dry eye dx and tx

Winter Wonderland: Not for Dry Eye Sufferers

BY WILLIAM TOWNSEND, OD, FAAO

In many disease states, patients tend to be more symptomatic during certain times of the year. Many of us perceive that this is true for dry eye sufferers. These individuals often complain that their signs and symptoms are more severe during winter. And despite all the fun activities of winter, dry eyes are no fun.

Ocular dryness associated with winter may be multifactorial, but one contributing factor is the overall reduction in relative humidity that occurs during colder months. For instance, the average morning relative humidity in New York City in January is 68 percent while in August it is 79 percent. To understand this, we need to review some basic atmospheric science. A literature search of dry eye associated with winter yields few results, so we must look to other sources.

Relative Humidity Basics

The process of evaporation in a closed environment will proceed until the number of water molecules escaping into the air equals the number of those returning to the liquid state. At this point, the atmosphere is said to be saturated. Temperature influences this process; molecular kinetic energy is greater at higher temperatures, so at higher temperatures more molecules can escape the surface and the saturated vapor pressure is correspondingly higher. In other words, at higher temperatures, the atmosphere can "hold" more moisture. Conversely, at lower temperatures, the atmosphere can sustain fewer water droplets.

The impact of this is apparent in the difference between relative humidity outside versus inside a warmed house. If the outside air temperature in winter is 0°F and the relative humidity is 75 percent, that same air inside a 70°F home will have a 4 percent relative humidity. The issue is not, as we often hear, that heating systems "dry out the air," but that warmed air can hold much more moisture. Without supplying extra humidity, indoor winter air simply does not contain much moisture. The result is increased evaporation from skin and mucous membranes, including from the ocular surface.

Everyday activities return some moisture to indoor air. One person's breathing produces one-quarter cup of water per hour. Cooking for a family of four introduces approximately five pints of water into the atmosphere, and showering puts one-half pint of water into the air. These help to offset the lowered indoor relative humidity. However, this same situation exists in a car where heated air lowers relative humidity and little water is introduced except through passengers breathing. The end result is that travel by car, air or commuter train results in ocular dryness.

Management Strategies

Managing and treating dry eye patients during the winter months poses a special challenge. A good starting point is questioning all patients about signs and symptoms related to dry eye. Equally important is educating patients about the cause of their symptoms. Finally, it's important to address all of the components that contribute to dry eye.

One potential source of relief is using a humidifier to increase relative humidity in the home or workplace. Patients may inquire as to the appropriate level of humidly to seek. It's important to not increase humidity too much, which can cause condensation inside the home, thus promoting growth of molds and dust mites.

Patients may need to increase the frequency of using artificial tears. They also need to reduce the deleterious effects of dry air currents by redirecting air vents at home, and especially in the car, away from their eyes. CLS

For references, please visit www.clspectrum.com/references.asp and click on document #158.

Dr. Townsend practices in Canyon, Texas and is an adjunct faculty member at UHCO. E-mail him at drbilltownsend@gmail.com.