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

Abstract

Several methods have been described to both evaluate and document progression in keratoconus and to show efficacy of cross-linking, however, there are no consistent generally accepted parameters. Modern corneal tomography, including both anterior and posterior elevation and pachymetric data can be used to screen for ectatic progression, be employed to detect earlier change and additionally to show efficacy of new treatment modalities, such as crosslinking.To describe specific quantitative values that can be used as progression and efficacy determinants, the normal noise measurements of the three parameters used in the ABCD keratoconus classification (corneal thickness at the thinnest point, anterior and posterior radius of curvature taken from the 3.0 mm optical zone centered on the thinnest point), were determined. Values were obtained from both a normal population and a known keratoconic population. The 80 and 95% one-sided confidence intervals for all three parameters were surprisingly small, suggesting that they may perform well as progression and efficacy determinants.

Corneal Ectasia Preferred Practice Pattern®

“…The US Food and Drug Administration (FDA) approved the procedure for patients between the ages of 14 and 65 years with progressive keratoconus or corneal ectasia following keratorefractive surgery. No consistent or clear definition of ectasia progression has been identified,98 yet several tomography-derived values (alone or in combination) have been evaluated as progression determinants 99,100. These include maximum keratometry, steepening of the anterior or posterior corneal surface and thinning and/or an increase in the rate of corneal thickness change from the periphery to the thinnest point are some of these determinants as well refractive changes such as increasing myopia and astigmatism.Technique OptionsThe original Dresden protocol for CXL ("conventional" CXL) involved removal of the corneal epithelial layer, application of topical riboflavin every 2 minutes for 30 minutes to saturate the cornea, followed by 30 minutes of UV-A light treatment with continued instillation of riboflavin (again, every 2 minutes) until UV-A treatment is completed 102.…”

Evolution of Keratoconus: From Diagnosis to Therapeutics

“…This flattens the overall reference surface and increases the elevation difference of the ectatic region, allowing it to be more easily identified [22]. In a normal cornea, this difference is relatively minor [22]. The difference between the elevation values using the BFS and the ERS was found to be highly predictive in screening for ectatic disease.…”

“…The ERS excludes a 3-4-mm zone centered around the thinnest point on the cornea from the reference surface. This flattens the overall reference surface and increases the elevation difference of the ectatic region, allowing it to be more easily identified [22]. In a normal cornea, this difference is relatively minor [22].…”

“…Because the first 3 parameters are centered around the thinnest point, they should identify changes sooner than other more global measurements (▶ Fig. 4) [22,44].…”

Is this keratoconus?

“…Subsequent assessments of the cornea by the same instrument will provide evidence of progression, which can help in the counselling and follow-up of patients who might benefit from corneal cross-linking. 9 However, it is important to consider the limits of reproducibility and accuracy with these instruments that should not be mistaken for progression. 10 In addition to measuring corneal shape and thickness, technologies have evolved that aim to measure corneal biomechanics.…”