Cornea, disorders of

Injuries or diseases affecting the cornea, the outer shell of the eyeball. Injuries include corneal abrasions, which sometimes become infected and progress to a corneal ulcer. Penetrating corneal injuries can cause scarring, which may lead to impairment of vision. Chemical injuries can result from contact with a corrosive substance and require immediate flushing of the eye with water. In actinic keratopathy, the outer layer of the cornea is damaged by ultraviolet light. In exposure keratopathy, damage is due to reduced protection by the tear film and blink reflex. The cornea can also be infected by viruses, bacteria, and fungi, the herpes simplex virus being especially dangerous. True inflammation of the cornea (called keratitis) is uncommon as the cornea contains no blood vessels. Other disorders include: keratomalacia as a result of vitamin A deficiency; keratoconjunctivitis sicca (dry eye); corneal dystrophies such as keratoconus; and oedema, in which fluid builds up in the cornea and impairs vision. Rare congenital defects include microcornea (smaller cornea than normal) or megalocornea (bigger than normal) and buphthalmos, or “ox-eye’’, in which the entire eyeball is distended as a result of glaucoma. Degenerative conditions of the cornea such as calcium deposition, thinning, and spontaneous ulceration occur mainly in the elderly, and are more common in previously damaged eyes.