• Cornea/External Disease

    Review of: Keratoconus staging by decades: A baseline ABCD classification of 1000 patients in the Homburg Keratoconus Center

    Flockerzi E, Xanthopoulou K, Goebels S, et al. British Journal of Ophthalmology, August 2021

    Records from the Homburg Keratoconus Center (Germany) were used to evaluate the utility of the Belin ABCD grading system for diagnosing keratoconus and staging progression.

    Study design

    This retrospective, cross-sectional, single-center study analyzed the distribution of the Belin ABCD keratoconus parameters in various age groups among 1917 corneas in 1000 patients at a single academic medical center. The Belin ABCD grading system defines keratoconus according to 4 parameters: A = anterior curvature 3 mm from the thinnest corneal pachymetry; B = posterior curvature 3 mm from the thinnest corneal pachymetry; C = thinnest corneal pachymetry; and D = best corrected distance visual acuity. Each parameter is staged on a scale of 0–4, with 0 denoting the lowest abnormality of the parameter and 4 denoting the highest abnormality.


    The highest prevalence of keratoconus was in patients aged between 21 and 30 years; the staging distributions were similar in all age groups. The anterior curvature parameter (A) was more often at a lower stage than the posterior curvature parameter (B) such that the frequency of A0 > B0, A1 > B1, and A2 > B2. The A4 | B4 combination was the most frequently observed combination (n = 451). More corneas were classified as B4 than A4, and A3 and B3 occurred at the same frequency. The most frequent staging of corneal pachymetry (C) was C0 and the most frequent stage of visual acuity (D) was D1. These data indicate that early keratoconus became manifest in the posterior cornea before the anterior cornea, with mild changes to corneal pachymetry and vision.


    These results may not accurately represent ABCD parameters of early keratoconus in the general population because the study site was a surgical referral center for advanced keratoconus and therefore subject to selection bias (e.g., the high frequency of the A4|B4 combination). The study measured best-corrected distance visual acuity with rigid gas-permeable lenses for 36% of the cohort but not for the remaining 64%, who  were assessed with spectacles. Contact lens holidays were 3 days rather than the customary 14 days, which may have introduced artifactual data from corneal warpage.

    Clinical significance

    By taking into account the posterior corneal curvature, which manifests abnormalities earlier than anterior corneal curvature, physicians can use the Belin ABCD grading system to help improve the diagnosis and assessment of keratoconus progression.