Comparison of computerized corneal topography systems in assessing keratoconus

Purpose. Computerized corneal topography systems have been used to diagnose, analyze and map the progress of keratoconus. Two topography systems in widespread clinical use include the PAR (rasterstereography) and the EyeSys (placido disc). The purpose of this study is to compare color coded topographic maps produced by these systems in assessing keratoconus. Methods. Corneal images of 16 patients (23 eyes) clinically diagnosed with keratoconus were obtained by using EyeSys and PAR. Qualitative and quantitative analyses were performed for the following parameters: shape, location, steepest and flattest points, cylindrical value, and distance of steepest point from visual axis were noted and compared with manifest refraction and autokeratometric readings. Results. When PAR and EyeSys measurements for steepest or flattest points are compared, an average difference was found that is statistically significant (p = 0.037). The values obtained from these systems in regard to the distance to the steepest region are statistically in agreement. Comparing steepest and flattest readings of PAR and EyeSys to the reference criterion from Topcon™ AutoKeratometer showed no significant discrepancy. Evaluating the qualitative measurements of shape and location we found no difference in these parameters for either system. Conclusions. Our findings suggest that comparing the PAR and EyeSys measurements of the steepest region have very little relationship to each other. In the absence of a reference criterion, it is impossible to state which, if either, of these methods is accurate. Comparing these two systems with the measurements obtained by the AutoKeratometer found them to be consistent with the reference values and thus serve as a helpful tool in assessing keratoconus, though further comparative investigative studies are needed.