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Two distinct regions of the cornea have key anatomic, physiologic, and pathologic differences.
Two distinct regions of the cornea have key anatomic, physiologic, and pathologic differences. The central cornea has a diameter of about 6 mm and is surrounded by the peripheral cornea, a 2- to 3-mm "donut."
Infiltrates in the central cornea generally are infectious in nature and may lead to ulceration if left untreated. Conversely, infiltrates in the peripheral cornea usually are sterile and self-limiting. Although not an absolute determinant of infectious versus non-infectious nature, the cause for this distinction is thought to be due to the location of capillaries surrounding the peripheral corneal, making immune cells more readily available to the peripheral cornea.
Differential diagnosis of sterile infiltrates and corneal ulcers begins with careful investigation into the history and physical findings, including clinical signs and the patient's symptoms, on a case-by-case basis. Sterile infiltrates typically are small (<1 mm), gray-white circumlimbal lesions separated from the limbus by about 1 mm of clear space.
Although patients sometimes are asymptomatic, they may present with mild chemosis, mild conjunctival hyperemia, mild ocular irritation (but lack of significant pain), and relatively normal vision.
A few "red flag" indicators of corneal ulcers include positive culture results, the presence of iritis, and purulent discharge. The presence of pain; photophobia; severe, diffuse injection; raised appearance; fluorescein staining; and surrounding haze all are important signs and symptoms to differentiate between an infectious keratitis ("corneal ulcer") and a sterile infiltrate ("catarrhal ulcer").
Bugs behind microbial keratitis
The pathogenicity of P aeruginosa in microbial keratitis appears to be related to its abilities to adhere to contact lenses, produce numerous virulence factors, and invade or kill corneal cells. Typically, receptors on corneal epithelial cells allow for recognition of the infection, and this activates the host's immune system to counteract with an inflammatory response. Continued recruitment of polymorphonuclear neutrophils, however, actually can affect the health of corneal cells and tissues and ultimately may lead to scarring and vision loss.6