Peripheral Ulcerative Keratitis (PUK) is a group of destructive inflammatory diseases involving the peripheral cornea in human eyes. The symptoms of PUK include pain, redness of the eyeball, photophobia, and decreased vision accompanied by distinctive signs of crescent-shaped damage of the cornea. The causes of this disease are broad, ranging from injuries, contamination of contact lenses, to association with other systemic conditions. PUK is associated with different ocular and systemic diseases. Mooren's ulcer is a common form of PUK. The majority of PUK is mediated by local or systemic immunological processes, which can lead to inflammation and eventually tissue damage. Standard PUK diagnostic test involves reviewing the medical history and a completing physical examinations. Two major treatments are the use of medications such as corticosteroids or other immunosuppressive agents and surgical resection of the conjunctiva. The prognosis of PUK is unclear with one study providing potential complications. PUK is a rare condition with an estimated incidence of 3 per million annually.

The most easily identifiable sign is a visible lesion of the cornea presented usually in a crescent shape. Common reasons for destruction are stromal degradation and epithelial defects on the inflammatory cells. There would be a change in conformation of the peripheral cornea, depending on the severity of corneal thinning. This process is usually accompanied by the possibility of concealing perforation. The formation of an oval-shaped ulcer at the margin of the cornea is also a sign.

Symptoms of PUK include pain, redness, tearing, increased sensitivity to bright light, impaired or blurred vision, and the feeling of foreign objects trapped in the eyes.

There are several associations of PUK to ocular and systemic diseases. Rheumatoid arthritis (RA),  Wegner's granulomatosis (WG), and Polyarteritis Nodosa (PAN) are the most common systemic conditions.

There are three major causes for PUK. One possible cause is injury due to any kind of scratches by sharp or hard objects on the surface of the cornea. The scratched area forms an opening in the cornea, allowing microorganisms to access the cornea and lead to infection. Contamination of contact lenses is another cause as fungi, bacteria and parasites, microscopic parasite acanthamoeba, in particular, could inhabit the surface of the carrying case of the contact lens. When placing the contact lens to one's eyes, invisible microorganisms may contaminate the cornea resulting in PUK. An extended period of wearing contact lenses could also cause damage on the cornea surface, allowing the entry of microorganisms to the cornea. Other than contamination of contact lenses, contamination occurring in water could also cause PUK. Especially in places like the ocean, rivers, lakes and hot tubs, massive amounts of bacteria, fungi, and parasites exist. When there is an injury on the cornea surface, contact with contaminated water could transfer unwanted microorganisms into the cornea resulting in PUK. Virus and bacteria are sources of infection to the cornea. Herpes virus and bacteria that cause gonorrhea are some examples.

The corneal epithelium consists of five to six layers of cells with a total thickness of around 0.52mm. The cornea thickens to 0.65mm towards the periphery of the cornea. Stroma, which accounts for 90% of the corneal thickness, refers to the middle layer between epithelium and endothelium. It is present in the peripheral cornea to act as a transitional zone between the sclera and cornea. Limbal vasculature, deriving from capillaries that surround the peripheral cornea, supplies the stroma. Various molecules normally diffuse from these capillaries at the periphery to the central cornea. With limited diffusion, there is a higher concentration of IgM, factor C1 of the complement cascade, and Langerhans cells.  

Any kind of inflammatory stimulus present in the peripheral cornea results in recruitment of neutrophil and activation of both classical and alternative pathways of immune response, namely the humoral and cell-mediated autoimmune responses. These responses will lead to the formation of antigen-specific antibodies to combat foreign antigens. However, antigen-antibody complexes formed may deposit in the vascular endothelium and activate complements leading to severe local inflammation. Under this circumstance, inflammatory cells, such as macrophages and neutrophils, enter the peripheral cornea. These inflammatory cells release enzymes protease and collagenases, causing potential disruption of the corneal stroma. The additional release of cytokines, for example, interleukin-1, from these cells further accelerates the process of stromal destruction.

Mooren's ulcer is a common form of PUK. One classification of Mooren's ulcer, based on the clinical presentation, includes bilateral indolent mooren's ulcer, bilateral aggressive mooren's ulcer and unilateral mooren's ulcer. Unilateral mooren's ulcer, meaning ulcer of one eye, mainly affects elderly above 60 years old. Rapid onset with redness and severe pain of the affected eye and either slow or extremely quick progression are some typical characteristics of unilateral mooren's ulcer. Bilateral aggressive mooren's ulcer is prevalent in Indian between age 14 to 40. The common presentation includes the appearance of lesions in one eye, followed by the development of lesions in another eye. Finally, bilateral indolent mooren's ulcer is common in patients of at least 50-year-old. It usually progresses slowly and causes little or no pain.