Progression of keratoconus assessed by fourier analysis of videokeratography data.

OBJECTIVE To quantitatively document changes in corneal refractive parameters in relation to the progression of keratoconus over years. DESIGN Retrospective observational case series. PARTICIPANTS Eighty-five eyes of 64 patients with keratoconus who had undergone videokeratography examinations at least twice with an interval of 1 year or longer between each examination. METHODS By means of Fourier series harmonic analysis, topography data were decomposed into spherical component, regular astigmatism, decentration component, and higher order irregularity. MAIN OUTCOME MEASURES Change rate of each parameter per year was calculated by use of the least squares method. RESULTS All four refractive parameters were significantly larger in the keratoconic eyes than in the age-matched normal controls (P < 0.001, Student's t test). The yearly rate of change was significantly positive for spherical component (P = 0.008) and higher order irregular astigmatism (P = 0.015) but not for the regular astigmatism and decentration component. In eyes that showed apparent progression on color-coded maps, spherical component showed a significantly positive change rate per year (P = 0.002), but other parameters did not. In eyes without apparent progression, the yearly change rate of higher order irregular astigmatism was significantly larger than zero (P = 0.049). CONCLUSIONS Keratoconus progression over years was quantitatively described. It was indicated that spherical equivalent is a manifest indicator for the apparent progression of keratoconus, and irregular astigmatism increases along with the natural course of the disease.