Quantitative evaluation of irregular astigmatism by Fourier series harmonic analysis of videokeratography data

PURPOSE. TO assess quantitatively corneal irregular astigmatism in association with best spectacle-corrected visual acuity. METHODS. Refractive powers on a mire ring measured with computerized videokeratography were decomposed, using the Fourier series harmonic analysis. Extracting spherical and regular astigmatic components, the remaining irregular astigmatic component was quantified on rings 2 through 9. A weighted average was calculated by using the Stiles-Crawford effect on the basis of the radius of each ring of each eye and was used as an index of the irregular astigmatic component. Data analyses were carried out in 108 eyes, including 53 normal eyes, 34 eyes with keratoconus, and 21 eyes that had undergone penetrating keratoplasty for keratoconus. Keratoconic eyes and eyes after keratoplasty were included in the study only if visual acuity, corrected with a hard contact lens, was 20/20 or better. Logarithm of best spectacle-corrected visual acuity, age, type of disease, refractive astigmatism, irregular astigmatic component, surface regularity index, and surface asymmetry index were analyzed. RESULTS. In results of multiple regression analysis, the irregular astigmatic component was significantly correlated with best spectacle-corrected visual acuity (r = -0.744; adjusted R-2 = 0.549; P < 0.001), whereas other explanatory variables showed no correlation with best spectacle-corrected visual acuity. CONCLUSIONS. This model of the irregular astigmatic component seems to be an efficient, quantitative means of describing corneal irregular astigmatism.