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Study: Evaporation contributes significantly to dry eye
One study shows that evaporation significantly affects tear turnover depending on the surrounding humidity.
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Relative humidity greatly affects aqueous tear evaporation, which in turn significantly affects tear loss, especially in dry eye patients.
A study presented by James P. McCulley, MD, FACS, FRCOphth, at Hawaiian Eye 2006 showed that none of the five drops he tested for dry eye significantly increased or decreased aqueous evaporative tear loss but that the humidity at the time of measurements did.
“Aqueous tear evaporation contributes significantly to tear loss,” he said. “The loss of aqueous tears is definitely area and humidity dependent.”
The study
Dr. McCulley and colleagues looked at 16 patients diagnosed with aqueous tear deficiency. None of the patients were on any other medications that would affect tear production.
Each patient’s baseline evaporation rate was tested using a custom-made “evaporometer,” according to Dr. McCulley’s presentation. On five separate days, patients were given one of five commercially available dry-eye drops.
The medications, which Dr. McCulley did not name, were administered in one 40 µL drop. The patients were then reassessed 15 minutes later.
“We found various effects of the optical preparations. Some increasing, some decreasing the aqueous evaporative tear loss, but none were statistically significant,” he said.
Instead, the researchers found something they were not expecting.
“What we found was that at a lower relative humidity of 25% to 35%, we had statistically significantly more evaporative aqueous tear loss than at a relative humidity of 35% to 45%, across the board,” he said.
Humidity and evaporation
“We know that relative humidity plays a role, but just how big a role?” Dr. McCulley asked.
The investigators attempted to re-create the decreased humidity in an airplane cabin, which they estimated to be as low as 9%, and compare that to a higher relative humidity.
Due to equipment limits, Dr. McCulley said they compared 20% to 25% humidity, which could represent the humidity in an airplane at takeoff, with 40% to 45% humidity, which might be found in Dallas.
“If we look at 20% to 25% vs. 40% to 45%, there is almost a 100% increase in aqueous tear loss and evaporation. That is big,” he said. “Then again, how real is that? How much does evaporative aqueous tear loss contribute to total aqueous tear loss?”
Dr. McCulley referred to his studies to find that aqueous tear loss in normal eyes is around 42% and up to 56% in aqueous-deficient eyes.
“This was a shock to me, and I would guess it’s a shock to most of you,” he said. He said, however, he has not yet identified a clinically non-inflamed dry eye patient in whom evaporation necessarily caused dry eye.
“But evaporation plays a huge role in aqueous tear loss. Forty percent to 60% of our aqueous tears are evaporating,” he said.
Treatments in development
In development, Dr. McCulley said, are anti-evaporatives, lipid mimetics and lipid replacements that may be more effective than current dry eye formulations. Specifically, Allergan is developing an androgen drop, and Systane Free (Alcon) is a gel drop that has the potential to decrease lubricity and friction, he said.
“What I’m anticipating, at least based on some preliminary data, is that this will also decrease evaporation,” he said. He added that his research group is in the process of evaluating these new drops, as well as the effect on evaporation longer than 15 minutes after the drop is instilled.
Until then, Dr. McCulley encouraged physicians to perform their own experiments.
“Go pick up a sample, put it in one eye and not the other” before flying in an airplane, he said. “See if you appreciate the difference at the end of your flight.”
For more information:
- James P. McCulley, MD, FACS, FRCOphth, can be reached at UT Southwestern Medical Center at Dallas, 5323 Harry Hines Blvd., Dallas, TX 75390; 214-648-3407; fax: 214-648-9061; e-mail: james.mcculley@utsouthwestern.edu.
- Katrina Altersitz is an OSN Staff Writer who covers all aspects of ophthalmology.