Early diagnosis of keratoconus: what difference is it making?

WHY EARLY DIAGNOSIS? In corneal refractive surgery, early diagnosis of keratoconus (subclinical asymptomatic keratoconus) is of great importance in patients seeking surgery because it can prevent progression of the pathology after surgery and make it symptomatic (corneal ectasia), thus creating the need for diagnostic tests that provide high sensitivity (ability to detect the disease in affected subjects) with the objective that no asymptomatic subclinical keratoconus ends up not being diagnosed and thus not undergo corneal refractive surgery. Recently, Uçakhan et al investigated several Pentacam parameters in subclinical keratoconus, keratoconus and normal eyes. They found that the Scheimpflug system could differentiate between ectatic and normal eyes. In this study, the optimum cut-off point for posterior elevation was found to be 26.5 mm (97.7% sensitivity and 81.0% specificity). In a study by Mihaltz et al, receiver operating characteristic (ROC) curve analysis indicated that posterior elevation was the most important criterion in the diagnosis of keratoconus. A threshold value of 15.5 mm had sensitivity of 95.1% and specificity of 94.3% for differentiating normal eyes from keratoconus. These authors found lower pachymetry readings in subclinical, early and moderate keratoconus; however, they did not find significant differences in these parameters between subclinical keratoconus and normal eyes. In the general population, the prevalence of keratoconus is 1/2000. 4 If the prevalence of a particular disease in a population is low, the positive predictive value tends to be low given that having a greater number of healthy people increases the number of false positives. As the prevalence of keratoconus in the general population is low, to reach a conclusive diagnosis in early stages we would have to reconfirm the result with a second independent test since a single positive result would not be conclusive. However, with the goal of designing screening protocols that improve the ratio of cost-effectiveness we are led to search for diagnostic criteria that maximise the prevalence of the disease in certain population groups in order to increase the positive predictive value of these diagnostic tests. The prevalence in patients with astigmatism greater than 2 Dp increased to 14.1% (6.3% of eyes had keratoconus and 7.8% had subclinical keratoconus).