Variability of the standard reference height and its influence on the stereometric parameters of the heidelberg retina tomograph 3

To investigate the variability of the standard reference height (SRH) in consecutive Heidelberg Retina Tomograph 3 (HRT3) examinations and its influence on five main stereometric parameters.

HRT3 results of all patients attending our glaucoma center from August to October 2007 were retrospectively reviewed. Only one eye per patient with at least three consecutive HRTs and a quality control label of acceptable or better was selected. An SRH difference >or=10% compared with baseline was considered as excess SRH variability.

A review of 641 patients yielded 150 patients (150 eyes) who met the inclusion criteria, representing a total of 556 examinations. The mean total number of HRTs per patient was 3.7 (range, 3-7), and the mean follow-up time was 3.5 years (range, 1.4-6.7). The mean proportion of consecutive HRT3 examinations with intraindividual excess SRH variability was 46% (95% CI, 40-52), whereas the mean intraindividual excess SRH variability was 24% (95% CI, 20-28). The median absolute SRH difference was 8.6% (interquartile range, 3.9%-16.2%). Furthermore, 55.5%, 75.3%, 74.7%, 0.0%, and 19.4% of the variability in rim area, rim volume, retinal nerve fiber layer thickness, cup shape measure, and height variation contour, respectively, could be attributed to SRH variability.

There is considerable SRH variability between HRT3 examinations of the same eye, and this could explain more than half the variability of the parameters RA, RV, and RNFL. These findings indicate that changes of HRT3 parameters should be considered with caution when excess SRH variability is present.