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P. King–Smith, B.A. Fink, J.J. Nichols; Contributions of Evaporation and Tear Flow to Tear Film Thinning and Breakup . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1950.
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Continued thinning of the pre–corneal tear film can presumably lead to tear film breakup. The rate of thinning of the precorneal tear film has been reported to average 3.8 µm/min, (Nichols et al., 2005, IOVS, 46, 2353) and is often considerably more rapid (e.g. 10 µm/min or more). (These measurements were made starting 2 seconds after a blink.) These thinning rates are greater than reported evaporation rates, so rapid thinning might be due to another process such as "tangential" tear flow along the corneal surface (our previous study showed no detectable flow into the epithelium). This suggestion is tested in this study.
Tear flow was measured using an imaging video interferometer based on Doane's (1989, Optom Vis Sci, 66, 383) design. A program was written to derive the time course of vertical (typically upward) movement of the lipid layer at a position on the horizontal midline of the cornea and at 2 mm below. Vertical velocity was derived by averaging velocities derived from cross–correlation analysis of successive frames. Video recordings were analyzed from 18 normal subjects (10 female, mean age 32) for a period of 8 seconds after a blink.
Rapid upward movement of the lipid layer, averaging about 0.8 mm/s, was observed for the first 2 seconds after a blink. However, movement was much slower between 2 and 8 seconds after a blink; on the horizontal midline and 2 mm below, mean upwards velocities were 0.050 and 0.053 mm/s respectively. Thus, considering the area between the horizontal midline and 2 mm below, the flow out of this area at the top should be approximately balanced by the flow into the area from below, if the tear film has uniform thickness. If tear thickness is greater centrally than 2 mm below, then more tears would flow out at the upper level than would flow in at the lower level, which could potentially explain the thinning; however, it will be shown that the gradient of tear thickness required by this hypothesis seems unreasonably high.
Tangential tear flow does not seem sufficient to explain the observed rates of tear film thinning. Thus, evaporation may be the main cause of tear thinning and breakup. However, observed thinning rates are greater than reported evaporation rates. An explanation of this discrepancy may be that reported evaporation rates could be reduced by the pre–ocular chambers typically used, which can restrict the free flow of air currents in front of the eye, thus reducing evaporation rates.
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