Comparison of ranked segment analysis (RSA) and cup to disc ratio in computer-assisted optic disc evaluation
Ranked segment analysis is a new method for evaluation of optic nerve head topography with the Heidelberg Retina Tomograph. This analysis ranks sector measurements around the optic disc and compares these measurements with rank specific significance limits. We evaluated the diagnostic precision of the ranked segment analysis in a large clinical sample and compared with vertical cup/disc ratio measurements. Vertical cup/disc ratio measurements were not corrected for disc size.
We analysed optic disc images from 153 normal and 75 glaucomatous eyes using the Heidelberg Retina Tomograph (software version 2.01). Ranked segment analyses and vertical cup/disc ratio measurements were obtained from each optic disc image, and ROC curves were plotted.
At the 95% specificity level, sensitivity was significantly lower for ranked segment analysis (70%) compared to vertical cup/disc ratio measurements (85%). Almost 30% of the discs in the glaucoma group were classified as normal using the ranked segment analysis.
Ranked segment analysis yielded unacceptably poor discrimination, in fact, worse than cup/disc ratio with its known clinical limitations. Glaucomatous disc damage typically occurs at the vertical poles of the disc. High rank measurements (low sector values) in normal eyes, on the other hand, are more common in the temporal disc sectors. Important spatial information is lost during the ranking procedure. This may partly explain the low sensitivity of the ranked segment analysis observed in our study. The results raise serious concerns regarding the clinical usefulness of ranked segment analysis.