[Effect of the contour line on cup surface using the Heidelberg Retina Tomograph]

Significance of topometric follow-up examinations of the optic nerve head in glaucomatous eyes depends on the reproducibility of the calculated parameters. Since the definition of the standard reference plane in software version 1.11 of the Heidelberg Retina Tomograph has been changed, intrapapillary parameters depend directly on the position of the contourline in the sector between -10 degrees to -4 degrees, and therefore on the observer variability to determine the disc border. We evaluated intra- and interobserver variability and present a simple approach to increase reproducibility.

The disc border of 4 glaucomatous eyes, 3 ocular hypertensive eyes and 3 eyes of healthy subjects were traced by two observers, 5 times using the free draw mode and 5 times by the addition of contourline circles.

We found a median variability of the mean disc radius in sector -10 degrees to -4 degrees of 51 microns, which defines the position of the standard reference plane, resulting in a median variability of the position of the standard reference plane of 33 microns which caused a variability of 81 microns2 of the cup area. Addition of contourline circles smoothing the final contourline along the border of the optic disc resulted in a decrease of the coefficient of variation of the standard reference plane of 3.76% (6.76% vs. 3.0%), of the cup area of 2.34% (3.87% vs. 1.53%) and of the rim volume of 3.41% (9.75% vs. 6.34%).

The calculation of the cup area using software version 1.11 of the Heidelberg Retina Tomograph depends on observer variability. The addition of contourline circles to define the final contourline along the disc border increases reproducibility. However, in follow-up of topometric examinations of the optic nerve head the software supported transfer mode should be used. Comparing topometric data of an individual optic disc in follow-up suppose the same definition of the contourline. Therefore, topometric data evaluated using software version 1.10 or earlier needs to be recalculated.