The effect of contour line position on optic nerve head analysis by Heidelberg Retina Tomograph.

PURPOSE: To determine the influence of the contour line position on the Heidelberg Retina Tomograph (HRT) parameters and on the diagnostic capacity of the system.

METHODS

Thirty subjects with clinical diagnosis of primary open angle glaucoma and 30 healthy subjects were recruited. For each patient only one eye was randomly chosen. All optic nerve heads (ONH) were analysed with HRT. An operator drew the right contour line in all the considered images, while a second user increased and decreased radius size of 0.05 and 0.1 mm, recalculating each time all the HRT parameters. Five different disc area subgroups were obtained and for each subgroup discriminant formulas were applied. Sensitivity, specificity, and diagnostic precision were considered for each subgroup.

RESULTS

Among the HRT parameters, rim area was the most sensitive to the contour line position changes when the radius variation was greater than +/-0.1 mm. For most of the HRT parameters, the differences between subjects with glaucoma and control group did not significantly change when the disc area increased or decreased. Highest diagnostic capacity was observed for Mikelberg and Mardin formulas when the contour line was narrowed, while the cup shape measure value had the best diagnostic capacity when the contour line was larger.

CONCLUSIONS

When the disc area was modified, the sensitivity, specificity, and diagnostic capacity of discriminant formulas changed according to the ONH size; however, in the glaucoma clinic, small errors (within +/-0.05 mm) in contour line position did not significantly influence the final HRT diagnostic capacity.