Definitions for Keratoconus Progression and Their Impact on Clinical Practice

Abstract

Purpose: There is currently no consensus on which keratoconus need cross-linking nor on how to establish progression. This study assessed the performance of diverse progression criteria and compared them with our clinical knowledge of keratoconus evolution. Methods: This was a retrospective, longitudinal, observational study. Habitual progression criteria, based on (combinations of) keratometry (KMAX), front astigmatism (AF), pachymetry (PMIN), or ABCD progression display, from 906 keratoconus patients were analyzed. For each criterion and cutoff, we calculated %eyes flagged progressive at some point (RPROG), individual consistency CIND (%examinations after progression detection still considered progressive), and population consistency CPOP (% eyes with CIND >66%). Finally, other monotonic and consistent variables, such as front steep keratometry (K2F), mean radius of the back surface (RmB), and the like, were evaluated for the overall sample and subgroups. Results: Using a single criterion (e.g., ∆KMAX >1D) led to high values of RPROG. When combining two, (KMAX and AF) led to worse CPOP and higher variability than (KMAX and PMIN); alternative criteria such as (K2F and RmB) obtained the best CPOP and the lowest variability (P<0.0001). ABC, as defined by its authors, obtained RPROG of 74.2%. Using wider 95% confidence intervals (95% CIs) and requiring two parameters over 95CI reduced RPROG to 27.9%. Conclusion: Previous clinical studies suggest that 20% to 30% of keratoconus cases are progressive. This clinical RPROG value should be considered when defining KC progression to avoid overtreatment. Using combinations of variables or wider margins for ABC brings RPROG closer to these clinical observations while obtaining better population consistency than current definitions.