The uncomfortable sensation of the dry eyes is caused by inadequate tears. Human tears are composed of water, oils, protein and electrolytes. The purpose of this mixture is to protect the eye from the infection and to keep its surface clean and smooth.
Causes of dry eyes include imbalance in the composition of the tears, insufficient production of tears, certain medical conditions and medications as well as some environmental factors.
Imbalance of the components of tears
The tear film which covers and protects the eye has three main ingredients- oil, water and mucus. Oil makes the outer layer of the film, it smoothens it and prevents fast evaporation of the water component. When this component is lacking, the water evaporates too quickly and causes the sensation of dry eyes. Oil is produced in the glands located on the edge of the eyelids.
Water makes up the middle layer, it cleans the surface of the eye and washes away irritants. This layer is produced by tear glands. Mucus component is the inner layer that enables the even spreading of the tear film. When mucus is not produced in sufficient quantities, dry spots, called cornea, may appear on the surface of the eye.
Low tear production
The condition in which the eye cannot produce enough tears is called keratoconjunctivitis sicca. This happens to elderly people, post-menopausal women, to people who have certain medical conditions like lupus, diabetes, rheumatoid arthritis, scleroderma and conditions related to the vitamin A deficiency. It can also be a consequence of laser eye surgery or LASIK surgery, but in this case the dryness is only a temporary side effect of the procedure. Damage to the tear glands can also decrease the production of tears.
The thin tear film is spread evenly across the surface of the eye by blinking of the eyelids. If for some reason blinking is made difficult, the film will not be spread adequately or the tears will evaporate too quickly, causing dryness of the eyes. These problems can include in-turning of the eyelids, or their out-turning.
Dry eyes can be caused by diuretics and blood pressure drugs, antihistamines, decongestants, birth control pills and some analgesics, like ibuprofen and naproxen, some antidepressants and certain medications used for the treatment of acne.
Factors that can contribute to dry eyes include windy or cold weather, dry air, high altitudes, activities that require concentration and cause eye strain, like reading, writing, working at a computer, driving and similar.