Defining progressive keratoconus

“Where I work, surgical procedures are often only funded if there is evidence of progressive keratoconus,” said Dr Tom Flynn of England. “But how does one define progression? Using visual acuity, refraction, tomography? I prefer tomography, but have always found it difficult to determine whether the tomographic progression is truly occurring, or whether it is at least in part due to measurement variability.”

Dr Flynn presented the results of a study that addresses this problem at the EuCornea Congress. 100 eyes of 100 subjects with keratoconus with no previous surgery or scarring underwent scanning with a Pentacam HR. “The goal was to measure both the intra- and interobserver agreement of corneal tomographic parameters,” said Dr Flynn, based on Kmax and thinnest location parameters.

The results showed that Kmax measurements on Pentacam are less repeatable, meaning comparison of repeat measurement by a single operator, and less reproducible, meaning comparison of measurements by two different operators, as keratoconus becomes more advanced. “When Kmax >55 D, this parameter may not be useful for identifying progression,” said Dr Flynn. “However,” he continued, “when Kmax <55 D, a change of more than one dioptre is likely to represent progression, as is thinning of 21µm or more at the thinnest area of the cornea.”

Corneal measurements made using Scheimpflug tomography are less reliable in keratoconic eyes than has been reported in normal eyes. These limitations should be borne in mind when using tomography to monitor for progression of keratoconus.