Assessing progression of keratoconus: novel tomographic determinants

Abstract

Several methods have been described in the literature to both evaluate and document progression in keratoconus, but there is no consistent or clear definition of ectasia progression. The authors describe how modern corneal tomography, including both anterior and posterior elevation and pachymetric data can be used to screen for ectatic progression, and how software programs such as the Enhanced Reference Surface and the Belin-Ambrosio Enhanced Ectasia Display (BAD) can be employed to detect earlier changes. Additionally, in order to describe specific quantitative values that can be used as progression determinants, the normal noise measurement of the three parameters (corneal thickness at the thinnest point, anterior and posterior radius of curvature (ARC, PRC) taken from the 3.0 mm optical zone centered on the thinnest point), was assessed. These values were obtained by imaging five normal patients using three different technicians on three separate days. The 95 % and 80 % one-sided confidence intervals for all three parameters were surprisingly small (7.88/4.03 μm for corneal thickness, 0.024/0.012 mm for ARC, and 0.083/0.042 mm for PRC), suggesting that they may perform well as progression determinants.

Evaluation of keratoconus progression

“…The cut-off value we recommend for the D-index is 0.42 to mark a keratoconus eye as progressive, so that further therapeutic steps can be made. D-index is proven to be a reliable parameter in detecting early and definite keratoconus and can highlight changes in the corneal surface earlier than, for example, Kmax 6. It has already been tested as a parameter with which to diagnose keratoconus, but our results indicate that it is suitable to detect disease progression as well since it generated the highest result in our ROC analysis.…”

“…Of course, this index has to show the best results, given that progression was defined by the same variables but nevertheless, elevation, steepening and corneal thinning are typical corneal changes when the cornea is affected by keratoconus, and so it makes sense to use these characteristics as well as use them for the definition of progression and detection of progression. Furthermore, previous groups, for example Duncan et al , found the CI for ARC and PRC to be very small in a normative cohort, suggesting that these parameters are suitable for detecting ectatic progression 6. D-index and KPI are a combination of several variables that can all be affected by the disease’s progression, and their meaning and suitability as a marker for progression is emphasised.…”

“…If we would now only use Kmax, which did not change for the definition of progression in this eye, we would then be marking wrongly—as non-progressive. Therefore, our general recommendation for the evaluation of keratoconus progression is to also use parameters that include several variables, for example the D-index, or to use at least two different parameters affected by keratoconus, for example, steepening of the front or back surface and corneal thinning 6. Recently, other groups such as Wonneberger et al evaluated the repeatability of Scheimpflug imaging for diagnosing progression.…”

“…A recent review showed that although many studies have analysed how to detect keratoconus, there are very few that describe how the progression can be evaluated and to date, there is still no reliable definition of ectatic progression 6 7. Typical changes of the cornea when affected by keratoconus are an increase in corneal elevation, decrease in corneal curvature and a decrease in corneal thickness, but not all of these changes have to occur within one patient; this is what makes defining progression remarkably difficult 6. This lack of consensus in how to define the progression of the disease endangers treatment occurring in time to stop the natural history of keratoconus and also carries the risk of possibly unnecessary treatment of non-progressive eyes.…”

Assessing Progression of Keratoconus and Cross-linking Efficacy: The Belin ABCD Progression Display

“…The standard deviation and one-sided confidence intervals for the normal database were previously reported. 56 The keratoconic data were generated by examining three repetitive exams from known keratoconus patients taken on the same day at the University of Auckland, Department of Ophthalmology (Auckland, New Zealand). All patients were fully removed from the Pentacam between exams.…”