Estimating basal rear osmolarity in normal and dry eye subjects

Abstract

PurposeTear osmolarity (tOsm) is used as a measure of severity in dry eye disease (DED) and has also been proposed as an index of body hydration. Currently in DED the level of tear hyperosmolarity is compared with that of a normal population. We propose that a more valid reference point for both DED and body hydration could be acquired by measuring tOsm after a period of evaporative suppression.MethodsEight normal (N) and eight DED subjects were recruited and their tOsm measured with a TearLab® osmometer in uncontrolled environmental conditions. They then entered a controlled environment chamber (CEC) set to 23°C and 0.08 m/s airflow and, either: (1) had tOsm measured after 45 min of eye closure and then at 15 min intervals for a further 45 min at 45% relative humidity (RH), or (2) had tOsm measured every 15 min for 45 min, at 70% RH, to suppress evaporation.ResultsTear osmolality was significantly reduced after eye closure in both N and DED subjects, to levels in the range of plasma osmolarity, between 285 and 295 mOsm/L (N p = 0.03; DED p = 0.006). The tOsm rose towards the start value on eye opening. In subjects exposed to 70% RH, which was not expected to suppress evaporation completely, a fall in tOsm also occurred, but to a lesser extent than achieved with eye closure. This was significant in DED patients (p = 0.026) but not in N subjects (p = 0.12).ConclusionsAs predicted, total suppression of tear evaporation resulted in a fall in tOsm. We hypothesise that complete suppression of evaporation, for a sufficient period, drives down tOsm to a basal tear osmolarity value that will be a better measure of body hydration than tOsm measured in open eye conditions. It will also provide a personal baseline against which to gauge the severity of tear hyperosmolarity in DED patients.