Mechanism of aberration balance and the effect on retinal image quality in keratoconus: optical and visual characteristics of keratoconus

Purpose: To evaluate ocular wavefront aberrations in keratoconus and describe their effect on visual quality.

Setting: Department of Ophthalmology Semmelweis University, Budapest, Hungary.

Design: Case-control study.

Methods: Ocular aberrations over a 4.5 mm pupil were measured with a Hartmann-Shack sensor in eyes with keratoconus or subclinical keratoconus or of refractive surgery candidates with normal corneas. Visual quality metrics were calculated using an optical design program. The correlation between Zernike polynomials was evaluated using correlation analysis. Predictors of subclinical keratoconus and keratoconus among ocular aberrations were evaluated. Visual quality (point-spread function, retinal spot radius, Strehl ratio) between groups was compared.

Results: Horizontal and vertical coma were significant predictors of subclinical keratoconus (P<.05, r(2)=0.85) and oblique astigmatism and vertical coma of keratoconus (P<.05, r(2)=0.64). Spot radius (P<.05, r(2)=0.66) and the Strehl ratio (P<.05, r(2)=0.64) were significant discriminators in all groups. On correlation analysis, interactions of ocular aberrations were different in keratoconus eyes than in control eyes. In keratoconus eyes, there was a negative correlation between defocus and spherical aberration (r = -0.41) and between astigmatism and secondary astigmatism (r=-0.68). In contrast, spherical aberration correlated positively with tetrafoil (r=0.47) and the positive correlation between vertical coma and vertical tilt decreased (r=0.67 versus r=0.34).

Conclusion: Image quality in keratoconus was significantly decreased at equal levels of total root mean square error. A combination of ocular aberrations in eyes with keratoconus decreased image quality.