Differences in retinal images quality between normal and subclinical keratoconus and its association to posterior corneal asymmetry

Purpose To compare retinal image quality in subclinical keratoconus and normal eyes using a double-pass aberrometer and to correlate it with posterior surface deformation. Methods Sixty normal corneas were compared to 20 subclinical keratoconus (SKC) corneas. Retinal image quality was assessed using a double-pass system in all the eyes. The objective scatter index (OSI) modulation transfer function (MTF) cutoff, Strehl ratio (SR), and Predicted Visual Acuity (PVA) values at 100%, 20%, and 9% were calculated and compared between the groups. Posterior corneal asymmetry was measured using a combined Placido Dual Scheimpflug Analyzer, and correlations were tested with all optical quality parameters. Results Significant decrease in optical quality parameters was observed in SKC eyes compared with that in normal eyes. Subclinical KC eyes expressed greater scattering (OSI = 0.66 +/- 0.36 vs 0.47 +/- 0.26) and reduced contrast images (MTF and SR) than normal eyes, with 38.82 +/- 9.4 and 0.22 +/- 0.04, and 44.35 +/- 7.1 and 0.24 +/- 0.04, respectively. The reduction in the image contrast parameters (MTF and SR) was strongly correlated to the level of posterior corneal asymmetry in SKC. The greater the posterior asymmetry, the more affected was the image contrast, with r = -0.63 and -0.59, respectively for MTF and SR. Conclusion Retinal image quality was significantly more affected in eyes with subclinical keratoconus than in normal eyes. The reduction in optical quality observed in subclinical keratoconus was strongly associated with the increased asymmetry of the posterior cornea.