Steepening of corneal curvature with contraction of the ciliary muscle

Purpose To measure the changes of corneal curvature during contraction of the ciliary muscle. Setting Department of Ophthalmology, St. Luke's International Hospital, Tokyo, Japan. Methods Twenty-eight eyes of 14 healthy volunteers under 40 years old were enrolled in this prospective randomized controlled study and divided into pilocarpine and control groups. Intraocular pressure (IOP), pupil diameter, and corneal topography were measured before and 40 minutes after instillation of topical pilocarpine 4% or balanced salt solution. Corneal topography was analyzed for the mean ring-power of Placido rings 1 through 25, average corneal power (ACP), and for spherical equivalent, regular astigmatism, asymmetry, and high-order irregularity by Fourier analysis. Results Pilocarpine had no effect on IOP, but it did cause a significant decrease in mean pupil diameter. Simultaneously, pilocarpine increased the mean ring powers for Placido rings 1 through 4 and the ACP (+0.13 diopters (D) ± 0.17 [SD]; P  = .017). By Fourier analysis, the mean spherical component for the central 3.0 mm of the cornea increased in the pilocarpine group (+0.08 ± 0.15 D; P  = .020). There were no changes in components of regular astigmatism, asymmetry, and high-order irregularity. Conclusions The central cornea steepened in curvature and increased in power owing to contraction of the ciliary muscle. The results suggest that changes in corneal curvature increase refractive power during accommodation.