Parameters characterising the corneal surface
Visual acuity
All keratoconus subjects together (77 eyes) demonstrated a medium correlation in absolute value[1] between visual acuity and the highest corneal point of the corneal anterior surface (r=0.30, p<0.01), while a correlation between visual acuity and the lowest corneal point was weaker (r=0.21, p=0.06) (see Fig. 3). A correlation between visual acuity and the difference between two points was similar to that of the highest corneal point (r=0.32; p<0.01). In subjects with a central keratoconus apex (29 eyes), visual acuity had no statistically significant correlations with any of the highest corneal point (r=0.10, p=0.61), the lowest corneal point (r=0.15, p=0.44), or the difference between these two points (r=0.15, p=0.45). Subjects with a peripheral keratoconus apex (48 eyes) had larger correlations between visual acuity and the corneal surface’s characterising parameters than did subjects with a central keratoconus apex. 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.
Contrast sensitivity
Parameters characterising the corneal surface had higher correlations with the log-contrast sensitivity than with visual acuity. In all keratoconus subjects together (77 eyes), the correlation between log-contrast sensitivity and change in elevation (slope) of the corneal surface varied across spatial frequencies of the log-contrast sensitivity. The correlation (in absolute values) between the highest corneal elevation and log-contrast sensitivity in different spatial frequencies ranged from r=0.25 (p=0.03) at 3 cpd to r=0.47 (p<0.01) at 9 cpd. 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.
In all keratoconus subjects together (77 eyes), the absolute value of the correlation between the lowest corneal point and log-contrast sensitivity ranged from r=0.33 (p=0.09) at 5 cpd to r=0.40 (p<0.01) at 11 cpd. 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.
As described above, for subjects with a central keratoconus apex, log-contrast sensitivity’s correlation with maximum elevation was lower than with minimum elevation — while for subjects with a peripheral keratoconus apex, the correlation coefficients were similar to maximum and minimum elevation.
Anterior surface slope
Visual acuity
The study focused on two axis characterising changes in the surface (slope) — the direction through the corneal centre and the keratoconus apex (ax), and the direction perpendicular to it (P ax). Visual acuity had higher correlations with the changes in elevation along the (ax) direction (see Table 2).
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).
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.
Contrast sensitivity
Higher correlation between change in elevation (slope) and log-contrast sensitivity for all keratoconus subjects together (77 eyes) were associated with the axisgoing through the keratoconus apex and the corneal centre (ax) rather than the direction perpendicular to it (P ax) (see Table 3). The highest correlation between log-contrast sensitivity and changes in the elevation can be observed in the central area of the cornea within a 1 mm radius around the corneal centre for the direction (CB), both for all keratoconus subjects together and individually with the central and peripheral apex.
Not all spatial frequencies of the log-contrast sensitivity are equally relevant to the quality of life of the keratoconus subjects. Since changes in the corneal elevation in keratoconus subjects most significantly affect changes at 6 cpd [21], the individual data of keratoconus subjects regarding log-contrast sensitivity’s correlation with the corneal elevation in the direction (CB) for an area of 1 mm were presented at 7 cpd (see Fig. 4), as well as in the direction (CA) (see Fig. 5).
The log-contrast sensitivity showed higher correlation with the corneal elevation than with visual acuity. The direction characterising log-contrast sensitivity most efficiently is that from the central part of the cornea to the opposite direction of the apex in a 1 mm radius (CB). The median value of the change in elevation in this direction significantly depends on the location of the apex in a keratoconus subject — either central or peripheral (see Fig. 6). Thus, knowledge of the elevation within a 1 mm radius of the corneal centre to the opposite direction of the apex (CB) might be a good indicator in determining whether the keratoconus apex could be central or peripheral.
[1]The study focused on absolute values of correlations rather than their positive or negative values.