Importance of the lipid layer in human tear film stability and evaporation

Purpose. Previous work on rabbits has demonstrated a four-fold increase in tear evaporation when the tear lipid layer is removed. However, in vitro work has suggested that the lipid layer does not play a role in retarding evaporation of the aqueous layer. The importance of the lipid layer in human tear film stability and evaporation was determined in the current study by measurement of these parameters in the same individuals. Methods. The left eyes of 161 normal and dry eye subjects (72 males, 89 females), with an age range of 13 to 85 years, were examined. Tear evaporation was derived from the vapor pressure gradient measured with a modified Servomed evaporimeter. Lipid layer structure and noninvasive break-up time (NIBUT) were assessed clinically, by specular reflection, with the Keeler Tearscope. Lipid layer structure was categorized into marmoreal (open and closed meshwork), now, amorphous, and colored fringe (normal and abnormal) patterns. These observed patterns reflect lipid layer thickness. Ambient temperature and relative humidity remained fairly constant throughout the experiment. Results. Tear evaporation rate was found to vary significantly with different lipid layer patterns. Eyes with no visible lipid layer, or exhibiting an abnormal colored fringe pattern (with clumping of lipid amidst areas of little or no lipid cover), demonstrated a significantly higher rate of evaporation of the tear film (p < 0.001). There were no significant differences amongst the remainder of the patterns. The NIBUT was also found to vary significantly with lipid layer pattern (p < 0.001), with the absent or abnormal colored fringe lipid patterns exhibiting the poorest stability. Conclusions. Where the human lipid layer is absent, or is not confluent, and the tear film is unstable, tear evaporation is increased four-fold. However, where there is a stable, intact lipid layer, regardless of lipid thickness, tear evaporation is retarded.