Multicentre study: reliability and repeatability of Scheimpflug system measurement in keratoconus

To assess the repeatability and reliability of the most important tomographic parameters for characterising keratoconus measured with a Pentacam HR (high resolution).

Overall, 230 eyes in 158 patients with keratoconus were analysed. We performed five consecutive corneal tomography examinations for each eye with a Pentacam HR in patients with keratoconus. Study eyes were classified into three groups depending on the maximum posterior elevation (max_BFS_post): grade 1 for cases of keratoconus with a max_BFS_post of 40 µm; grade 2 for those with a max_BFS_post of between 41 and 75 µm and grade 3 for those with a max_BFS_post of over 75 µm. We calculated the intraclass correlation coefficients (ICCs) and repeatability limits of parameters from tomography and aberrometry.

All the parameters were found to have excellent ICCs (0.9). The repeatability limits for the key parameters were higher than 0.5D for the power parameters, 20° for the axis of corneal astigmatism and 10 µm for the thinnest corneal thickness. Further, we obtained repeatability limits of above 0.1 µm for the aberrometry values and overall greater than 15° for the coma axis. All the values increase with the severity of keratoconus, except for that of the coma axis which falls with keratoconus grade.

The reliability indicated by ICCs supports the view that the Pentacam HR is useful for the diagnosis of keratoconus. The repeatability limits suggest that new criteria should be established for monitoring progression taking into account the real measurements that can be made using this system.