Improvement of reproducibility of macular volume measurements using the Heidelberg retinal tomograph

Aims-To develop a more reproducible method of macular volumetric analysis i n order to facilitate serial monitoring of changes in retinal oedema with t ime. Methods-The Heidelberg retinal tomography (HRT) machine was used to scan th e macula of 20 normal subjects and 40 diabetics with macular oedema. The vo lume above reference plane (VARP) within a 2 nun diameter and 3 mm diameter circle was measured twice in each eye. The position of the circle to be me asured was carefully defined relative to major retinal vessels. As a modifi cation to the previously published technique, fluctuation of the height of the contour Line relative to the focal plane of the eye is compensated for by an adjustment of the reference plane. The position of the circle was con trolled relative to major retinal vessels using an acetate. The reproducibi lity of VARP measurement was assessed by three variables-the standard error of the difference on two isolated VARP measurements, the standard error of the difference between the average of three readings on two different occa sions, as well, as the coefficient of variation (COV). Results-Both the 2 nun and 3 mm circles showed good reproducibility in VARP measurements. In normal subjects, the COV of VARP for 2 nun circles and 3 mm circles were 13% and 12% respectively. In patients with macular oedema, the COV of VARP for 2 nun circles and 3 mm circles were 9% and 11% respecti vely. Conclusion-The modification of VARP measurements between scans of the same eye has improved the COV from 31% to 9% in eyes with diabetic macular oedem a.