Dynamics and function of the tear film in relation to the blink cycle

Abstract

Great strides have recently been made in quantitative measurements of tear film thickness and thinning, mathematical modeling thereof and linking these to sensory perception. This paper summarizes recent progress in these areas and reports on new results. The complete blink cycle is used as a framework that attempts to unify the results that are currently available. Understanding of tear film dynamics is aided by combining information from different imaging methods, including fluorescence, retroillumination and a new high-speed stroboscopic imaging system developed for studying the tear film during the blink cycle. During the downstroke of the blink, lipid is compressed as a thick layer just under the upper lid which is often released as a narrow thick band of lipid at the beginning of the upstroke. “Rippling” of the tear film/air interface due to motion of the tear film over the corneal surface, somewhat like the flow of water in a shallow stream over a rocky streambed, was observed during lid motion and treated theoretically here. New mathematical predictions of tear film osmolarity over the exposed ocular surface and in tear breakup are presented; the latter is closely linked to new in vivo observations. Models include the effects of evaporation, osmotic flow through the cornea and conjunctiva, quenching of fluorescence, tangential flow of aqueous tears and diffusion of tear solutes and fluorescein. These and other combinations of experiment and theory increase our understanding of the fluid dynamics of the tear film and its potential impact on the ocular surface.

Effects of tear film dynamics on quality of vision

“…Recent papers have reported on the relationship of tear breakup to the lipid layer rather than to the wettability of the corneal surface, specifically in eyes with the spot breakup pattern 47 48. Further studies using mathematical analysis and modelling of the tear film and breakup should be performed to understand the clinical implications and to improve the diagnosis and treatment of dry eye 49. Moreover, use of shearing interferometry50 51 or high-speed videotopographic measurement52 would help clarify the effect of quality and quantity of tear film on QoV in each breakup pattern.…”

Evaporation and Hydrocarbon Chain Conformation of Surface Lipid Films

“…30–36 The most likely cause of local high evaporation is that the lipid layer has a poor evaporation resistance in that region compared to higher resistance in surrounding regions. However, as pointed out, many subjects exhibited ocular surface cooling without fluorescein tear thinning and breakup and some showed no evidence of ocular surface cooling or fluorescein tear thinning and breakup.…”

“…35 The dynamics and function of the tear film in relation to the blink cycle has been reviewed and the authors favor the idea that the changes in the rate of evaporation through the lipid layer plays a role in tear film instability and dry eye. 36 …”

Super-resolved thickness maps of thin film phantoms and in vivo visualization of tear film lipid layer using OCT

“…The functions of the tear film lipid layer have not yet been fully elucidated. Although for a long time it has been accepted that its main purpose is retarding evaporation [30], in vitro studies struggle to affirm this [29,31]. Nevertheless, the lipid layer is suspected to play a role in evaporative dry eye [30,32].…”