Purpose To comparatively evaluate the image quality in normal and forme fruste keratoconus patients using a double-pass system.

Methods Fourteen normal eyes and 15 eyes with forme fruste keratoconus were included. Corneal tomographic parameters were obtained with optical coherence tomography. Image quality characteristics were determined by an optical quality analysis system (OQAS). Two readings were obtained from each eye. The following parameters were analysed: modulation transfer function cutoff frequency (MTF), Strehl ratio and OQAS values (OV) at contrasts of 100% (OV-100), 20% (OV-20) and 9% (OV-9). Repeatability coefficient (RC), coefficient of variation (CVw) and intraclass correlation coefficient (ICC) were evaluated.

Results There were no significant differences in central corneal thickness, anterior best-fit sphere and posterior best-fit sphere in both groups. There was a significant difference between the mean MTF (p=0.001), Strehl ratio (p<0.001), OV-100 (p=0.001), OV-20 (p<0.001) and OV-9 (p<0.001) values between both groups. The coefficients of variation were 4.10 (MTF) and 13.77 (Strehl ratio) in normal eyes and 14.53 (MTF) and 14.97 (Strehl ratio) in eyes with forme fruste keratoconus. The ICC values were 0.99 and 0.95 for MTF and, 0.80 and 0.91 for Strehl ratio, in normal and forme fruste keratoconus eyes, respectively. The corresponding repeatability values were 3.53 and 6.15 for MTF and, 0.06 and 0.04 for Strehl ratio, respectively.

Conclusions OQAS has the potential to be a useful diagnostic tool in cases with early keratoconus. The repeatability of a double-pass (DP) system was good in normal as well as forme fruste keratoconus eyes.