Postblink changes in total and corneal ocular aberrations

Abstract

Objective: The purpose of this study was to determine objectively the changes in optical aberrations induced by the progressive tear film irregularity after a blink and their effects on retinal image quality.

Design: Prospective, consecutive, nonrandomized comparative trial (self-controlled).

Participants: Twenty healthy subjects.

Main outcome measures: Corneal and total ocular aberrations were determined in 20 subjects at various time intervals (approximately 0, 10, and 20 seconds) after a blink. Corneal and total aberrations were measured with an Orbscan II topographer (Orbtek Inc, Salt Lake City, UT) and a Zywave aberrometer (Bausch & Lomb, Irvine, CA), respectively. All data were decomposed using Zernike polynomials to yield the root mean square wavefront deviations, in micrometers (microm), for different pupil diameters (3.5, 4.5, 5.5, and 6.5 mm). A merit function (Mf), defined as the volume under the 2-dimensional modulation transfer function as computed from the total wavefront error, was taken as the image quality metric.

Results: Both corneal and total aberrations (third order and higher) showed a statistically significant increase with time after the blink for all pupil diameters except 3.5 mm. The magnitude of the increase was greater at larger pupil diameters. For a 6.5-mm pupil, the aberrations increased on average by a factor of 2.5 (total) and 2.5 (corneal). Increases in total aberration were related closely to increases in corneal aberration (P<0.0001). For a 6.5-mm pupil, after 20 seconds, the reduction in optical quality (Mf) among the subjects studied was 21+/-8%.

Conclusions: After a blink, the gradual increase in optical aberration associated with the increasingly irregular tear film may cause a progressive reduction in the optical quality of the eye. These changes in aberration with time may partly limit the improvements in visual performance that are achievable by customized corneal ablation.