Retinal Nerve Fiber Layer Measured by Heidelberg Retina Tomograph and Nerve Fiber Analyzer

PURPOSE: To compare retinal nerve fiber layer (RNFL) thickness measured by Heidelberg retina tomograph (HRT) and nerve fiber analyzer (GDx). METHODS: Twenty eyes of 20 consecutive healthy subjects were recruited for this study. Each subject had a normal visual field and a normal optic nerve head, which was assessed by slit-lamp biomicroscopy using a 90 degrees lens. Using the HRT and GDx, RNFL measurement was calculated as for software vs 2.01 and vs 1.0.14, respectively. Retinal nerve fiber layer thickness was evaluated for the entire annulus surface every 5 degrees degrees. RNFL was assessed by HRT and GDx. HRT RNFL measurement was calculated at 0 microm from the edge, while GDx RNFL measurement at 1.75 disc diameter as for software. The difference between the highest points and the deepest points was calculated and compared. Furthermore, because of the possibility of different scales in the two systems, the following ratio was calculated: superior/inferior, superior/temporal, superior/nasal, inferior/temporal, and inferior/nasal. RESULTS: When the entire RNFL thickness was considered, a significant (p<0.001) difference was found between the HRT and GDx measurements. A difference of 200 microm was found between the highest and the deepest HRT points while a difference of 40 microm was found between the highest and the deepest GDx points. CONCLUSIONS: HRT and GDx RNFL measurements were statistically different in each sector. However, ratio parameters showed no difference between the obtained values except for superior/temporal ratio and inferior/temporal ratio.