Estimating basal tear osmolarity in normal and dry eye subjects

Purpose: Tear osmolarity (tOsm) is used as a measure of severity in dry eye disease (DED) and has been proposed as an index of body hydration. In DED the level of tear hyperosmolarity is compared with that of a control population. It is proposed here that a better index of body hydration and a more valid reference point in DED can be acquired by measuring the tOsm after a period of evaporative suppression. Method: 8 normal and DED subjects were recruited, their tOsm measured in uncontrolled environmental 'clinic conditions'. Then in experiment 1 they entered a controlled environment chamber and had tOsm measured after 45 minutes of eye closure and then, with the eyes open, at 15 minute intervals for a further 45 minutes, at a relative humidity (RH) of 45%. Alternatively, in experiment 2, they had tOsm measured every 15 minutes for 45 minutes during exposure to 70% RH, as a separate measure to suppress evaporation. Results: A significant decrease in tOsm occurred in both normal and DED subjects after lid closure in experiment 1 (normal RE p = 0.015; normal LE p = 0.006; DED RE p = 0.0002; DED LE p = 0.01). The tOsm also fell slightly after exposure to 70% RH in experiment 2 significant in the LE of normal group only (normal LE p = 0.045). Conclusions: Suppression of tear evaporation resulted in a fall in tOsm, close to that of plasma osmolarity (285- 295 mOsm/L). It is proposed that this new measure, termed Basal Tear Osmolarity (BTO), could provide a valuable index of plasma osmolarity and hence of body hydration and in DED, a personal baseline against which to gauge the severity of tear hyperosmolarity.