Dry eye syndrome is a common disorder of the normal tear film that results from decreased tear production, excessive tear evaporation, and an abnormality in the production of mucus or lipids normally found in the tear layer, or a combination of these. 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’s syndrome, rheumatoid arthritis, or lupus. Evaporative loss of the watery tear layer is usually a result of an insufficient overlying lipid layer.
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Causes of Dry Eye Syndrome
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. While reading, watching TV, or performing a task that requires close attention with the eyes, a person may not blink as often. This decreased blinking allows excessive evaporation of the tears. Certain conditions, such as stroke or Bell’s palsy, make it difficult to close the eyes. As a result, the eyes may become dry from tear evaporation.
Abnormal production of mucin by the conjunctiva may occur. This can result from chemical (alkali) burns to the eye or because of different autoimmune diseases, such as Stevens-Johnson syndrome and cicatricial pemphigoid. This abnormal production leads to poor spreading of the tears over the surface of the eye. The surface of the eye can dry out and even become damaged, even though more than enough watery tears may be present.
Insufficient lipid layers are the result of meibomian gland dysfunction, rosacea, or following oral isotretinoin medication. Meibomian glands are the oil glands in the eyelids that produce the lipid layer. If these oil glands become blocked or if the oil is too thick, there may not be enough oil to cover the watery tear layer to prevent its evaporation.
In addition, if an infection is present along the eyelids or the eyelashes (called blepharitis) the bacteria may break down the oil, so there may not be enough oil. This may lead to evaporative loss of tears and dry eyes.