PROSPECTIVE-STUDY DESIGN FOR THE HEIDELBERG RETINA TOMOGRAPH - THE EFFECT OF CHANGE IN FOCUS SETTING

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 interpre ts such change as being axial in origin. This results in an apparent a lteration in the size of fundus features with time and in reduced sens itivity of the instrument to subtle changes relating to disease progre ssion. The aim of this study was to determine the effect of changing H RT focus settings on the absolute scaling of topography images. Method s: Seven image series were acquired of the optic nerve head of a singl e emmetropic eye at each of a range of panel focus settings under thre e conditions: (A) emmetropia, (B) trial lens-induced ametropia, (C) co ntact lens-induced ametropia. A masked observer measured the separatio n between two defined retinal vascular bifurcations for each topograph y image. Results: The measured distance decreased with a positive shif t 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). Ov er equivalent focus ranges, data from the three groups were not signif icantly different, lying within the 95% confidence limits at each sett ing. The mean standard deviation for the distance measure was 10 mu m. Conclusion: Adjustment in focus settings to compensate for refractive change in eyes of stable axial length and keratometry induces a chang e in the topography measures using the HRT. It is recommended that, fo r the detection of subtle change over time, refractive changes are cor rected without a change in the panel focus setting.