Influence of the contourline on the cup area using the Heidelberg Retina Tomograph

Background 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. Method 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. Results We found a median variability of the mean disc radius in sector -10 degrees to -4 degrees of 51 mu m, which defines the position of the standard reference plane, resulting in a median variability of the position of the standard reference plane of 33 mu m which caused a variability of 81 mu m(2) 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%). Conclusion 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.