Postblink changes in total and corneal ocular aberrations

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-control led). 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 (Am), 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. (C) 2004 by the American Academy of Ophthalmology.