Development of the standard reference plane for the Heidelberg retina tomograph

Background: Topometry of the optic disc is the quantitative assessment of t he structure of the optic nerve head by means of three-dimensional paramete rs. The parameter values depend on definitions of intraocular reference pla nes. Purpose: To describe the development of intraocular reference planes i n laser scanning tomography for the Heidelberg Retina Tomograph (HRT) using image intrinsic data with a fixed offset reference plane (320 mu m) and to present a contour-line-based "flexible" standard reference plane ("SRP") f or calculation of intrapapillary stereometric parameters taking the interin dividual variability of optic disc topography into account. Methods: Ten-de gree triple images were obtained by laser scanning tomography from 99 glauc oma eyes and 180 normal eyes. The images were evaluated to assess the varia bility of height measurements of an optic disc border contour-line segment (6 degrees width) corresponding to the site of the papillo-macular bundle a s indicated by the average optic disc surface inclination angle. Results: T he average optic disc sur-face inclination angle was -7 degrees+/-3 degrees below the horizontal meridian (0 degrees). The 6 degrees wide contour-line segment for the SRP was chosen according to the average surface inclinatio n angle (-10 degrees to -4 degrees). The reproducibility of the SRP-segment height measurements was 16.0+/-10.8 mu m for normal eyes and 23.4+/-18.0 m u m for glaucoma eyes. To ensure that the automatic reference level determi nation for intrapapillary parameters remained below the disc border height, we defined the SRP level at a 50 mu m offset (>2 SD of average segment hei ght reproducibility in glaucoma) added to the individual height position of the 6 degrees contour line segment. Conclusion: The flexible standard refe rence plane allows for automatic determination of intrapapillary variables once a disc border contour line is interactively defined. In contrast to a fixed offset reference plane (e.g. 320 mu m below the mean retina height), the interindividual variability of optic disc topography (oblique insertion , glaucomatous surface flattening) is respected at the cost of the need for an accurate optic disc border outline.