Pupil displacement in corneal topography images is related to the eye's refractive error.

Abstract

Abstract: : Purpose: Corneal topography maps are often used to determine the cornea's contribution to the overall wavefront aberration of the eye. For this comparison to be accurate, corneal and ocular wavefront aberration maps should be centered on the same reference axis, the line of sight, which passes through the pupil center (Salmon and Thibos, JOSA A, 2002). However, the center of a corneal topography map is often displaced from the center of the subject's pupil. This displacement is known as videokeratoscope shift and its magnitude is directly related to angle lambda (Mandell, CLAO, 1994), the angle between the line of sight and the pupillary axis. Angle lambda has been reported to vary with refractive error (Donders, 1864) so we examined the relationship between videokeratoscope shift and the eye's refraction. Methods: Corneal topography maps were obtained on 38 eyes using a Humphrey Atlas Model 995 instrument, which employs the Placido ring technique. A series of four images were obtained on each eye. A Matlab (Mathworks) edge detection routine was used to find the pupil margin in each raw topographer mire image; the pupil center was defined as the centroid of that edge. The scaling coordinates of the topography map were used to calculate the horizontal and vertical displacement of the pupil center from the center of the ring mires. Results:The average horizontal pupil displacement was 0.195 mm temporal to the topography ring center, while the average vertical displacement of the pupil was 0.069 mm above the ring center. Horizontal pupil displacement varied with the eye’s refractive error, such that myopic eyes had smaller horizontal pupil displacements than emmetropic eyes. The regression of the horizontal pupil displacement (y) in mm against the spherical equivalent refraction (x) in Diopters was y = 0.025*x + 0.28; r = 0.57, p = 0.0002. A positive displacement of the pupil means that the pupil center was temporal to the topographer mire ring center. Vertical pupil displacement did not vary with refractive error. Conclusions: Our results indicate that angle lambda is smaller in myopic eyes than in emmetropic eyes. This finding has implications in the study of the role of aberrations in refractive error, because asymmetric aberrations would be affected by the horizontal displacement between visual and optical axes.

Keywords: cornea: clinical science • optical properties • topography