CHANGES IN CORNEAL CURVATURE WITH VIEWING DISTANCE

Walter Huang

Abstract

PURPOSE. To investigate changes in corneal curvature with viewing distance using a highly sensitive corneal topography system. METHOD. Twelve young adults, ranging in age from 23 to 32 years (mean age=26.2) with spherical equivalents between 0 and-3.00 D and less than or equal to 0.25 D of astigmatism, participated in this study. Measurements of the corneal curvature of the right eye were taken using the Humphrey Atlas Eclipse Corneal Topographer System Model 995, while subjects used their left eye to fixate distance and near targets seen at 4.0 m and 0.20 m, respectively, through a mirror. Five measurements were taken for each condition. For each measurement, the corneal surface was best fit with the reference sphere and the shape factor. The reference sphere is determined by the radius of curvature at the vertex. The shape factor is a measure of the asphericity of the cornea with a lower value approaching sphericity. A repeated trials analysis of variance was performed on the data for distance versus near and refractive error category (emmetrope versus low myope).

RESULTS. Results showed a significant effect of viewing distance on shape factor (F=19.48, df=1/10, P=0.0013), with shape factor values significantly smaller when viewing at near. No significant differences were found between reference sphere values in the two viewing conditions or between emmetropes and low myopes.

CONCLUSIONS. Based on reference sphere measurements, there is no change in curvature at the vertex of the cornea between distance and near. However, the shape factor does change, indicating that the curvature steepens in the more peripheral cornea when viewing at near. This change must be due to forces on the eye that are associated with near viewing (ciliary muscle contraction and/or extraocular muscle contraction).

Details

Year: 2001

Program Number: Poster 91

Author Affiliation: University of California

Co-Authors: Jane Gwiazda, Frank Thorn, Ji Chang He, Richard Held

Co-Author Affiliation: The New England College of Optometry, The New England College of Optometry, The New England College of Optometry, The New England College of Optometry

Room: Exhibit Hall C