Dry eye, also known as dry eye syndrome, is a chronic irritation of the ocular surface as a result of anomalies in tear quantity and/or composition.
Tears, produced by the lacrimal glands are composed of water and salt. In addition, small amounts of protein and lipids (fats) are made by cells (cells with mucus, located in the conjunctiva, the thin membrane that covers the white of the eye) and some small glands (Meibomian glands) located in the eyelids. Regular blinking of the eyelids can mix and spread these different components, thus forming a moisturizing, lubricating and protective film that adheres to the surface of the eye, called "tear film.
In certain circumstances the production of tears, proteins or lipids is disturbed:
The amount of tears may be insufficient, either by reduction of their production or through excessive evaporation.
The production of lipids or proteins can be altered, not allowing them to form an effective protective film on the surface of the eye. In this case, sometimes tear production even increases, inducing tearing without improving the situation, since these other essential components of the tear film are lacking.
In both cases, the result is the same: the surface of the eye is not hydrated or lubricated enough which produces chronic irritation and causes symptoms or complications to occur.