Introduction
As advances in diagnosis and treatment of keratoconus emerge, classification and staging become essential to monitor progression of the disease and response to treatment. There is currently no consensus on classification systems for keratoconus, and several classification systems have been proposed. Although cone morphology remains the simplest descriptive classification, other more complex classifications systems related to disease severity and risk for progression are more commonly used in clinical decision-making.
The scope of this chapter is the review of the different classification systems to provide the reader with various approaches when making decisions on treatment and assessing results.
Cone Morphology
A simple way to approach classification is based on cone morphology. This approach is relevant in contact lens fittings and potential surgical planning. , Cone morphology can be evaluated on the tangential anterior curvature map. The types of cones include round (nipple) and oval cones. Nipple cones are typically smaller in diameter, and central or paracentral, often located inferonasally ( Fig. 7.1 ), whereas oval cones are ellipsoid in shape and usually located in the inferotemporal meridian ( Fig. 7.2A,B ). Although morphology has relevance, this classification does little to impart significant understanding of the severity of current disease or future prognosis.
Keratoconus Disease Severity Classification Systems
Many systems have been proposed ( Table 7.1 ); the most commonly referred to are the Amsler-Krumeich classification, the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) study classification, , and the more recent Belin ABCD classification. Other less utilized proposed systems include the Keratoconus Severity Score, the Alió-Shabayek classification, and the anterior segment optical coherence tomography (AS-OCT) classification.
| System Name | Date Instituted | Primary Variables | Technology Used | ||
|---|---|---|---|---|---|
| Amsler-Krumeich , | 1938, modified 1998 | K m , corneal thickness, manifest refraction, scarring | Manual keratometry | ||
| CLEK study , | 1996 | K m | Manual keratometry | ||
| Alió-Shabayek | 2006 | K m , thickness, RMS HOA, scarring | Placido-disk videokeratography | ||
| Keratoconus Severity Score | 2006 | Slit-lamp findings, topographic pattern, corneal power, RMS HOA | Placido-disk topography | ||
| AS-OCT | 2013 | Corneal thickness, opacity | Fourier-domain optical coherence tomography | ||
| Belin ABCD | 2016 | ARC, PRC, thickness, CDVA, scarring | Scheimpflug tomography |
AMSLER-KRUMEICH CLASSIFICATION (1938, MODIFIED 1998)
The Amsler-Krumeich ( Table 7.2 ) is the oldest and most commonly used classification system. Corneas are graded according to severity of ectatic disease, ranging from stage 1 to 4, with 4 being the most severe. This system utilizes apex anterior corneal curvature, apex corneal thickness, manifest refraction, and presence or absence of corneal scarring.
| Severity | K m (sim k, D) | Thickness (μm) | Myopia and Astigmatism (D) | Cornea | ||
|---|---|---|---|---|---|---|
| 1 | <48 | >500 | <5.00 | Eccentric steepening, no central scars | ||
| 2 | 48–53 | 400–500 | 5.00 to <8.00 | No central scars | ||
| 3 | 54–55 | 200–400 | 8.00 – 10.00 | No central scars | ||
| 4 | >55 | <200 | Not measurable | Central scars |
The main limitations of this system are that it predates many of the current imaging modalities currently used to diagnosis keratoconus, relies on subjective clinical judgment as diseased corneas may fit into more than one stage, and does not provide easy monitoring of progression across stages.
Most importantly, as measurements are taken at the apical surface, they may not reflect the severity of the cone, which in the majority of cases is displaced.
CLEK STUDY CLASSIFICATION (1996)
The CLEK study was an observational study undertaken to identify risk for severity and progression of keratoconus. Based on keratometric readings, patients were classified as mild (steep keratometry [K] < 45 diopters [D]), moderate (steep K between 45 D and 52 D), or severe (steep K >52 D) ( Figs. 7.3 – 7.5 ). , Other factors that were recorded were high- and low-contrast visual acuity, manifest refraction, fluorescein patterns in habitual contacts lens users, and slit-lamp biomicroscopic changes, most importantly related to scarring ( Table 7.3 , Figs. 7.6 – 7.8 ), first definite apical clearance contact lens (FDACL; the flattest rigid contact lens that demonstrated apical clearance), and patient-reported quality of life. ,
| Grade | Overall Scarring | ||
|---|---|---|---|
| 1.0 | Trace and not on line of sight, <1.5 mm total size | ||
| 2.0 | Easily noticeable and approaching line of sight, 1.5–2.5 mm total size | ||
| 3.0 | Dense but translucent and impinging on line of sight, total size 2.5 mm or greater | ||
| 4.0 | Opaque and on line of sight, size 2.5 mm or greater |
ALIÓ-SHABAYEK CLASSIFICATION (2006)
With the adoption of newer technology, higher-order aberrations were incorporated into a classification system using Placido-disk topography to measure Zernike coefficients. The increase in higher-order aberrations in keratoconic corneas is mainly due to coma-like aberrations. , In this classification system, the root mean square value of coma-like aberrations is added to mean keratometry, corneal thickness, and biomicroscopic evidence of corneal scarring ( Table 7.4 ).
| Severity | K m (sim k, D) | Thickness (μm) | RMS of Coma-LikeAberrations (μm) | Cornea | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| 1 | <48 | 1.50–2.50 | No central scars | |||||||
| 2 | 48–53 | >400 | 2.50–3.50 | No central scars | ||||||
| 3 | 53–55 | 300–400 | 3.50–4.50 | No central scars | ||||||
| 4 | >55 | 200–300 | >4.50 | Central scars |
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