Scanning-slit topography in patients with keratoconus

To evaluate the anterior and posterior corneal surfaces using scanning-slit topography and to determine the diagnostic ability of the measured corneal parameters in keratoconus.

Orbscan II measurements were taken in 39 keratoconic corneas previously diagnosed by corneal topography and in 39 healthy eyes. The central minimum, maximum, and astigmatic simulated keratometry (K) and anterior axial power values were determined. Spherical and cylindrical mean power diopters were obtained at the central and at the steepest point of the cornea both on anterior and on posterior mean power maps. Pachymetry evaluations were taken at the center and paracentrally in the 3 mm zone from the center at a location of every 45 degrees. Receiver operating characteristic (ROC) analysis was used to determine the best cut-off values and to evaluate the utility of the measured parameters in identifying patients with keratoconus.

The minimum, maximum and astigmatic simulated K readings were 44.80±3.06 D, 47.17±3.67 D and 2.42±1.84 D respectively in keratoconus patients and these values differed significantly (P<0.0001 for all comparisons) from healthy subjects. For all pachymetry measurements and for anterior and posterior mean power values significant differences were found between the two groups. Moreover, anterior central cylindrical power had the best discrimination ability (area under the ROC curve=0.948).

The results suggest that scanning-slit topography and pachymetry are accurate methods both for keratoconus screening and for confirmation of the diagnosis.