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Peter Ewen King-Smith, Padmapriya Ramamoorthy, Carolyn G Begley, Richard J Braun; Evaporation is not the only mechanism of tear film breakup. Invest. Ophthalmol. Vis. Sci. 2016;57(12):6172.
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© ARVO (1962-2015); The Authors (2016-present)
To describe how, in addition to evaporation, divergent flow of tears can contribute to breakup.
Evidence for contributions of divergent flow of tears to tear film breakup was evaluated from fluorescein video recordings from over 100 subjects (method of King-Smith et al., 2013, IOVS 54, 6003). Two characteristics can be used to distinguish fluorescence dimming due to divergent flow, from that due to evaporation. First, evaporation is relatively slow, causing steady dimming over several seconds, whereas divergent flow can be much more rapid. Second, evaporation causes fluorescence dimming by quenching. Quenching does not occur when fluorescein concentration is much below a “critical concentration” of 0.2% (Nichols et al., 2012, IOVS, 53, 5426); thus dimming at low concentrations is probably due to divergent flow. Here, results for one subject – a 45 year old male with mild dry eye (OSDI score 19%) – illustrate characteristics associated with divergent tear flow.
Fig. 1 shows images obtained after instillation of 1 μL of 0.1% fluorescein, yielding a concentration, after dilution by tears, much below the critical concentration. Fig. 1A, recorded just after a blink, shows dark patches which may be caused by spreading of thick “globs” (g) of lipid under an outward surface tension gradient. The dark spots appear too soon and the fluorescein concentration is too low for them to be explained by evaporation. Fig. 1B shows that, later, globs may be stretched into “comets”, (c), by upward flow of the lipid layer. Fig. 1C indicates binding (b) of the lipid layer to the corneal surface; this binding effect is deduced from Fig. 1D, recorded after the next blink, which shows bright “afterimages” (a) interpreted as grooves generated in the corneal surface by shear stress from the lipid layer. Fig. 2 shows a lipid layer image (Braun et al., 2015, Prog Ret Eye Res, 45, 132) of comets and globs from another subject.
Divergent flow of tears can make important and characteristic contributions to tear film breakup.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
Fig. 1. A, B and C were recorded 0.3, 1.4 and 8.5 seconds after a blink. D (contrast increased) was recorded after the next blink. g, globs; c, comets; b, binding of lipid layer to corneal surface; a, afterimages; f, lens fluorescence; r, illumination reflection.
Fig. 2. Comets (c) and globs (g) in the lipid layer of a 41 year old white female (normal OSDI), recorded 0.2 seconds after a blink (contrast doubled).
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