title: Keratoconus: The ABCD Grading System.
abstract: OBJECTIVE
  To propose a new keratoconus classification/staging system that utilises current tomographic data and better reflects the anatomical and functional changes seen in keratoconus.
  
  
  METHODS
  A previously published normative database was reanalysed to generate both anterior and posterior average radii of curvature (ARC and PRC) taken from a 3.0 mm optical zone centred on the thinnest point of the cornea. Mean and standard deviations were recorded and anterior data were compared to the existing Amsler-Krumeich (AK) Classification. ARC, PRC, thinnest pachymetry and distance visual acuity were then used to construct a keratoconus classification.
  
  
  RESULTS
  672 eyes of 336 patients were analysed. Anterior and posterior values were 7.65 ± 0.236 mm and 6.26 ± 0.214 mm, respectively, and thinnest pachymetry values were 534.2 ± 30.36 µm. The ARC values were 2.63, 5.47 and 6.44 standard deviations from the mean values of stages 1-3 in the AK classification, respectively. PRC staging uses the same standard deviation gates. The pachymetric values differed by 4.42 and 7.72 standard deviations for stages 2 and 3, respectively.
  
  
  CONCLUSIONS
  A new keratoconus staging incorporates anterior and posterior curvature, thinnest pachymetric values, and distance visual acuity and consists of stages 0-4 (5 stages). The proposed system closely matches the existing AK classification stages 1-4 on anterior curvature. As it incorporates posterior curvature and thickness measurements based on the thinnest point, rather than apical measurements, the new staging system better reflects the anatomical changes seen in keratoconus.
authors.affiliation: Ophthalmology & Vision Science, University of Arizona, Marana, Arizona, United States.
authors.forename: M W
authors.initials: MW
authors.lastname: Belin
authors.affiliation: Ophthalmolgoy & Vision Science, University of Arizona, Tucson, Arizona, United States.
authors.forename: J K
authors.initials: JK
authors.lastname: Duncan
mesh_terms: D000328:Adult; D000368:Aged; D000465:Algorithms; D063171:Corneal Pachymetry / Q000379:methods / Q000592:standards*; D019781:Corneal Topography / Q000379:methods / Q000592:standards*; D018450:Disease Progression; D005260:Female; D006801:Humans; D007640:Keratoconus / Q000145:classification* / Q000175:diagnosis* / Q000473:pathology; D008297:Male; D008875:Middle Aged; D017410:Practice Guidelines as Topic; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012720:Severity of Illness Index; D014792:Visual Acuity