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

Aims—To develop a more reproducible method of macular volumetric analysis in order to facilitate serial monitoring of changes in retinal oedema with time. Methods—The Heidelberg retinal tomography (HRT) machine was used to scan the macula of 20 normal subjects and 40 diabetics with macular oedema. The volume above reference plane (VARP) within a 2 mm diameter and 3 mm diameter circle was measured twice in each eye. The position of the circle to be measured was carefully defined relative to major retinal vessels. As a modification 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 controlled relative to major retinal vessels using an acetate. The reproducibility of VARP measurement was assessed by three variables—the standard error of the difference on two isolated VARP measurements, the standard error of the diVerence between the average of three readings on two diVerent occasions, as well as the coeYcient of variation (COV). Results—Both the 2 mm and 3 mm circles showed good reproducibility in VARP measurements. In normal subjects, the COV of VARP for 2 mm circles and 3 mm circles were 13% and 12% respectively. In patients with macular oedema, the COV of VARP for 2 mm circles and 3 mm circles were 9% and 11% respectively. 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 oedema. (Br J Ophthalmol 2000;84:1194–1197) The assessment of changes in the retina is an integral part of the management of posterior segment diseases. DiYculties arise when relatively small volumes of retina are involved in the disease process, and diabetic macular oedema is the paradigm of such a clinical scenario. There is a need for a simple, accurate, non-invasive, and practical way to follow patients who may develop changes in central retinal volume. A number of researchers, including ourselves, have attempted to measure either retinal thickness or retinal volume values. Our initial method of volume measurement resulted in rather high coeYcient of variation (COV) results in diabetics with macular oedema. As a result of this, we have developed a modification to our original technique of measuring retinal volumes using the Heidelberg retinal tomograph (HRT) in an attempt to improve the reproducibility found in our initial methodology. The HRT is a scanning laser ophthalmoscope with confocal optics. It takes 32 images of the fundus in 1.6 seconds in a plane perpendicular to the optical axis. The computer software then aligns the images so as to eliminate the eVect of eye movement and compiles a three dimensional image of the surface scanned. This instrument has originally been used for the analysis of optic nerve head topography but it has recently been adapted for macular investigations. 3 5 6 Method Forty eyes of diabetic patients with macular oedema (20 with previous focal laser photocoagulation, 20 without) and 20 eyes of normal subjects were scanned using the HRT. Macular oedema was defined as retinal thickening evident on contact lens examination or slit lamp biomicroscopy with a 78D lens within 2 disc diametres from the centre of the fovea. Normal subjects were volunteers who had no fundal abnormalities clinically. Cycloplegia was achieved before scanning with topical cyclopentolate 1%, and phenylephrine 2.5% was used to achieve adequate dilatation in all subjects. The refractive error of the patients ranged from −6 to +6 dioptres. The study was performed with approval of the ethics committee of Queen’s Medical Centre. Informed consent was obtained from each participant before HRT scanning. The scans were performed by an experienced operator (LMT). The scanning technique has been described in detail in other reports. 6 To assess the repeatability of volume Br J Ophthalmol 2000;84:1194–1197 1194 Department of Ophthalmology, Queen’s Medical Centre, University Hospital, Nottingham NG7 2UH, UK