SOURCES OF VARIABILITY OF TOPOMETRIC DATA WITH A SCANNING LASER OPHTHALMOSCOPE

Objective: To determine in a two-part study whether misalignment betwe en 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 acq uired with the Heidelberg Retina Tomograph in eight patients with glau coma. 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 volu me 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 fro m three slightly different directions. Results: Average cup volume,var iability of cup volume estimates, and variability in the direction of imaging correlated significantly among the patients with glaucoma (mul tiple R(2)=.95; P<.001). Average rim volume, variability of rim volume estimates, variability in the direction of imaging, and variability i n the mean height of the contour line also correlated significantly (m ultiple R(2)=.88; P=.03). In the rabbit eye, the cup volume and the ri m 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.