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J. J. Nichols, E. Hinel, P. King-Smith; Lipid Layer Thickness and Tear Film Thinning Before and After Application of a Lipid Emulsion Drop. Invest. Ophthalmol. Vis. Sci. 2008;49(13):115.
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To examine the relation between the normal lipid layer thickness and tear film thinning (i.e., evaporation) before and after application of a lipid emulsion eye-drop.
Non-contact lens wearers were recruited to participate. A newly modified wavelength-dependent interferometry system was used to measure lipid layer thickness (by absolute reflectance) and precorneal tear film thinning (evaporation) simultaneously. At baseline, subjects were asked to blink one second after the beginning of the spectral recording and then hold their eyes open for an additional 19 seconds (a second recording was completed after two minutes of rest and the values were averaged). One drop of a lipid emulsion eye drop (Soothe, Bausch and Lomb) was then placed in each eye of the subject, who then completed a demographic form and The Ocular Surface Disease Index (OSDI). Two tear film interferometry measurements were then taken 15 minutes after the initial Soothe eye drop instillation.
Thirty-eight subjects (33 normal, 5 dry eye by OSDI score) completed the study (age = 35.9 ± 13.7 years, 53% female). At baseline, the median lipid layer thickness was 29.0 nm, while the median rate of tear film thinning was -1.78 µm/min. The correlation between lipid layer thickness and the rate of tear film thinning was strong (Spearman’s R = 0.40, p = 0.01). Fifteen minutes after application of the lipid emulsion eye-drop, the median lipid layer thickness was 36.0 nm, while the median rate of tear film thinning was -1.95 µm/min. Again, the correlation between lipid layer thickness and the rate of tear film thinning was strong (Spearman’s R = 0.37, p = 0.02). While there was a statistically significant increase in median lipid layer thickness following the application of the lipid emulsion eye-drop (median increase = 8 nm, Wilcoxan’s test, p = 0.004), there was not a corresponding reduction in the rate of tear film thinning (median change = -0.25 µm/min, Wilcoxan’s test, p = 0.67).
Lipid layer thickness is highly correlated with the rate of tear film thinning, which we have previously shown is primarily due to evaporation. These results provide further evidence of the functional role of the lipid layer in resisting aqueous evaporation whereby thicker lipid layers are associated with more evaporative resistance. Although the Soothe lipid emulsion eye-drop was associated with an increase lipid layer thickness, this thickness was not substantial enough to reduce evaporation. This may be due to the short-lived effect of this lipid emulsion eye-drop.
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