A major cause of blindness worldwide, corneal ulcers produce corneal scarring or perforation. They occur in the central or marginal areas of the cornea, vary in shape and size, and may be singular or multiple. Marginal ulcers, caused by a sensitivity to Staphylococcus aureus, are the most common form. Prompt treatment (within hours of onset) can prevent visual impairment.
Corneal ulcers generally result from bacterial, protozoan, viral, or fungal infections. Common bacterial sources include Staphylococcus aureus, Pseudomonas aeruginosa, Streptococcus viridans, Streptococcus (Diplococcus) pneumoniae, and Moraxella liquefaciens; viral sources, herpes simplex type 1, and varicella-zoster viruses; and common fungi, such as Candida, Fusarium, and Cephalosporium.
Other causes include trauma, exposure, reactions to bacterial infections, toxins, and allergens. Tuberculoprotein causes a classic phlyctenular keratoconjunctivitis; vitamin A deficiency results in xerophthalmia; and fifth cranial nerve lesions result in neurotropic ulcers.