Dry eye is a disorder of the tear film due to tear deficiency or excessive tear evaporation which causes damage to the interpalpebral ocular surface (i.e. exposed eye surface) and is associated with symptoms of ocular discomfort. These symptoms include: eye fatigue, discomfort, redness, itching, dryness, irritation, crusting of lids, scratchiness, excessive tearing, discharge, blurry vision, pain, light sensitivity, blinking abnormalities, and foreign body sensation.

The exposed surface of the eye becomes unhealthy and rough because of the dryness. This unhealthy surface is the reason for all the dry eye symptoms. Although the underlying problem is dry eyes, you may wonder why then the eyes are tearing so much. The reason why the eye tears, is that eye irritation stimulates 'reflex tearing' from the tear glands.

Symptom assessment is a key component of dry eye diagnosis - to the extent that many believe dry eye syndrome to be a symptom-based disease. Several questionnaires have been developed to determine a score that would allow for dry eye diagnosis.

McMonnies & Ho questionnaire is the one that is often used in clinical studies of dry eyes. There are 14 questions that can give a score from 0 to 45. Scores above 14.5 are consistent with dry eye diagnosis. Scores above 14.5 have a 87% sensitivity and specificity for dry eye diagnosis (Adv Exp Med Biol 1998;438:835-8). The maximum possible score is 45. The closer your score is to 45, the more certain is the dry eye diagnosis and the more severe is your dry eye disease.


Dry Eye Questionnaire

Air Travel and Dry Eye
The relative humidity in most aircraft cabins is low, typically from 10 to 20% with an average of 15-19%, depending on the aircraft. The cabin relative humidity is well below the preferred values of 30- 60% suggested in American Society of Heating, Refrigerating and Air-Conditioning Engineers Standard 62-1999.

Low relative humidity may cause drying of the skin, mucous membranes, and conjunctivae (Dry Eye). Dryness of eye may lead to tearing and pain, especially in those wearing contact lenses. A study of the effects of low humidity on the human eye found that at a relative humidity of either 10% or 30%, eye pain (described as scratchiness, pain, or burning) increased over time up to the fourth hour of exposure (total exposure duration of 10 hour) for both a naked eye and an eye covered with a soft contact lens. In aircraft cabins, symptoms of low humidity, such as eye and nasal irritation, seem to occur within 2 hours after exposure begins. Furthermore, all symptoms increase in severity with time, and an adaptive response to the low-humidity cabin environment is not evident.
The adverse effects of low relative humidity experienced by crew and passengers are temporary and are alleviated when they leave the aircraft. The time required for rehydration depends on individual physiology and the ambient environment. (Reference: The Airliner Cabin Environment and the Health of Passengers and Crew. Board on Environmental Studies and Toxicology, Division on Earth and Life Studies. 2002, 344 pp.; ISBN 0-309-08289-7; available from National Academy Press)

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