title: Keratoconus: an analysis of corneal asymmetry.
abstract: BACKGROUND
  Keratoconus, a non-inflammatory corneal ectasia, is reported to have bilateral involvement in over 90% of patients. The purpose of this study was to quantify the extent of asymmetry of disease at presentation to a regional corneal clinic.
  
  
  METHODS
  Eighty three patients diagnosed at presentation, using a combination of videokeratography, slit lamp examination, and refractive findings were retrospectively selected. On this basis, 73 patients were designated as having evidence of keratoconus in both eyes. In order to quantify the degree of asymmetry between fellow eyes in these bilateral patients, intraclass correlation was calculated for best spectacle corrected visual acuity (BSCVA) and for 13 different topographical indices generated using videokeratography. In order to examine the link between each index and visual function, the intrapatient differences in each index were compared to the intrapatient differences in BSCVA using Pearson's correlation.
  
  
  RESULTS
  BSCVA showed a high degree of asymmetry between fellow eyes with a correlation coefficient of r = 0.006. With the exception of area analysed, all of the topographical indices also showed disparity between paired eyes (r = 0.01 to r = 0.25). Pearson's analysis found that the intrapatient differences in the standard deviation of the power (SDP), average corneal power (ACP), central corneal power (K), as well as the composite keratoconus prediction index (KPI) inversely correlated with the intrapatient differences in best spectacle corrected acuity (r = -0.76,-0.75,-0.69, and -0.73 respectively).
  
  
  CONCLUSIONS
  This study demonstrates, quantitatively, the asymmetry of disease found in patients at the point of initial diagnosis of keratoconus. It also suggests that increases in indices which reflect various aspects of corneal power as well as the composite index KPI correlate with a decrease in BSCVA.
authors.affiliation: Department of Ophthalmology, University of Ulster, Coleraine, Co Londonderry, UK. dm.burns@ukgateway.net
authors.forename: D M
authors.initials: DM
authors.lastname: Burns
authors.forename: F M
authors.initials: FM
authors.lastname: Johnston
authors.forename: D G
authors.initials: DG
authors.lastname: Frazer
authors.forename: C
authors.initials: C
authors.lastname: Patterson
authors.forename: A J
authors.initials: AJ
authors.lastname: Jackson
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D003315:Cornea / Q000503:physiopathology; D019781:Corneal Topography; D006801:Humans; D007640:Keratoconus / Q000473:pathology* / Q000503:physiopathology; D008875:Middle Aged; D012189:Retrospective Studies; D012720:Severity of Illness Index; D014792:Visual Acuity