In a healthy eye, there is a layer of liquid tears, oil and mucus that makes up the tear film. This tear film coats your eye and prevents it from becoming dry. When one or more elements of this tear film isn't working as it should, you have dry eye disease. The part of the tear film that is not functioning properly determines which type of dry eye you have.
In a healthy eye, there is a layer of tears that bathe the surface of the eye and keep it moist. These tears are called basal tears and are different from the tears that are produced when you cry. Aqueous Deficient Dry Eye is any condition that prevents your body from forming the basal tears that normally always coat your eye or causes these tears to be of a poor quality that they are no longer useful for their intended purpose. Tears are formed by the lacrimal gland, which sits above the eye. When this gland is not working as it should, or your eye is inflamed, tear production can be affected. Other medical conditions can also prevent your body from making healthy tears and we explore those causes here.
Evaporative Dry Eye occurs when the oily layer of your tear film is not working as it should. In your upper and lower eyelids there are little glands, called Meibomian Glands that secrete an oily substance called mebium. The oily mebium helps to keep the tears created by the lacrimal gland on the surface of your eye. Every time you blink, the Meibomian Glands release mebium across the surface of your eye to keep the tears on your eye.
Sometimes the oil in these glands becomes hard or the oil loses quality. When this happens, the meibomian glands can become clogged and there is no longer enough oil to keep the tears on the surface of your eye.
It is estimated that roughly 90% of dry eye cases are caused by Evaporative Dry Eye, while the remaining 10% of cases are caused by Aqueous Deficient Dry Eye. It is also possible to have both Aqueous Deficient and Evaporative dry eye, or for one type of dry eye to contribute to the development of the other by causing inflammation.
There is also a rare type of dry eye called Mucin Deficient Dry Eye. This can occur when the mucus layer on your eye is not functioning properly. It is a rare form of dry eye, with the main causes being due to either a chemical injury to the eye or a Vitamin A deficiency.