Sources of variability of topometric data with a scanning laser ophthalmoscope

Objective: To determine in a two-part study whether misalignment between the patient and the laser scanner is a major source of variability with the Heidelberg Retina Tomograph (Heidelberg [Germany] Engineering). Methods: Three topographic images of the right optic nerve were acquired with the Heidelberg Retina Tomograph in eight patients with glaucoma. The correlations between average cup volume, variability of cup volume estimates, and direction of imaging were evaluated. Furthermore, the correlations between average rim volume, variability of rim volume estimates, and direction of imaging were evaluated. Next, the optic nerve cup volume and rim volume estimates of a rabbit's left eye were compared between three series of five topographic images acquired from three slightly different directions. Results: Average cup volume,variability of cup volume estimates, and variability in the direction of imaging correlated significantly among the patients with glaucoma (multiple R(2)=.95; P<.001). Average rim volume, variability of rim volume estimates, variability in the direction of imaging, and variability in the mean height of the contour line also correlated significantly (multiple R(2)=.88; P=.03). In the rabbit eye, the cup volume and the rim volume differed significantly among the three image series (analysis of variance P<.001 and P=.04, respectively). Conclusion: Misalignment between the patient and the laser scanner may account for significant variability with the Heidelberg Retina Tomograph.