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Progression of keratoconus assessed by fourier analysis of videokeratography data
Tetsuro Oshika1, Tatsuro Tanabe, Atsuo Tomidokoro
1Department of Ophthalmology, University of Tokyo School of Medicine, Tokyo, Japan. oshika-tky@umin.ac.jp
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.