August 2016

 

CORNEA

 

Moving keratoconus classification into the modern era


by Michael W. Belin, MD

 
   

Figure 1. Sample application of new ABCD grading system Source: Michael Belin, MD

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.

The last decade has seen a dramatic advancement in the treatment of ectatic disease. Corneal collagen crosslinking (CXL), intracorneal rings (ICR), and deep anterior 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 modalities. 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The most widely used, albeit outdated, Amsler-Krumeich (AK) system fails to make use of the technological advances in corneal imaging. Specifically, the posterior corneal surface and full pachymetric data are not utilized. In the AK system, the severity of keratoconus is graded from stage 1–4 using spectacle refraction, central keratometry, presence or absence of scarring, and central corneal thickness. This staging, based solely on the anterior 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 anterior corneal surface. The goal of the ABCD classification was to develop a classification/staging system that had some similarities to the AK system for anterior data, but addressed the following deficiencies: 1. absence of posterior data;

2. relying on apical corneal thickness as opposed to thinnest point;

3. failure to distinguish normal from possible pathology; and 4. lack of visual acuity measurements.

In the Belin/Ambrosio Enhanced 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 portion 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 opposed 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 grading 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 corrected 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 independently, recognizing subclinical disease, and adding a stage 0 to better reflect an absence of possible disease. The grading system is dependent on tomography to produce both posterior data and thinnest point pachymetry, but this information could be available from any commercial tomographic unit (e.g., Scheimpflug, slit scanning, OCT). It is currently available on the Pentacam (Oculus, Wetzlar, Germany) on the “Topometric/Keratoconus Staging” display.

The greatest hindrance to a clinical adoption is the lack of familiarity ophthalmologists have in using radius of curvature instead of diopters. Radius of curvature was selected to allow the same measurement of both the anterior and posterior surfaces as radius of curvature is independent on index of refraction. The posterior corneal surface is a negative lens with a low power due to the cornea/aqueous interface. Reporting the true dioptric power of the posterior cornea would be less intuitive. A sample application of the new ABCD grading system is shown in Figure 1. This is the left cornea of a 25-year-old asymptomatic male with 4.5 D of astigmatism and best spectacle distance vision (BSCVA) of 20/25. The upper left map is the anterior sagittal curvature with a mean K of 45. The Amsler-Krumeich classification would be stage I, which fails to convey a true anatomic picture. The BAD display (upper right) demonstrates the prominent posterior ectasia with a final “D” of 6.49 (highly abnormal). The ABCD classification (bottom) is “A0 B2 C1 D1,” which conveys the relatively normal anterior surface with a more prominent posterior ectasia. The 1-line loss of BSCVA is reflected by D1.

The new ABCD classification system conveys anatomical and functional data that is missing from the Amsler-Krumeich classification. It conveys information on both anterior and posterior corneal surfaces, is centered on the thinnest point, which is typically the region of the cone, and adds a visual acuity measurement as well as an indication of corneal scarring. The ABCD classification allows for a much improved description of the keratoconic cornea than was previously possible. It also may allow for more tailored treatment plans as different surfaces of the cornea may be more amenable to different medical or surgical intervention. The ABCD classification system is currently available on the Pentacam.

Editors’ note: Dr. Belin is professor of ophthalmology and vision science, University of Arizona, Tucson, Arizona. He has financial interests with Oculus.

Contact information

Belin: mwbelin@aol.com