Keratoconus apex positions impact on visual acuity and contrast sensitivity

The change of corneal shape in keratoconus subjects can impact the optical quality of the eye on the retina and it reduces the contrast sensitivity by light scattering. The aim of our study was to estimate the keratoconus subjects' contrast sensitivity and visual acuity depending on keratoconus apex position. We included 45 keratoconus subjects (77 eyes), which have keratoconus in the first to the third stage, in our study. There were 33 eyes with keratoconus apex in the central part of the cornea and 46 eyes with keratoconus apex in the periphery of the cornea. Contrast sensitivity and visual acuity were measured at 3 m with and without the best possible spectacle correction using the FrACT program 3.9.3. The contrast sensitivity was measured at following frequencies - 1, 3, 5, 7, 9, 11, 13, and 15 cpd. The results showed that keratoconus subjects have lower contrast sensitivity in all spatial frequencies than subjects without pathological changes. The lowest contrast sensitivity was in keratoconus subjects with a central apex position compared to contrast sensitivity with peripheral apex position. The difference of contrast sensitivity between subjects with and without pathology increased up to 11 cpd but remained rather constant at the highest spatial frequencies - 11, 13, and 15 cpd. There was statistically significant difference between the median of visual acuity in subjects with central apex (0.42 decimal system) and subjects with peripheral apex (0.60 decimal system).