Dry eye known medically as keratoconjunctivitis sicca or keratitis sicca, is a condition where there is a problem with the production of tears.
Usually eyes feel irritated, scratchy, dry and uncomfortable. Eyes may be red and there may be a burning sensation, or it may feel as if there is something in the eye like grit or an eyelash. Sometimes there may be periods of blurred vision but these normally go away after a short while or on blinking. Sometimes eyes may water too much and often eyes won’t actually feel dry but you may notice that there are no tears when you are upset or when peeling onions.
Tears are important as they form a layer on the very front of the eye. The tear film takes a part in the focusing of light into the eye but it mainly lubricates the eye keeping it moist and smooth. The tear film is actually made up of three layers each with a different function.
The layer closest to the eye is called the mucin layer. It coats the cornea which is the clear window at the front of the eye; it forms a foundation for the other layers of the tear film.
The middle layer is called the aqueous layer. This is the watery layer that most people think of when they think of tears. This layer is produced by the lacrimal gland which is underneath the upper eyelid. This layer provides moisture and supplies oxygen and other important nutrients to the cornea. This layer is mainly water.
The outer layer is called the lipid layer. This is an oily film which seals the tear film to the eye and helps to prevent any evaporation. Evaporation is the technical name for the watery layer being lost into the air.
The mucin and lipid layers are produced by small glands around the eye. Each time we blink the tears are spread onto the front of the eye. Excess tears are drained away by tiny drainage holes on the inside of the eyelids, these holes channel the excess tears into the nose. This is why crying sometimes makes your nose run.
Dry eye has a number of causes. It happens mostly as a part of the natural ageing process but can also be caused by problems with blinking or problems with the glands which produce the tears. Some drugs can cause dry eye, like antihistamines and oral contraceptives. Contact lenses carry an increased risk if you have dry eye.
Sometimes dry eye is also a symptom of other conditions affecting other parts of the body particularly arthritis or a condition called Sjogrens syndrome. Sjogrens syndrome is a condition where, as well as dry eyes, people can also have a dry mouth. This condition can also involve a type of arthritis.