Evaporation-driven instability of the precorneal tear film

Abstract

Tear-film instability is widely believed to be a signature of eye health. When an interblink is prolonged, randomly distributed ruptures occur in the tear film. "Black spots" and/or "black streaks" appear in 15 to 40 s for normal individuals. For people who suffer from dry eye, tear-film breakup time (BUT) is typically less than a few seconds. To date, however, there is no satisfactory quantitative explanation for the origin of tear rupture. Recently, it was proposed that tear-film breakup is related to locally high evaporative thinning. A spatial variation in the thickness of the tear-film lipid layer (TFLL) may lead to locally elevated evaporation and subsequent tear-film breakup. We examine the local-evaporation-driven tear-film-rupture hypothesis in a one-dimensional (1-D) model for the evolution of a thin aqueous tear film overriding the cornea subject to locally elevated evaporation at its anterior surface and osmotic water influx at its posterior surface. Evaporation rate depends on mass transfer both through the coating lipid layer and through ambient air. We establish that evaporation-driven tear-film breakup can occur under normal conditions but only for higher aqueous evaporation rates. Predicted roles of environmental conditions, such as wind speed and relative humidity, on tear-film stability agree with clinical observations. More importantly, locally elevated evaporation leads to hyperosmolar spots in the tear film and, hence, vulnerability to epithelial irritation. In addition to evaporation rate, tear-film instability depends on the strength of healing flow from the neighboring region outside the breakup region, which is determined by the surface tension at the tear-film surface and by the repulsive thin-film disjoining pressure. This study provides a physically consistent and quantitative explanation for the formation of black streaks and spots in the human tear film during an interblink.

Flow Evaporimeter To Assess Evaporative Resistance of Human Tear-Film Lipid Layer

“…Upon assuming an average TFLL thickness of 100 nm, 26 the water diffusion permeability in TFLL is thus estimated by eq 4 as 0.95 × 10 −10 m 2 /s in agreement with our previous estimation based on clinical observation on evaporation rate reduction due to lipid layer deficiency. 33 Quantitative extraction of human-TFLL resistance from measured flow-evaporimeter data suggests that R L is not a strong function of air flow or relative humidity. We find that the Berkeley Flow Evaporimeter successfully garners TFLL resistance.…”

“…Equation 1, however, strictly applies at steady state. Fortunately, the periodic temperature excursions of the anterior eye surface are minimal; 33,69,70 evaporation rate is, thus, unaffected. Although the resistance of the TFLL to tear evaporation, R L , depends on film thickness, lipid affinity to water molecules, and possibly other lipid physicochemical properties such as viscosity and elasticity, 34 it remains unclear how the human TFLL retards tear evaporation in vivo due to the limitation of available and reliable studies.…”

“…However, available calculations for the evaporating-tear surface temperature demonstrate minimal deviation from eye temperature. 33,69,70 Thus, T S for humans was set to 35°C independent of evaporation rate.…”

“…34 No currently available evaporimeter has the features cited as necessary to obtain valid tear-evaporation rates and quantitative evaporation resistances of the TFLL on humans. This means that all results obtained to date on, for example, the importance of tear evaporation in dry eye, meibomian-gland disease, 62,63 tear-salinity control, 16,17,33 contact-lens wear, 35,52,57,64 and topical drug delivery 65,66 are suspect and in need of reexamination. For these reasons, we design an accurate, safe, and convenient flow evaporimeter that meets the stated design criteria and is suitable for clinical evaluation of dry-eye-related maladies.…”

“…Recently, several helpful mathematical models of tear-evaporation dynamics 16,17,33 and in vitro experiments 34 have been established. Unfortunately, these results are insufficient to validate evaporation rates from human tears.…”