title: The evidentiary significance of case reports: eye rubbing and keratoconus.
abstract: OBJECTIVE
  Individual case reports (observational or interventional) are thought to have a very low ranking for evidentiary significance. This clinical report examines the relationship between the evidentiary significance of case reports and their presentation.
  
  
  METHODS
  An observational report of unilateral keratoconus (KC), that developed in association with a corresponding unilateral chronic habit of abnormal rubbing, is examined to identify features that can raise the level of evidentiary significance.
  
  
  RESULTS
  A feature of this type of case is the opportunity to compare the eye with KC to the unaffected eye, for which there was an unremarkable rubbing history. The unaffected eye serves as a retrospective unintentional control for the affected eye. Detailed assessment to establish the normality of the apparently normal eye is crucial to the differential diagnosis between a markedly asymmetric presentation and a true unilateral presentation. Any history of systemic diseases that may be associated with the presenting disease can be important. Longitudinal follow-up, especially if postonset data are supplemented with preonset data, can contribute to the evidentiary base by allowing for comparison with the natural history of the relevant disease. Intervention to remove a suspected causal factor, when combined with appropriate follow-up, can provide additional evidence.
  
  
  CONCLUSIONS
  Depending on their presentation, some types of case report can have a significant evidence-based function in assessing causal hypotheses and, accordingly, provide a higher evidentiary ranking than generally assigned to case reports. The potential for a higher evidentiary ranking may be more important when randomized clinical trials are contraindicated for ethical reasons, or there are challenging problems with study design. Wider agreement regarding the quantitative and qualitative criteria for diagnosing the earliest stages of KC would aid the assessment of the evidentiary significance of case reports of this disease.
authors.affiliation: School of Optometry and Vision Science, University of New South Wales, Kensington, Australia. cmcmonnies@unsw.edu.au
authors.forename: Charles W
authors.initials: CW
authors.lastname: McMonnies
mesh_terms: D000328:Adult; D005500:Follow-Up Studies; D005820:Genetic Testing; D006801:Humans; D007640:Keratoconus / Q000209:etiology* / Q000235:genetics; D008137:Longitudinal Studies; D008297:Male; D008487:Medical History Taking; D015588:Observer Variation; D010812:Physical Stimulation / Q000009:adverse effects*