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A condition caused by the reduction in quality and quantity of tears. Common symptoms include pain from eye irritation; a sandy or gritty sensation; decreased tolerance to contact lenses; and sensitivity to light. In early stages, the symptoms may seem to appear and disappear, but generally worsen as the day progresses. The condition is typically treated with artificial tears and/or prescription drops or ointments. Closure of the tear drainage system (punctal occlusion) may also improve symptoms.
Dry eyes affect millions of people throughout the world. The condition is most common in women after menopause, but it can affect both sexes and all age groups, including children.
The symptoms of dry eyes are well known to sufferers — irritation, redness, sensitivity to light, and the constant sensation of foreign matter in the eye. However, these symptoms are less frequently recognized by health care professionals, who often neglect or minimize them. A clearer understanding of the nature and causes of dry eyes may enable sufferers to cope with their condition and participate in the care and treatment of their eyes.
The tear film protects the eye by cleansing and flushing harmful chemicals and environmental contaminants from the surface. This anti-infectious film, which covers the surface of the eye, is like a three-layer sandwich.
The outer, or lipid, layer is very thin and oily. This layer, which floats on the surface of the tear film, is secreted by the meibomian glands located just behind the lashes in the lids. The outer layer prevents evaporation of the tear film; abnormalities and diseases of the eyelids, such as blepharitis, styes, and chalazia, cause increased evaporation and dry eyes.
The middle, or aqueous, layer is secreted by the lacrimal glands and accounts for most of the tear film. This layer provides the eye surface with oxygen and various nutrients. Sjogren's syndrome, a systemic disease characterized by dry eyes and dry mouth, affects the lacrimal gland leading to a decrease in aqueous tear secretion.
The inner, or mucin, layer is secreted by goblet cells normally present on the surface of the eye. This layer coats the eye's surface (epithelium) with a substance called mucin, which enables tears to "stick" to the surface of the eye. Deficiencies in mucin can cause dry eyes in patients with eye disorders caused by chemical injuries, Steven-Johnson syndrome, and ocular pemphigoid and systemic diseases caused by nutritional deficiencies.
The eyelids are also important in maintaining the tear film. Eyelids, which open and close normally, spread tears over the surface of the eye (epithelium). That does not occur in patients with abnormal lid position, poor lid closure, or an incomplete blink.
The epithelium keeps bacteria and other microorganisms, chemicals and pollutants from entering the eye. The epithelium and the tear film form the major refracting surface of the eye and are responsible for sharp, clear vision.
Damage to the surface of the eye accounts for the pain, foreign body sensation, difficulties with bright lights, decreased or blurred vision, increased risk of infection and problems with allergic or toxic reactions to topical eye drops in patients with dry eyes.