The Impact of Irregular Corneal Shape Parameters on Visual Acuity and Contrast Sensitivity
Authors: Liduma Sanita , Luguzis Artis , Krumina Gunta 
Published: July 2020

Abstract
Background: To understand which irregular corneal parameters determine the visual quality in keratoconus subjects. Methods: The cross-sectional study examined thecovered eyes of 44 subjects, graded from the first to third keratoconus stages by Amsler-Krumeich classification. To characterize the shape of cornea, wWe obtained measurements in two ways: (a) by projectinged two perpendicular axes onto a cornea (– the main axis passed first, through the central point of the cornea (visual axis projection) and keratoconus apex; second, as , while the second axis was perpendicular to the main axis) to – and read the elevation values at points on theses axeis as parameters characterising the corneal surface; (b) by projectinged circles with different diameters around the central part of the cornea (1, 2, and 3 mm) and reading elevation values at points equally displaced on these circles as parameters characterising an anterior surface slope. Irregular corneal shape parameters’ correlations with visual acuity and contrast sensitivity were determined The measurements were used to calculate various elevation change (slope) parameters, in order to understand which corneal slope parameter has the strongest correlation with visual acuityquality and contrast sensitivity.Results: According to the acquired data, the visual acuity of a corrected eye does not have a strong correlation with the measured keratoconus apex slope. Parameters characterising the corneal surface’s correlations with contrast sensitivity were from r=0.25 (p=0.03) at 3 cpd to r=0.47 (p<0.01) at 9 cpd for the highest elevation and from r=0.33 (p=0.09) at 5 cpd to r=0.40 (p<0.01) at 11 cpd for the lowest elevation in all subjects together, while for visual acuity the parameters were r=0.30 (p<0.01) for the highest elevation and r=0.21 (p=0.06) for the lowest elevation in all subjects together. The correlation between contrast sensitivity and the highest and lowest corneal point in all measured cornea was stronger for subjects with a peripheral corneal apex than for those with a central apex. In keratoconus subjects, Ccontrast sensitivity displayed a strong correlation with keratoconus slope in the central part of the cornea (with a radius of 1 mm). Correlations in different spatial frequencies ranging from 0.48 (p<0.01) at 3 cpd to 0.61 (p<0.01) at 9 cpd.Conclusion: Contrast sensitivity has a higher correlation with corneal shape parameters than with is more important parameter which describes the visual quality of keratoconus subjects than visual acuity. Subjects with a peripheral corneal apex had stronger correlations with visual acuity and contrast sensitivity than did subjects with a central apex. In keratoconus subjects, Tthe strongest correlation was for contrast sensitivity and elevation (slope) in the region most important region which determines the visual quality in keratoconus subjects is the region within a 1 mm radius of the corneal centre in the opposite direction of the keratoconus apex (direction (ax) CB).