Eyeworld

EW CORNEA 34 August 2016 by Michael W. Belin, MD T he last decade has seen a dramatic advancement in the treatment of ectatic disease. Corneal collagen crosslinking (CXL), intra- corneal rings (ICR), and deep anteri- or lamellar keratoplasty (DALK) are options that earlier did not exist or were rarely used. While our treatments have advanced, our methods of staging or classifying ectatic disease have been stagnant for more than 60 years. The lack of a modern classification system has hindered our ability to properly evaluate different treatment mo- dalities. The Global Consensus on Keratoconus and Ectatic Diseases (2015) concluded that no clinically adequate classification system for keratoconus currently exists. The most widely used, albeit outdated, Amsler-Krumeich (AK) system fails to make use of the Moving keratoconus classification into the modern era ABCD Criteria A B C D ARC (3 mm zone) PRC (3 mm zone) Thinnest Pach um BDVA Scarring STAGE 0 > 7.25 mm (< 46.5 D) > 5.90 mm (< 57.25 D) > 490 um ≥ 20/20 (≥ 1.0) – STAGE I > 7.05 mm (< 48.0 D) > 5.70 mm (< 59.25 D) > 450 um < 20/20 (< 1.0) –, +, ++ STAGE II > 6.35 mm (< 53.0 D) > 5.15 mm (< 65.5 D) > 400 um < 20/40 (< 0.5) –, +, ++ STAGE III > 6.15 mm (< 55.0 D) > 4.95 mm (< 68.5 D) > 300 um < 20/100 (< 0.2) –, +, ++ STAGE IV < 6.15 mm (> 55.0 D) < 4.95 mm (> 68.5 D) ≤ 300 um < 20/400 (< 0.05) –, +, ++ Table 1. Scarring: clear, no scarring (–), scarring, iris details visible (+), scarring, iris obscured (++). Diopters are shown for anterior radius of curvature; anterior equivalent diopters are shown for posterior radius of curvature. technological advances in corneal imaging. Specifically, the posterior corneal surface and full pachymet- ric data are not utilized. In the AK system, the severity of keratoconus is graded from stage 1–4 using spec- tacle refraction, central keratometry, presence or absence of scarring, and central corneal thickness. This staging, based solely on the anteri- or corneal surface, is inadequate as newer treatment modalities, such as crosslinking, may be utilized earlier in the disease process and at times prior to clinical changes on the ante- rior corneal surface. The goal of the ABCD classifica- tion was to develop a classification/ staging system that had some simi- larities to the AK system for anterior data, but addressed the following deficiencies: 1. absence of posterior data; 2. relying on apical corneal thick- ness as opposed to thinnest point; 3. failure to distinguish normal from possible pathology; and 4. lack of visual acuity measure- ments. In the Belin/Ambrosio En- hanced Ectasia Display (BAD) corneal elevation data is depicted against a reference surface called the "Enhanced Reference Surface." The concept behind the Enhanced Reference Surface was to generate a surface that more closely resembles the more normal peripheral cornea. A small diameter optical zone (3.0 mm) centered on the thinnest por- tion of the cornea is excluded from the standard 8.0 mm best fit sphere (BFS). The new enhanced surface utilizes all the elevation data from within the 8.0 mm central cornea and outside the exclusion zone. The enhanced reference surface more closely resembles the normal cornea because the exclusion zone centered on the thinnest point incorporates the major ectatic region. A similar concept can be used to stage or classify keratoconus. As op- posed to excluding a 3.0 mm zone, we utilize this zone centered on the thinnest point as this area represents the ectatic region better than a single point parameter such as Kmax or maximal elevation. The new grad- ing system called ABCD looks at the anterior (A) and posterior, or back (B) radius of curvature taken from the 3.0 mm zone centered on the thinnest point, thinnest corneal (C) pachymetry, distance (D) best cor- rected vision, and adds a modifier (–) for no scarring, (+) for scarring that does not obscure iris details, and (++) for scarring that obscures iris details (Table 1). This grading system is relatively simple to use and has the advantage of grading each component inde- pendently, recognizing subclinical disease, and adding a stage 0 to better reflect an absence of possible Figure 1. Sample application of new ABCD grading system Source: Michael Belin, MD

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