Reproducibility of volumetric measurements of normal maculae with the Heidelberg retina tomograph.

AIMS

The Heidelberg retina tomograph (HRT) is a scanning laser ophthalmoscope with confocal optics. The reproducibility of the optic nerve head topography is accurate and reliable. The authors describe a new technique for the assessment of macular thickening by volumetric quantification and present the results of its reproducibility in normal subjects.

METHODS

Topographic images of the macula, centred on the fovea were obtained in one eye of 44 normal subjects. The volumes above the reference plane bound by a 1 mm, 2 mm, and 3 mm diameter circle were measured. The reference plane was adjusted to the lowest point of the height variation of the contour line at each examination. The reproducibility of repeated measurements within a 2 mm diameter circle was assessed in 20 eyes selected at random. Three HRT scans of each eye were obtained. The measurements of volume above reference plane of each scan were repeated three times on three separate days.

RESULTS

The intrascan coefficients of variability measured 7.12-9.57%. The 95% confidence interval for the geometric mean ratio of single volume measurements was 0.92 to 1.24 for scans 1 and 2, 0.89 to 1.17 for scans 1 and 3, and 0.81 to 1.12 for scans 2 and 3. When the mean of three measurements of one scan were compared with the mean of three measurements of a second scan, the 95% confidence interval for their geometric mean ratio was 0.89 to 1.20 for scans 1 and 2, 0.89 to 1.16 for scans 1 and 3, and 0.84 to 1.13 for scans 2 and 3. The average standard deviation (SD) for one measurement per scan was 0.02 mm3, and 0.019 mm3 for two or three measurements per scan. Linear regression demonstrated a significant increase in SD as volumetric measurements increased (p = 0.003). Age did not significantly affect the SD of volumetric measurements (p = 0.797). The authors found no significant differences in volumetric measurements across all ages for all three circles (p = 0.314, p = 0.471, p = 0.267).

CONCLUSION

Good reproducibility for volumetric measurements at the macula was found with the HRT using the above technique in normal subjects. This method may be extremely useful for the identification and quantification of diabetic macular oedema and for monitoring the effects of argon laser photocoagulation.