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 corneal ulcer is an open sore on the cornea.
 

Corneal ulcers may begin with a corneal injury, which then becomes infected with bacteria, fungi, or the protozoan Acanthamoeba (which lives in contaminated water). Viral ulcers (often due to a herpes virus) can be triggered to recur by physical stress or may recur spontaneously. Ulcers can also occur if a foreign object lodges in the eye or the eye is irritated by a contact lens, especially when contact lenses are worn during sleep or are not adequately disinfected. A deficiency of vitamin A and protein may lead to the formation of a corneal ulcer; however, such ulcers are rare in the United States.

When the eyelids do not close properly, the cornea may become dry and irritated; such irritation can lead to injury and the development of a corneal ulcer. These ulcers usually become infected. Corneal injury may result from in-growing eyelashes (trichiasis) or an inturned eyelid (entropion).

Symptoms

Corneal ulcers cause pain, usually a feeling like a foreign object is in the eye, with aching and sensitivity to bright light and increased tear production. A white spot of pus may appear in the cornea. Sometimes, ulcers develop over the entire cornea and may penetrate deeply. Additional pus may accumulate behind the cornea. The deeper the ulcer, the more severe the symptoms and complications. The conjunctiva usually is bloodshot.

Corneal ulcers may heal with treatment, but they may leave a cloudy scar that impairs vision. Other complications may include deep-seated infection, perforation of the cornea, displacement of the iris, and destruction of the eye

Diagnosis and Treatment

A corneal ulcer is an emergency that should be treated immediately. To see an ulcer clearly, a doctor may apply eye drops that contain a dye called fluorescein, which temporarily stains the ulcer and allows it to be examined more clearly. Treatment depends on the underlying cause. For instance, antibiotic, antiviral, or antifungal drugs may be needed. Corneal transplantation (keratoplasty) is sometimes needed also (see
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Source :
http://www.merck.com/mmhe/sec20/ch230/ch230c.html