What is Dry Eye Syndrome?

Your ophthalmologist may have told you that you have Dry Eye Syndrome, and suggested products to relieve its symptoms.  But what is Dry Eye?

Dry Eye Syndrome is not a disease, although it may occasionally be caused by a disease.  It is instead a condition of the eye where, for a number of reasons, the tear glands do not produce adequate lubrication of the eye’s surface.

The tear glands produce two types of tears, reflex tears and lubricating tears. Each type is produced under different conditions, and each has a different function.

Reflex tears are produced when the eye responds suddenly to an external source of irritation.  Cutting an onion, watching a sad movie, or being poked in the eye causes an emergency response where the tear ducts flood the eye with water in an attempt to dilute or remove the irritant.  Because so large a volume of tearing is produced the tear drainage system is overwhelmed, and tears run down our faces.  Another cause of reflex tearing can be an imbalance in the composition of lubricating tears (discussed below.)  If lubricating tears do not contain sufficient lubricating oils they may dry too quickly to be effective.  This can cause irritation, which causes reflex tears, which cause the eyes to water.  It sounds strange, but excessively watering eyes can be a symptom of dry eye syndrome.

The other type of tears, lubricating tears, are much more complex.  They consist of three primary ingredients, mucus, water and oil.  The inner layer of mucus, secreted by goblet cells in the eyelids, covers the surface of the cornea and helps spread the tears evenly over the eye.  Water, the middle layer, is secreted by the lachrymal glands, and the outer layer of oils comes from the Meibomian glands inside the eyelids.  This last oily layer helps retard the evaporation of the water layer, and also helps seal the eyelids when they are closed.
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The most common cause of dry eye is an inadequate volume of water in the middle layer of the tears.  This can be caused by a number of different factors:

  • Aging.  As we get older many of our body’s systems no longer work as effectively as they used to.  It is thought that a majority of people over the age of 70 experience occasional symptoms of dry eye.

  • Women are particularly susceptible to dry eye because of hormonal changes that occur with age.

  • Environmental factors.  Smoking, sitting at a computer for long periods of time, low humidity environments can all accelerate the rate of evaporation of the water layer, leading to dry eye.

  • Drugs.  A number of prescription and over the counter drugs can cause dry eye.  Antihistamines and decongestants are among the offenders.

  • Medical conditions.  A number of diseases and medical conditions can cause dry eye as a side effect.



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