Comparison of the quantitative contrast sensitivity function between early keratoconus and normal eyes

To compare the characteristics of the quantitative contrast sensitivity function (qCSF) in eyes with early keratoconus (Early-KC) and normal control (NC) eyes and investigate the associated factors.

A cross-sectional study.

This study included 43 eyes of 43 patients with Early-KC (including subclinical keratoconus [SKC] and forme fruste keratoconus [FFKC]) and 77 NC eyes of 77 participants with corrected distance visual acuity (CDVA) all ≥ 20/20. Contrast sensitivity (CS) was assessed using the qCSF tests. Subgroup analysis was performed according to keratoconus type(SKC and FFKC) and astigmatism(cylindrical refraction >-1.0D or ≤-1.0D).

Sex ratio, spherical refraction, and spherical equivalent (SE) varied significantly between the two groups (all P < 0.01). The area under log CSF (AULCSF), CSF Acuity, and CS at low (1.0 and 1.5 cycles per degree [cpd]) and high (12.0 and 18.0 cpd) spatial frequencies decreased significantly in the Early-KC group than that in the NC group (all P < 0.05). The subgroup analysis revealed a similar decrease in the SKC group (all P < 0.05). AULCSF, CSF Acuity, and CS at high spatial frequencies of patients with cylindrical refraction ≤-1.0D in the Early-KC group decreased significantly (all P < 0.05) than those in the NC group. The index of vertical asymmetry and index of height decentration correlated negatively with CS at 1.5 cpd (r= -0.321 and -0.306; both P < 0.05).

CS decreased significantly at low and high spatial frequencies in Early-KC, though with normal CDVA. The qCSF test can sensitively reflect visual performance in early keratoconus.