Low humidity causes dramatic evaporation of aqueous tears

Evaporation of aqueous tears has been reported to be about 10% of tear turnover, according to James McCulley, MD, from the Department of Ophthalmology, University of Texas Southwestern Medical Center, Dallas, United States.

"However, it may very well be that the percentage is significantly higher," Dr. McCulley said. He explained that under certain conditions, such as on airplanes, in a desert, in windy conditions, and in high altitudes, the humidity can be very low.

"The principle function of the lipid layer, the outermost layer of the tear film, is to prevent tear overflow and evaporation of the aqueous tears," he said.

In a clinical study, Dr. McCulley and colleagues evaluated patients with dry eye and normal subjects in various conditions of relative humidity to determine the aqueous loss to evaporation. Patients studied either had or had no evidence of meibomian gland dysfunction and clinically aqueous-deficient dry eyes and interpalpebral fissures.

Interestingly, patients with keratoconjunctivitis with meibomian gland dysfunction had increased tear volume. Those with classic keratoconjunctivitis without meibomian gland dysfunction had about the same tear volume as normal subjects. Tear flow results were similar in the three groups. Both dry eye groups had decreased tear turnover, i.e., stagnant tears, but not decreased tear volume.

"The aqueous tear loss to evaporation, measured by evaporameter, in relatively high humidity (40% to 45%) is substantial, but if the relative humidity was decreased by 20% there was almost a 100% increase in the loss of aqueous tears through evaporation," Dr. McCulley said. "There were no significant differences among the groups in the loss of tears to evaporation, which was remarkable in this patient population with aqueous tear deficiency and meibomian gland dysfunction. Low humidity is a very potent force."

When the investigators assessed the effect of evaporative tear loss on tear turnover at a relatively low humidity of 40% to 45%, there is about a 20% to 30% loss of aqueous tears to evaporation in contrast to previous published data of 10%.

"This is a much greater impact on aqueous tear loss to evaporation than previously thought," he said. "If the relative humidity is decreased, the amount of loss of aqueous tears over time increases to 40% to 60%, respectively, in normals and in some patients with aqueous-deficient dry eyes."

Patients with dry eye have this greater loss because of the stagnant tear turnover.

Dr. McCulley suggested that hyperosmolarity might be involved in dry eye disease based on a comparison of the effects of normal saline in a control group and an artificial tear containing HP-guar.

"HP-guar resulted in a significant decrease in evaporation 30 minutes after instillation and at 60 minutes the trend continued," he concluded.