The Impact of Irregular Corneal Shape Parameters on Visual Acuity and Contrast Sensitivity

Abstract

Background: To understand which irregular corneal parameters determine the visual quality in keratoconus subjects. Methods: The cross-sectional study examined the eyes of 44 subjects, graded from the first to third keratoconus stages by Amsler-Krumeich classification. We obtained measurements in two ways: (a) by projecting two perpendicular axes onto a cornea (first, through the central point of the cornea and keratoconus apex; second, as the perpendicular axis) to read the elevation values at points on these axes as parameters characterising the corneal surface; (b) by projecting 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 in order to understand which corneal slope parameter has the strongest correlation with visual acuity and contrast sensitivity.Results: .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, contrast sensitivity displayed a strong correlation with slope in the central part of the cornea (with a radius of 1 mm) 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 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, the strongest correlation was for contrast sensitivity and elevation (slope) in the region within a 1 mm radius of the corneal centre in the opposite direction of the keratoconus apex (direction (ax) CB).

The impact of irregular corneal shape parameters on visual acuity and contrast sensitivity

“…Visual acuity had statistically significant correlations with either the highest corneal point (r = 0.44, p < 0.01) or the lowest corneal point (r = 0.34, p = 0.02), and the average difference between both points (r = 0.44, p < 0.01). Thus, subjects with a peripheral keratoconus apex had a larger correlation with the maximum elevation of the corneal surface than did the subjects with a central keratoconus apex [ 20 ].

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“…In subjects with a central keratoconus apex (29 eyes), correlations between log-contrast sensitivity and elevation of the highest corneal point ranged from r = 0.10 ( p = 0.61) at 3 cpd to r = 0.38 ( p = 0.05) at 9 cpd. As to the subjects with a peripheral keratoconus apex (48 eyes), the correlation between the log-contrast sensitivity and elevation of the highest corneal point ranged from r = 0.33 ( p = 0.02) at 3 cpd to r = 0.53 ( p < 0.01) at 9 cpd [ 20 ].…”

“…In subjects with central keratoconus apex (29 eyes) the absolute value of correlation between the lowest corneal point and log-contrast sensitivity ranged from r = 0.32 (p = 0.09) at 7 cpd to r = 0.49 ( p < 0.01) at 15 cpd. As to the subjects with a peripheral keratoconus apex (48 eyes), correlation ranged from r = 0.32 ( p = 0.03) at 3 cpd to r = 0.47 (p < 0.01) at 9 cpd [ 20 ].…”

“…Analysing the change in elevation along the axis going through the corneal centre and the keratoconus apex (ax) — separately both in the direction from the corneal centre to the opposite direction of the keratoconus apex (CB) and towards the keratoconus apex (CA) — visual acuity had higher correlations with the (CB) direction (see Table 2 ) [ 20 , 21 ].…”

“…The highest correlation of visual acuity in all keratoconus subjects together (77 eyes) was that with the changes in elevation along the axis going through the corneal centre and the keratoconus apex in a 1 mm radius (ax direction CB) (see Table 2 ). The situation was similar in the subjects with a peripheral apex (48 eyes): namely, the highest correlation of visual acuity was that with an elevation change within a 1 mm radius along the axis passing through the keratoconus apex (ax), while the subjects’ eyes with a central keratoconus apex (29 eyes) do not demonstrate statistically significant correlations between the shape of cornea parameters and visual acuity at any distance from the corneal centre [ 20 , 21 ].…”