Dry Eye Syndrome

Dry Eye Syndrome Overview

To help keep your eyes comfortable and your vision optimal, a normal, thin film of tears coats your eyes. Three main layers make up this tear film.

• The innermost layer is the thinnest. It is a layer of mucin (or mucus). This very thin layer of mucus is produced by and coats the cells in the conjunctiva (the clear skin that lines the eye). The mucus helps the overlying watery layer to spread evenly over the eye.
  
• The middle (or aqueous) layer is the largest and the thickest. This layer is essentially a very dilute saltwater solution. The main (or lacrimal) glands and the accessory tear glands produce this watery layer. This layer’s function is to keep the eye moist and comfortable, as well as to help flush out any dust, debris, or foreign objects that may get into the eye. Defects of the aqueous layer are the most common cause of dry eye syndrome, also referred to as keratoconjunctivitis sicca (KCS).
  
• The most superficial layer is a very thin layer of lipids (fats or oils). These lipids are produced by the meibomian glands and the glands of Zeis (oil glands in the eyelids). The main function of this lipid layer is to help decrease evaporation of the watery layer beneath it.

Dry eye syndrome (DES) is a common disorder of the tear film, affecting a significant percentage of the population, especially those older than 40 years. The estimated number of people affected ranges from 10-14 million in the United States. Worldwide, the incidence rate closely parallels that of the United States. DES can affect any race, and it may be slightly more common in women than in men.

Dry Eye Syndrome Causes

DES is a common disorder of the normal tear film that results from one of the following:

• Decreased tear production
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• Excessive tear evaporation
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• An abnormality in the production of mucus or lipids normally found in the tear layer

Aqueous (watery) tear deficiency is caused by either poor production of watery tears or excessive evaporation of the watery tear layer.

• Poor production of tears by the tear glands may be a result of age, hormonal changes, or various autoimmune diseases, such as primary Sjögren syndrome, rheumatoid arthritis, or lupus.
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• Evaporative loss of the watery tear layer is usually a result of an insufficient overlying lipid layer.
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• Some medications, such as antihistamines, antidepressants, beta-blockers, and oral contraceptives, may decrease tear production.

If blinking is decreased or if the eyelids cannot be closed, the eyes may dry out because of tear evaporation.

• When you read, watch TV, or perform a task that requires close attention with your eyes, you do not blink as often. This decreased blinking allows excessive evaporation of the tears.
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• Certain conditions, such as stroke or Bell palsy, make it difficult to close your eyes on your own. As a result, your eyes may become dry from tear evaporation.