Influence of corneal diameters on the accuracy of Scheimpflug-derived corneal tomographic indices in Chinese patients with forme fruste keratoconus or thin corneas

Background: To investigate the influence of corneal diameters (CD) on the diagnostic accuracy of Pentacam tomographic indices in eyes with forme fruste keratoconus (FFKC) or thin corneas (TC). Methods One hundred and one eyes of 101 patients with FFKC (FFKCG), 104 eyes of 104 patients with corneal thickness < 490µm (TCG), and 200 eyes of 200 normal subjects (NG) were analyzed in the study. Pentacam ectasia indices were compared between different groups. Results The results for the multiple linear regression analysis between CD groups and corneal indices adjusted for flat central power (Kf), steep central power (Ks) and maximum corneal power (Kmax) showed that, compared with ≤ 11.9mm, back corneal elevation (BE) decreased 2.35 per unit, deviation of normality of the back elevation (Db) decreased 0.45 per unit (P = 0.007) and overall deviation of normality (BAD-D) decreased 0.26 per unit (P = 0.027) on average for subjects with CD > 11.9 in the FFKCG. The comparisons for the classifications (normal/abnormal) of the individual indices showed that for CD ≤ 11.9mm, the rates of abnormal cases in the FFKCG were significantly higher than NG for seven indices; for CD > 11.9mm, the rates of abnormal cases were higher than NG only for two indices. For CD ≤ 11.9mm, the rates of abnormal cases in the TCG were higher than NG for nine indices; for CD > 11.9mm, the rates of abnormal cases were higher than NG for seven indices. Conclusion BAD indices could underestimate the risk of ectasia in patients with large cornea, especially those with FFKC.