOSE: To investigate the agreement between neuroretinal rim area (RA) measurements using the Heidelberg Retina Tomograph I (HRT Classic) and Heidelberg Retina Tomograph II (HRT II). To compare apparent RA changes in follow-up series of HRT II topographies when using either an HRT Classic or HRT II mean topography as baseline. DESIGN: Cross-sectional study and “no-change,” short time series study. PARTICIPANTS: Forty-three ocular hypertensive and 31 primary open angle glaucoma subjects. METHODS: Five HRT Classic and 5 HRT II examinations were acquired from 1 eye of each subject, across 2 visits within 6 weeks. For the cross-sectional study, follow-up RA measurements from HRT Classic and HRT II were compared, using the same HRT Classic mean topography as the baseline. The linear rates of RA change were compared in 2 short time series with either an HRT Classic or an HRT II mean topography as baseline, and 4 follow-up HRT II mean topographies. Intervals between topographies were arbitrarily set at 1 year for meaningful comparisons of rates. Rates of RA change over time were calculated by linear regression. Separate analyses were performed using 3 available reference planes (RP). MAIN OUTCOME MEASURES: Global and sectoral RA measurements in HRT Classic and HRT II mean topographies; linear rates of RA change. RESULTS: HRT Classic minus HRT II mean differences (95% limits of agreement) were 0.09 (−0.17, 0.35) mm(2), 0.09 (−0.13, 0.32) mm(2), and 0.11 (−0.24, 0.46) mm(2) for the Moorfields, 320 µm, and standard RPs, respectively (P<0.001 for all RPs, Wilcoxon rank sum test). In the time series, the mean differences (95% limits of agreement) of RA rates of change (HRT Classic baseline minus HRT II baseline) were −0.01 (−0.06, 0.03) mm(2)/y, −0.01 (−0.06, 0.04) mm(2)/y, and −0.0(2) (−0.09, 0.05) mm(2)/y using the Moorfields, 320 µm, and standard RPs, respectively. CONCLUSION: Although HRT software is backward-compatible, follow-up RA measurements made in the same eye using HRT Classic and HRT II devices display statistically and clinically meaningful systematic differences when HRT Classic topographies are used as a baseline.