Corneal refractive changes due to short-term eyelid pressure in downward gaze.

PURPOSE: To assess corneal refractive changes after 15-minute visual tasks and their association with eyelid morphology. SETTING: Contact Lens and Visual Optics Laboratory, School of Optometry, Queensland University of Technology, Brisbane, Queensland, Australia. METHODS: Eighteen young subjects with normal ocular health were recruited. Corneal topography was measured with a videokeratoscope before and after 4 conditions consisting of 2 downward gaze angles (20 degrees and 40 degrees) and 2 types of visual tasks (reading and steady fixation). Anterior eye photography in downward gaze was used to determine the eyelid angle, tilt, and position with respect to the cornea. RESULTS: Corneal refractive power changed significantly after the 15-minute downward gaze tasks. The largest mean corneal spherocylindrical change was +0.33 -0.30 x 84 after reading in the 40-degree downward gaze (4.0 mm corneal diameter). The refractive changes were significantly larger after the 40-degree tasks than after the 20-degree tasks (P<.001). The changes in refractive root-mean-square error were significant for all conditions, except the 20-degree steady fixation task, with 4.0 and 6.0 mm analysis diameters (P<.05). Significant correlations were found between some aspects of eyelid morphometry and corneal refractive change. CONCLUSIONS: The pressure of the eyelids on the cornea in short-term downward gaze resulted in optically and clinically relevant corneal changes. Correlation between the refractive corneal changes and eyelid parameters suggests that the angle, shape, and position of the eyelids influence the nature of the corneal changes. When high accuracy is required, refraction should be qualified by the visual tasks undertaken before assessment.