Because of its potential to permanently impair vision or perforate the eye, a corneal ulcer is considered an ophthalmologic emergency. Although corneal ulcers may occasionally be sterile, most are infectious in etiology. Ulcers due to viral infection occur on a previously intact corneal epithelium. Bacterial corneal ulcers generally follow a traumatic break in the corneal epithelium, thereby providing an entry for bacteria. The traumatic episode may be minor, such as a minute abrasion from a small foreign body, or may result from such causes as tear insufficiency, malnutrition, or contact lens use. Increased use of soft contact lenses in recent years has led to a dramatic rise in the occurrence of corneal ulcer, particularly due to Pseudomonas aeruginosa.  In addition, with the introduction of topical corticosteroid drugs in the treatment of eye disease, fungal corneal ulcers have become more common.
Peripheral ulcerative keratitis (PUK) is a complication of rheumatoid arthritis (RA) that can lead to rapid corneal destruction (corneal melt) and perforation with loss of vision. An example is shown in the image below.
Mooren ulcer is a rapidly progressive, painful, ulcerative keratitis, which initially affects the peripheral cornea and may spread circumferentially and then centrally. Mooren ulcer can only be diagnosed in the absence of an infectious or systemic cause.
Risk factors include contact lens use, HIV, trauma, ocular surface disease, and ocular surgery.  Overnight contact lens wear has been shown to be associated with increased risk. Other identified risk factors include age, gender smoking, low socioeconomic class, and inadequate contact lens hygiene.
Common bacterial isolates cultured from patients with keratitis include P aeruginosa, coagulase-negative staphylococci, Staphylococcus aureus, Streptococcus pneumoniae, and Enterobacteriaceae (including Klebsiella, Enterobacter, Serratia, and Proteus). Klebsiella pneumoniae mucoid phenotype and its ability to form biofilm may be important in producing a corneal ulceration. Agents, such as N- acetylcysteine, may have a role in treatment because they inhibit biofilm formation.
Fungi (Fusarium) and amoeba (Acanthamoeba) have been found in a small number of patients but frequently present with more severe symptoms.
Mooren ulcer is an idiopathic ulceration of the peripheral cornea, which may be due to an autoimmune reaction or it may be associated with the hepatitis C virus.
Approximately 25,000 Americans develop infectious keratitis annually. The annual incidence of microbial keratitis associated with contact lens use is approximately 2-4 infections per 10,000 users of soft contact lenses and 10-20 infections per 10,000 users of extended-wear contact lenses. Approximately 10% of these infections result in the loss of 2 or more lines of visual acuity. 
A study from the United Kingdom reports factors associated with an increased risk of a corneal invasive event: wearing extended-wear hydrogel lenses, male gender, smoking, and the late winter months (March > July). 
Authors from the United Kingdom also report an 8 times higher incidence of corneal invasive event in contact lens wearers who sleep in contact lenses compared with wearers who use lenses only during the waking hours. 
Corneal scarring and vision loss are possible.
Studies from the United Kingdom suggest that males who wear extended-wear contact lenses are at increased risk of forming a corneal ulcer.
Other studies suggest that males are at increased risk due to the higher probability of sustaining ocular trauma.
Corneal injury or infection can affect people of all ages. A bimodal distribution is observed. The age groups with a higher prevalence of disease are likely tied to risk factors, those in the first group (< 30 y) who are more likely to be contact lenses wearers and/or sustain ocular trauma, and those in the second group (>50 y) who are more likely to undergo eye surgery.
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