The adult human cornea measures 10–12 mm horizontally and 10–11 mm vertically. The average corneal thickness is 555 ± 37 μm centrally,1 and 600–690 μm peripherally.2 The cornea has an important function in focusing light on the retina; it contributes 43.25 Diopters (approximately 75% of the total) to the total refractive power of the eye. The cornea is covered by the tear film, which functions as first-line defense against infections. The normal tear film contains components of the complement cascade, immunoglobulins, and cytokines. It lubricates and supplies the avascular cornea with essential nutrients. Proper lubrication depends on blinking, mediated by intact eyelids. Blinking helps to remove debris from the ocular surface and distributes the tear film evenly on the cornea. The corneal layers from front to back include the epithelium (4–5 layers) and its basement membrane, Bowman's layer, stroma, Descemet's membrane, and endothelium (single layer).
The transition from the cornea to the sclera (covered by the conjunctiva) is called “limbus.” In the limbal area, blood vessels are present and stem cells for the corneal epithelium reside here. Behind the cornea, the anterior chamber contains a clear fluid, the aqueous humor. Anterior segment structures visible with the naked eye include the iris and crystalline lens.
Inflammation of the cornea is called keratitis. Keratitis may be infectious or noninfectious. Noninfectious keratitis is seen in ocular surface disorders such as dry eye syndrome and collagen vascular disorders such as rheumatoid arthritis. Infectious keratitis is a medical emergency, because of its potential to decrease vision permanently. Therefore, infectious keratitis requires prompt diagnosis and treatment.
Finally, whenever treating a child with ocular problems, one should recognize that amblyopia (impaired vision) can develop in an eye that receives a degraded image. Amblyopia is treated with appropriate restoration of a clear visual axis, correction of refractive errors (glasses, contact lenses), and specific treatment, such as occlusion therapy or optical/medical penalization. The risk of amblyopia development is higher in younger patients.
Superficial versus Ulcerative Keratitis
In superficial keratitis, only the epithelial layer is involved. Mild stromal edema may be present, but the examiner can see through it and appreciate iris details. In ulcerative keratitis, an epithelial defect is present, and the underlying stroma shows whitish discoloration. This is also called a “corneal ulcer.” Visualization of anterior segment structures is impeded by an ulcer. It is important to inspect the cornea with the slit lamp to assess size of the epithelial defect and depth of underlying stromal ulceration. A corneal ulcer can lead to significant thinning and even perforation of the cornea. The anterior chamber of the eye is examined for cells or layering of white blood cells (hypopyon), characteristically found in more severe cases of infectious keratitis.