Effect of humidity variations in a controlled environment chamber on tear evaporation after dry eye therapy.

PURPOSE

Many dry eye (DE) patients are sensitive to adverse environments where tear evaporation rate (TER) increases. Pilot study-A controlled environment chamber was used to determine the time of exposure required for TER to reach steady state equilibrium at 40% relative humidity (RH). Study 1-To assess the difference between normal and DE subjects in their tear physiology response. Study 2-To determine, under varying environmental conditions, the efficacy of an emulsion eye drop on tear physiology.

METHODS

Pilot study-TER adaptation time was determined by exposing 3 normal and 3 DE subjects to RH of 40% at 72°F for 0, 5, 10, 15, 20, and 25 minutes. Study 1-The difference in noninvasive tear breakup time (NITBUT) and TER responses between DE and normal subjects were determined at various RH from 5% to 70% (at 72°F) for 20 subjects (10 normal subjects; 10 DE subjects). Study 2-To assess the efficacy of an emulsion eye drop, the same 20 subjects were dosed four times per day for 7 days with a drop containing emulsified castor oil and reassessed.

RESULTS

Pilot study-Evaporation at 40% RH showed a peak (around 5 minutes) followed by a decline to steady state level at 10 minutes. Dry eye subjects showed greater evaporation than normal subjects at 40% and 5% RH but not at 70%, where TER declined to zero in both groups. No significant change in NITBUT was found in either group for the various exposure times of the test period (P>0.05). Study 1-TER was higher in DE compared with normal subjects at 5% or 40% RH, however reduced to almost zero in both groups at 70% RH. A significant difference in NITBUT was found between the DE and normal groups at each humidity (P<0.05). Study 2-An emulsion-based drop effectively lowered the TER, especially in DE patients. For NITBUT, a significant improvement in both normal and DE subjects was found at 5% and 40% but not at 70% RH levels.

CONCLUSIONS

Pilot study-TER measurements required at least 10 minutes in the chamber to obtain a steady-state TER with no significant change to NITBUT. Study 1-TER has a reverse correlation with environmental humidity in the range of 5% to 70%, with TER reduced to zero at 70% RH. Dry eye subjects had a higher TER at all RH levels below 70%, and NITBUT is significantly different between DE and normal subjects at all humidities. Study 2-Emulsion-based drops reduced TER in DE patients by an amount equivalent to that obtained by raising environmental humidity by 30%. Noninvasive tear breakup time was improved in both normal and DE subjects at lower RH levels.