Dry eye is a process which affects the surface of the eye with a deterioration of the cornea and conjunctiva due to poor wetting and lubrication of these structures because of the poor quality or poor tear.
The direct contact of the air with the ocular surface determines changes in surface cells (metaplasia epidermizante), their metabolism is altered and the consequences are a more fast death and those cells disappearence. Moreover, the lack of tear results in loss of natural lubricant and the rubbing of the eyelids over eyes takes place, a constant microtrauma which further aggravates the process of cell dry-out, an inflammatory response is induced with release of toxic substances (cytokines) against the tissues.
We now know that the deterioration of the tear, so when it is a quantitative deficiency, as in the case of deterioration in its quality, is initially manifested by an increase in osmolarity of the particles that are dissolved in the aqueous part of tears. Hyperosmolarity causes negative conditions, toxic to the cells of the ocular surface, starting an inflammatory process that closes a vicious circle which alters increasingly tear features, makes it evaporate more quickly and dries the ocular surface because of the action of air directly on the ocular surface.
Up to now, we thought that the cases of dry eye were more frequent due to lack of tear but it has been shown that the cases with a tear of poor quality or, as happens even more often , a mixed mechanism are more frequent since in most patients, the factors we have seen above, are given more than once. This is important because it pretty changes how to approach the diagnosis and treatment, as we shall see.