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The cornea is the clear, transparent and protective outer layer of the eye. It provides a clear medium for light to pass into the eye, through the pupil and on to the retina at the back of the eye. Its elliptical curvature refracts light, contributing about two-thirds of the eye’s total focusing power.
Unlike most tissues in the body, the transparent cornea contains no blood vessels, which might interfere with its ability to pass light. Instead it relies on tears and the fluid inside the eye to nourish it or protect it against infection. This ensures that the layers of the cornea are free of any opaque areas.
As delicate as it is, the cornea can be the site for many diseases, ranging from injuries and infections to degenerative and genetic defects. Corneal illnesses can present in different ways, but many of them cause excessive tearing, red eye and severe pain, especially if the nerves around it are affected. Blurred vision can occur whenever a disease affects either the clarity or the curvature of the cornea. Irritation and light sensitivity are common symptoms in many disorders affecting the surface of the cornea.
If injured, the cornea has a remarkable ability to heal minor scratches. However if the scratch penetrates more deeply, the healing process can take longer.
Keratitis is an inflammation of the cornea that sometimes occurs with infection after viruses, bacteria, or fungi enter the cornea. This can result from injuries or uncommonly from contact lens use. Keratitis can cause all of the symptoms described above, along with discharge from the eyes, and is usually treated with antibiotic or antifungal eye drops.
Other disorders can cause degenerative changes, like Keratoconus, in which the cornea thins and changes shape. The disease is usually inherited or secondary to trauma and can cause visual changes due to distortion of the curvature of the cornea. The nearsightedness is initially treated with glasses but can require rigid contact lenses and sometimes a corneal transplant.
The cornea can be affected by allergies, other genetic disorders and dystrophies, autoimmune disorders, nutrition deficiencies, inflammatory diseases, and growths. It can also be damaged secondarily by other eye conditions such as dry eye, eyelid disorders, and glaucoma.
An eye doctor usually obtains a complete medical history and performs a careful examination of the eye and the cornea with a slit lamp microscope to diagnose corneal disorders. Eye drops and colored dyes are used to diagnose certain conditions. More advanced tests including topography and keratometry (to study the shape of the cornea), pachymetry (to study the thickness of the cornea), specialized microscopy, and laboratory studies, are required in some cases.
Just like the varied conditions, treatments might range from medications, laser treatment or surgery, depending on the condition. Infectious corneal disease caused from bacteria and viruses can be prevented by good hygiene and regular vaccinations, protecting the eye from injury and limiting physical contact with people who have contagious forms of conjunctivitis. Glasses and sunglasses with the most complete ultraviolet protection can minimize damage from the sun’s rays, including eye surface cancers. Following strict guidelines for contact lens hygiene can help decrease the risk of corneal infections related to contact lens use. Safety glasses protect against many types of trauma. A healthy diet with plenty of omega-3-fatty acids and sufficient vitamin A are especially important for maintaining a protective tear film layer. Although many corneal diseases, like those from hereditary factors (like dystrophies), cannot be prevented, vision can often be preserved with early detection and treatment.