Pink eye is a colloquial term for conjunctivitis which is a term that refers to the inflammation of the outer layer of the eye and of the inside of the eyelids. There are four types of conjunctivitis which are bacterial, viral allergic and chemical. Symptoms common to all types are red eye (hyperaemia), irritation (chemosis) and watering (epiphora). Eyesight is not usually affected.
Viral conjunctivitis often occurs alongside infection of the upper respiratory tract, the common cold or a sore throat. The term pink eye is commonly used to denote any type of conjunctivitis but if it is used specifically it is with reference to the viral type.
Bacterial conjunctivitis is often diagnosed by the presence of mucopurulent discharge that causes the eyelids to stick together and crusting of the eye. These obvious signs do not have to be present for the infection to be bacterial in origin though as not all bacteria that cause conjunctivitis are of the pus producing pyogenic type.
Allergic conjunctivitis is caused by an allergen, often pollen or dander, that in turn causes the release of histamine and other substances by mast cells. These substances stimulate the dilation of blood vessels, increase secretion of tears and irritate the nerve endings. The whole series of events is due to the fact that the body is mounting defenses against an organic external threat where there is, in reality, none.
Chemical injury leading to conjunctivitis happens when an acid or an alkali gets in the eye. Mild burns cause the condition while more severe burns cause the cornea to turn white. Litmus paper is used to ascertain whether an acid or alkali burn is the culprit and if the pH is outside the 6-8 range lots of irrigation until the pH returns to nuetral is the recommended treatment.