The cornea is the eye's outermost layer. It is the clear, dome-shaped surface that covers the front of the eye.
What Is the function of the cornea?
Because the cornea is as smooth and clear as glass but is strong and durable, it helps the eye in two ways:
- It helps to shield the rest of the eye from germs, dust, and other harmful matter. The cornea shares this protective task with the eyelids, the eye socket, tears, and the sclera, or white part of the eye.
- The cornea acts as the eye's outermost lens. It functions like a window that controls and focuses the entry of light into the eye. The cornea contributes between 65-75 percent of the eye's total focusing power.
When light strikes the cornea, it bends or refracts the incoming light onto the lens. The lens further refocuses that light onto the retina, a layer of light-sensing cells lining the back of the eye that starts the translation of light into vision. For you to see clearly, light rays must be focused by the cornea and lens to fall precisely on the retina. The retina converts the light rays into impulses that are sent through the optic nerve to the brain, which interprets them as images.
The refractive process is similar to the way a camera takes a picture — the cornea and lens in the eye act as the camera lens. The retina is similar to the film. If the image is not focused properly, the film (or retina) receives a blurry image.
The cornea also serves as a filter, screening out some of the most damaging ultraviolet (UV) wavelengths in sunlight. Without this protection, the lens and the retina would be highly susceptible to injury from UV radiation.
About 120 million people in the United States wear eyeglasses or contact lenses to correct nearsightedness, farsightedness, or astigmatism. These vision disorders—called refractive errors—affect the cornea and are the most common of all vision problems in this country.
Refractive errors occur when the curve of the cornea is irregularly shaped (too steep or too flat). When the cornea is of normal shape and curvature, it bends or refracts, light on the retina with precision. However, when the curve of the cornea is irregularly shaped, the cornea bends light imperfectly on the retina. This affects good vision. The refractive process is similar to the way a camera takes a picture. The cornea and lens in your eye act as the camera lens. The retina is similar to the film. If the image is not focused properly, the film (or retina) receives a blurry image. The image that your retina" sees" then goes to your brain, which tells you what the image is.
When the cornea is curved too much, or if the eye is too long, faraway objects will appear blurry because they are focused in front of the retina. This is called myopia, or nearsightedness. Myopia affects over 25 percent of all adult Americans.
Hyperopia, or farsightedness, is the opposite of myopia. Distant objects are clear, and close-up objects appear blurry. With hyperopia, images focus on a point beyond the retina. Hyperopia results from an eye that is too short.
Astigmatism is a condition in which the uneven curvature of the cornea blurs and distorts both distant and near objects. A normal cornea is round, with even curves from side to side and top to bottom. With astigmatism, the cornea is shaped more like the back of a spoon, curved more in one direction than in another. This causes light rays to have more than one focal point and focus on two separate areas of the retina, distorting the visual image. Two-thirds of Americans with myopia also have astigmatism.
Refractive errors are usually corrected by eyeglasses or contact lenses. Although these are safe and effective methods for treating refractive errors, refractive surgeries are becoming an increasingly popular option.
What are some diseases and disorders affecting the cornea?
Sometimes the cornea is damaged after a foreign object has penetrated the tissue, such as from a poke in the eye. At other times, bacteria or fungi from a contaminated contact lens can pass into the cornea. Situations like these can cause painful inflammation and corneal infections called keratitis. These infections can reduce visual clarity, produce corneal discharges, and perhaps erode the cornea. Corneal infections can also lead to corneal scarring, which can impair vision and may require a corneal transplant.
As a general rule, the deeper the corneal infection, the more severe the symptoms and complications. It should be noted that corneal infections, although relatively infrequent, are the most serious complication of contact lens wear.
Minor corneal infections are commonly treated with anti-bacterial eye drops. If the problem is severe, it may require more intensive antibiotic or anti-fungal treatment to eliminate the infection, as well as steroid eye drops to reduce inflammation. Frequent visits to an eye care professional may be necessary for several months to eliminate the problem.
Fuchs' dystrophy occurs when endothelial cells gradually deteriorate without any apparent reason. As more endothelial cells are lost over the years, the endothelium becomes less efficient at pumping water out of the stroma. This causes the cornea to swell and distort vision. Eventually, the epithelium also takes on water, resulting in pain and severe visual impairment.
Epithelial swelling damages vision by changing the cornea's normal curvature, and causing a sight-impairing haze to appear in the tissue. Epithelial swelling will also produce tiny blisters on the corneal surface. When these blisters burst, they are extremely painful.
Corneal transplants are the most common and successful of all transplant surgery done today. The cornea is the clear front of the eye through which light is focused so we can see. If the cornea is damaged it may become swollen or scarred, losing its smoothness and clarity and causing glare or blurred vision. A corneal transplant is indicated if vision cannot be corrected satisfactorily by other methods or painful swelling cannot be relieved by medications or special contact lenses Once your and your ophthalmologist decide you need a corneal transplant, your name is put on a list at the local eye bank. Usually, the wait is short. Before the cornea is released for transplant, the eye bank tests the donor for the viruses that cause hepatitis and AIDS and checks the cornea for clarity.
Surgery is often done on an outpatient basis, with either local or general anesthetic, depending on your age, medical condition, and eye disease. The operation is painless, consisting of removal of the diseased or injured cornea and the completion of any additional work, such as the removal of a cataract, that is necessary; then the clear donor cornea is sewn into place. You may go home after a short stay in the recovery area. You will probably have to wear an eye shield or glasses as prescribed by your doctor. It may be several months before healing is complete and your doctor removes the stitches. Corneal transplants may be rejected %5 to %30 of the time, but most rejections, if treated promptly, can be stopped with minimal injury.
A successful corneal transplant requires care and attention on the part of both patient and physician. However, no other surgery has so much to offer when the cornea is deeply scarred or swollen. The vast majority of people who undergo corneal transplants are happy with their improved vision.