The HRT II (software version 3.0, Heidelberg Engineering, Dossenheim, Germany) was used to acquire CSLO images in the study. It uses confocal scanning laser principles to obtain a three-dimensional (3-D) topographic image of the optic nerve. Its principles of working have been described in detail elsewhere.
7 For each patient, three topographical images were obtained and were combined and automatically aligned to make a single mean topography used for analysis. Magnification errors were corrected using patients’ corneal curvature measurements. An experienced examiner outlined the optic disc margin on the mean topographic image while viewing stereoscopic photographs of the optic disc. Good quality images required a focused reflectance image with a standard deviation not greater than 50 μm.
Topographical parameters included with the HRT software and investigated in this study were disc area, cup area, rim area, cup/disc area ratio, rim/disc area ratio, cup volume, rim volume, mean cup depth, maximum cup depth, mean height contour, height variation contour, cup shape measure, mean RNFL thickness, RNFL cross-sectional area, linear cup/disc ratio, and two linear discriminant functions (LDFs), from Mikelberg et al.
8 (LDF Mikelberg) and Bathija et al.
9 (LDF Bathija). The software on HRT II also incorporates the Moorfields regression analysis (MRA) which is a comparison of the subject’s rim area to a predicted rim area for a given disc area and age, based on confidence limits of a regression analysis derived from healthy subjects included in the instrument’s normative database.
10 This database contains information from 733 eyes of white subjects, 215 eyes of African American subjects, and 104 eyes from Indian subjects. These subjects were selected based on the presence of normal IOP (<23 mm Hg), normal visual fields, no family history of glaucoma, and no history of ocular disease (Mike Sinai, PhD, Heidelberg Engineering, written communication, May 2006). Each sector is classified as within normal limits (WNL) if the measurement falls within 95% CI, borderline (BL) if the measurement falls between the 95% to 99.9% CI, and outside normal limits (ONL) if the measurement falls below the 99.9% CI. The MRA also provides results for the global rim area (MRA global) and a final classification (MRA classification). A normal MRA classification requires the MRA analysis of all sectors and the global rim area to be within normal limits. A borderline MRA classification occurs when at least one of the sectors or the global rim area is borderline, and an outside normal limits result occurs when at least one sector or the global rim area is outside normal limits.
The HRT 3.0 software utilizes a new manufacturer-developed automated analysis for the detection of glaucomatous damage called glaucoma probability score (GPS) which is independent of the contour line traced by the examiner around the optic disc margin.
11 It is based on a 3-D model of the entire topographical image, including the optic disc and surrounding peripapillary retinal nerve fiber layer. Five shape-based parameters are used in the model to characterize the shape of the optic disc and RNFL. Three parameters are used to characterize the optic disc: cup size (width), cup depth (depth), and rim steepness (slope). Two parameters are used to characterize the RNFL: the vertical RNFL curvature (superior to inferior curvature) and the horizontal RNFL curvature (nasal to temporal curvature). A 3-D model incorporating information from the five parameters described is then constructed for the optic disc being examined. The values of the parameters are then fed into a machine learning classifier analysis called a relevance vector machine (RVM), which compares a patient’s results to previously defined healthy and glaucomatous models. According to the manufacturer, the final GPS result is the probability or likelihood that the scan has structural characteristics that are consistent with glaucoma.