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S. Huth, D. Tran, C. Skotnitsky, L. Lasswell, P. Mahmud, T. Kim; Wavelength-Dependent Optical Interferometry Measurements of Change in Thickness of Apical Corneal Tear Film Following Eye Drop Instillation. Invest. Ophthalmol. Vis. Sci. 2008;49(13):108.
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Wavelength-dependent optical interferometry can measure tear film thickness at the apex of the cornea with accuracy to approximately 30 nm, or about 1% of total thickness. To date, such a measurement following instillation of lubricant eye drops has not been reported. The purpose of this study was to determine changes in tear film thickness and time to return to baseline after drop instillation; and changes in tear film thickness after using drops for 14 days.
Twenty-two participants completed a randomized, double-masked, crossover study comparing blink tears Lubricant Eye Drops (AMO, Santa Ana, CA) to Systane Lubricant Eye Drops (Alcon; Fort Worth, TX). To quantify changes in tear film thickness and ocular retention time, 40 µL of the drop was instilled on Day 1, and the tear film of the right eye was measured with a wavelength-dependent optical interferometer at 1, 3, 6, 9, 12, 15, 20, 30, and 60 minutes. Means were plotted at each time point to profile thickness over time following instillation. Thickness was measured on Days 1 and 14 to evaluate changes. The Wilcoxon rank-sum test was used for comparisons between the groups.
Thickening of tear film was observed in all subjects after instillation, followed in 18 (blink) and 21 (Systane) subjects by a first return to baseline thickness (time = t1), and thereafter thinning below baseline in 13 (blink) and 10 (Systane) subjects, which was in turn followed in 7 subjects in each group by a second return to baseline (time = t2). Subjects not conforming to this sequence did not return to baseline at either t1 or t2. Four subjects (blink) and 1 subject (Systane) did not return to baseline (t1) at 60 min. Six subjects (blink) and 3 subjects (Systane) did not return to baseline (t2). In the blink tears group, mean t1 and t2 values were 31.6 ± 39.5 min. and 45.3 ± 39.1 min. In the Systane group mean t1 and t2 values were 28.1 ± 47.4 min. and 44.9 ± 48.4 min. No significant difference was found between drops for t1 (p = .561) or t2 (p = .999). Baseline tear film thickness did not change significantly after two weeks’ use of either drop in either group.
Thinning of tear film following application of lubricant eye drops is a previously unobserved phenomenon that may result from washout of tear film solutes. Return to baseline times, t1 and t2 were not significantly different between the two eye drops of equal 10 cP viscosity, which may indicate that changes in tear film thickness are viscosity driven despite differences in polymers and chemistries.
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