The role of tear physiology in ocular surface temperature

Purpose To determine whether the more rapid cooling of the tear film in dry eyes is related to other tear film parameters, a battery of tear physiolog y tests was performed on dry eye patients and control subjects. Methods Tear evaporation rate was measured with a modified Servomed (vapour pressure) evaporimeter and ocular temperature with an NEC San-ei 6T62 Ther mo Tracer in 9 patients diagnosed as having dry eye and in 13 healthy contr ol subjects. Variability in temperature across the ocular surface was descr ibed by the temperature variation factor (TVF). Lipid layer structure and t ear film stability were assessed with the Keeler Tearscope and tear osmolal ity was measured by freezing point depression nanolitre osmometry. Results The data were explored by principal component analysis. The subject s with and without dry eye could be separated into two distinct groups enti rely on the basis of their tear physiology. Dry eye patients exhibited high er tear evaporation rates, osmolalities and TVF, lower tear film stabilitie s and poorer-quality lipid layers than the control subjects. A significant linear relationship was found to exist between tear evaporation rate and TV F for all subjects (R-2 = 0.242, p = 0.024). Conclusions Rapid cooling of the tear film in dry eyes appears to be relate d to the reduced stability of the tears and the increased rate of evaporati on. The higher latent heat of vaporisation, associated with the increased e vaporation in dry eyes, may account for the increased rate of cooling of th e tear film in this condition.