What are dry eyes?
The eye depends on the presence of a tear film to provide constant moisture and lubrication to maintain vision and comfort. Tears are a combination of:
- Water, for moisture;
- Oils, for lubrication and to prevent evaporation of tear liquid;
- Mucus, for even spreading of tears on the surface of the eye; and,
- Antibodies and special proteins, for resistance to infection.
These components are secreted by special glands located around the eye. When there is an imbalance or deficiency in this tear system, or when the tears evaporate too quickly, a person may experience dry eye.
When tears do not lubricate the eye enough, you may have the following in your eye:
- A gritty sensation, like a feeling of a foreign body or sand
- Redness and blurring of vision.
Sometimes, a person with dry eyes will have excess tears running down the cheeks, which may seem confusing. This happens when the eye isn't getting enough lubrication. The eye sends a distress signal through the nervous system for more lubrication. In response, the eye is flooded with emergency tears.
However, these tears are mostly water and do not have the lubricating qualities or the rich makeup of normal tears. They will wash dirt away from the eye, but they will not coat the eye surface properly. In addition, because these emergency tears tend to arrive too late, the eye needs to regenerate and restore itself, and treatment is necessary.
What causes dry eyes?
The majority of patients with dry eye have chronic inflammation (swelling) in the tear glands (lacrimal glands) that line the eyelid and in the conjunctiva (the thin lining on the inside of the eyelids and the front part of the eye). Just like inflammation in a knee, the lungs, or liver, this chronic inflammation can permanently damage the tear gland tissue to the point that treatment becomes ineffective.
In addition to an imbalance in the tear-flow system of the eye, dry eye can be caused by the drying out of the tear film. This can be made worse by dry air created by air conditioning, heat, or other environmental conditions. Many patients also have ocular rosacea (meibomian gland dysfunction), an abnormality of the glands on the edge of the eyelid (meibomian glands) that are supposed to produce the oil to prevent evaporation of the tears. When a patient has both dry eye and ocular rosacea, not only does he or she produce too few tears, but the tears he or she does make evaporate too quickly.