Evidence for the Major Contribution of Evaporation to Tear Film Thinning between Blinks

According to the Report of the Dry Eye Workshop,1 tear film breakup is one of the two “core mechanisms” of dry eye, the other being hyperosmolarity. Whereas it is evident that hyperosmolarity is caused by increased evaporation and/ or reduced tear production,1 the causes of tear film breakup are less evident. To help elucidate the mechanisms of breakup, we have used spectral interferometry to study the thinning of the tear film over an extended period after blinking.2 After some transient thinning or thickening for approximately the first 2 seconds after a blink (probably associated with an upward drift of the aqueous layer of the tear film3), we found that the tear film thins as a rather linear function of time with rates of up to approximately 20 μm/min. This high thinning rate is fast enough to explain tear film breakup; for example, if the tear film were 3 μm thick initially (2 seconds after a blink), then, at a thinning rate of 20 μm/min, it would thin to zero thickness in a further 9 seconds for a total time of 11 seconds, which is comparable to observed noninvasive breakup times.4 Thus it is arguable that tear film breakup is the result of this linear thinning between blinks. Note that our thinning rate measurements correspond to just one location at the center or the cornea; thus, even if little or no thinning is observed at this location, thinning could be rapid enough in other locations (e.g., due to lipid layer deficiency) to explain observed noninvasive breakup times. From considerations of the three-dimensional nature of space, it may be concluded that thinning of the tear film could be due to flow of tears (or water) in three directions: outward (i.e., evaporation), inward, into the epithelium, and tangentially, over the ocular surface.2 Tear film thinning did not cause a detectable thickening of the corneal epithelium,2 which would indicate a flow of tear fluid into the epithelium; the analysis of Levin and Verkman5 indicates that any flow across the corneal surface would tend to be in the opposite direction, driven by osmotic flow from the epithelium into hyperosmolar tears. Thus evaporation and tangential flow will be the possible mechanisms of tear thinning considered here. It was noted that reported rates of evaporation were typically less than half of the observed average thinning rate, suggesting that tangential flow made a greater contribution than evaporation.2 However, a problem with this conclusion is that tangential flow would be expected to have variable effects; if tears tend to flow out of some areas (divergent flow), this will indeed cause local thinning, but in other areas, convergent flow of tears will cause thickening. Thus tangential flow could cause either thinning or thickening and would tend to contribute to the variability of thinning rates rather than to the mean rate.6 Additionally, movement of the lipid layer at times >2 seconds after a blink was found to be slow, and the corresponding movement of the underlying aqueous layer was insufficient to explain the observed rate of tear film thinning.7 Movement of the underlying aqueous layer, due to gravity and pressure gradients, is possible without any movement of the lipid layer, but, again, these effects were found to be considerably smaller than the observed thinning rate.7 To help explain the discrepancy between observed evaporation and thinning rates, it has been suggested that evaporation rate measures using open chambers or unventilated closed chambers restrict air currents over the cornea, and hence evaporation is slower than in the more open environment used for tear thinning rate measurements.6 The proposal that tear film thinning is largely due to evaporation is supported by the inverse correlation between tear thinning rate and lipid thickness (increasing lipid thickness would be expected to reduce evaporation).8 In this study, the contribution of evaporation to tear film thinning was studied by measuring the thinning of the tear film behind air-tight goggles. These were expected to eliminate or greatly reduce tear film evaporation; thus, it was hypothesized that little or no thinning would be observed with wearing goggles if thinning is mainly due to evaporation.