Background: Image scaling on the Heidelberg Retina Tomograph (HRT) is based on an axial model of ametropia. In the longitudinal follow-up of patients, refractive changes in ametropia may occur; the HRT interprets such change as being axial in origin. This results in an apparent alteration in the size of fundus features with time and in reduced sensitivity of the instrument to subtle changes relating to disease progression. The aim of this study was to determine the effect of changing HRT focus settings on the absolute scaling of topography images.
Methods: Seven image series were acquired of the optic nerve head of a single emmetropic eye at each of a range of panel focus settings under three conditions: (A) emmetropia, (B) trial lens-induced ametropia, (C) contact lens-induced ametropia. A masked observer measured the separation between two defined retinal vascular bifurcations for each topography image.
Results: The measured distance decreased with a positive shift in focus. There was a significant difference in distance with change in focus setting for all conditions (A P < 0.008, B and C P < 0.0001). Over equivalent focus ranges, data from the three groups were not significantly different, lying within the 95% confidence limits at each setting. The mean standard deviation for the distance measure was 10 microns.
Conclusion: Adjustment in focus settings to compensate for refractive change in eyes of stable axial length and keratometry induces a change in the topography measures using the HRT. It is recommended that, for the detection of subtle change over time, refractive changes are corrected without a change in the panel focus setting.