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Keratoconjunctivitis sicca, also called keratitis sicca, sicca syndrome, xerophthalmia, dry eye syndrome, or simply dry eyes, is an eye disease caused by decreased tear production or increased tear film evaporation commonly found in humans and some animals. The phrase “keratoconjunctivitis sicca” is Latin, and its literal translation is “dryness of the cornea and conjunctiva”.

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. This definition of dry eyes was adopted by the National Eye Institute workshop on dry eyes.

The eye becomes dry either because there is not enough tears being produced or because there is abnormally high rate of evaporation of tears.

In the light of new knowledge about the roles of tear hyperosmolarity and ocular surface inflammation in dry eye and the effects of dry eye on visual function, The Dry Eye Workshop (DEWS) in 2007 adopted a revised definition of dry eyes. According to the DEWS definition, Dry eye is a multifactorial disease of the tears and ocular surface that results in symptoms of discomfort, visual disturbance, and tear film instability with potential damage to the ocular surface. It is accompanied by increased osmolarity of the tear film and inflammation of the ocular surface. Dry eye is recognized as a disturbance of the Lacrimal Functional Unit (LFU), an integrated system comprising the lacrimal glands, ocular surface (cornea, conjunctiva and meibomian glands) and lids, and the sensory and motor nerves that connect them (Link fo DEWS Report).

According to the DEWS report, prevalence of dry eye ranges from 5%-30% in people aged 50 years and older. Prevalence of moderate- to-severe dry eye lies closer to the lower estimate of the range, whereas prevalence of mild dry eyes lies closer to the higher estimate of the range. It is estimated that about 3.2 million women and 1.7 million men, for a total of 4.9 million Americans 50 years and older have dry eye. Tens of millions more have mild dry eyes that may be notable only when some adverse contributing factor is present, such as low humidity or contact lens wear.

There are two kinds of tears: those that lubricate the eye; and those that are produced in response to irritation or emotion. Excessive tears occur when a foreign body irritates the eye or when a person cries.

Tears that lubricate the eye are produced around the clock, and they have an important everyday function. A film of tears, spread over the eye by a blink, makes the surface of the eye smooth and optically clear. Without the tear film, good vision would not be possible.

The tear film consists of three layers: an outer oily layer, a watery layer, and a layer of mucus.Some people do not produce enough lubricating tears to keep the eye wet and comfortable. Tear production normally decreases with age, or it can be due to a disease or medication.Dry eyes can make it difficult or impossible to wear contact lenses.