What is the cornea?
The cornea is the eye's outermost layer. It is the clear, dome-shaped surface that covers the front of the eye.
Structure of the Cornea
Although the cornea is clear and seems to lack substance, it is actually a highly organized group of cells and proteins. Unlike most tissues in the body, the cornea contains no blood vessels to nourish or protect it against infection. Instead, the cornea receives its nourishment from the tears and aqueous humor that fills the chamber behind it. The cornea must remain transparent to refract light properly, and the presence of even the tiniest blood vessels can interfere with this process. To see well, all layers of the cornea must be free of any cloudy or opaque areas.
The corneal tissue is arranged in five basic layers, each having an important function. These five layers are:
Epithelium The epithelium is the cornea's outermost region, comprising about 10 percent of the tissue's thickness. The epithelium functions primarily to: (1) Block the passage of foreign material, such as dust, water, and bacteria, into the eye and other layers of the cornea; and (2) Provide a smooth surface that absorbs oxygen and cell nutrients from tears, then distributes these nutrients to the rest of the cornea. The epithelium is filled with thousands of tiny nerve endings that make the cornea extremely sensitive to pain when rubbed or scratched. The part of the epithelium that serves as the foundation on which the epithelial cells anchor and organize themselves is called the basement membrane.
Lying directly below the basement membrane of the epithelium is a transparent sheet of tissue known as Bowman's layer. It is composed of strong layered protein fibers called collagen. Once injured, Bowman's layer can form a scar as it heals. If these scars are large and centrally located, some vision loss can occur.
Beneath Bowman's layer is the stroma, which comprises about 90 percent of the cornea's thickness. It consists primarily of water (78 percent) and collagen (16 percent), and does not contain any blood vessels. Collagen gives the cornea its strength, elasticity, and form. The collagen's unique shape, arrangement, and spacing are essential in producing the cornea's light-conducting transparency.
Under the stroma is Descemet's membrane, a thin but strong sheet of tissue that serves as a protective barrier against infection and injuries. Descemet's membrane is composed of collagen fibers (different from those of the stroma) and is made by the endothelial cells that lie below it. Descemet's membrane is regenerated readily after injury.
The endothelium is the extremely thin, innermost layer of the cornea. Endothelial cells are essential in keeping the cornea clear. Normally, fluid leaks slowly from inside the eye into the middle corneal layer (stroma). The endothelium's primary task is to pump this excess fluid out of the stroma. Without this pumping action, the stroma would swell with water, become hazy, and ultimately opaque. In a healthy eye, a perfect balance is maintained between the fluid moving into the cornea and fluid being pumped out of the cornea. Once endothelium cells are destroyed by disease or trauma, they are lost forever. If too many endothelial cells are destroyed, corneal edema and blindness ensue, with corneal transplantation the only available therapy.
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 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:
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.
How does the cornea respond to injury?
The cornea copes very well with minor injuries or abrasions. If the highly sensitive cornea is scratched, healthy cells slide over quickly and patch the injury before infection occurs and vision is affected. If the scratch penetrates the cornea more deeply, however, the healing process will take longer, at times resulting in greater pain, blurred vision, tearing, redness, and extreme sensitivity to light. These symptoms require professional treatment. Deeper scratches can also cause corneal scarring, resulting in a haze on the cornea that can greatly impair vision. In this case, a corneal transplant may be needed.
What are some diseases and disorders affecting the cornea?
Some diseases and disorders of the cornea are:
What is a corneal transplant? Is it safe?
A corneal transplant involves replacing a diseased or scarred cornea with a new one. When the cornea becomes cloudy, light cannot penetrate the eye to reach the light-sensitive retina. Poor vision or blindness may result.
In corneal transplant surgery, the surgeon removes the central portion of the cloudy cornea and replaces it with a clear cornea, usually donated through an eye bank. A trephine, an instrument like a cookie cutter, is used to remove the cloudy cornea. The surgeon places the new cornea in the opening and sews it with a very fine thread. The thread stays in for months or even years until the eye heals properly (removing the thread is quite simple and can easily be done in an ophthalmologist's office). Following surgery, eye drops to help promote healing will be needed for several months.
Corneal transplants are very common in the United States; about 40,000 are performed each year. The chances of success of this operation have risen dramatically because of technological advances, such as less irritating sutures, or threads, which are often finer than a human hair; and the surgical microscope. Corneal transplantation has restored sight to many, who a generation ago would have been blinded permanently by corneal injury, infection, or inherited corneal disease or degeneration.
What problems can develop from a corneal transplant?
Even with a fairly high success rate, some problems can develop, such as rejection of the new cornea. Warning signs for rejection are decreased vision, increased redness of the eye, increased pain, and increased sensitivity to light. If any of these last for more than six hours, you should immediately call your ophthalmologist. Rejection can be successfully treated if medication is administered at the first sign of symptoms.
A study supported by the National Eye Institute (NEI) suggests that matching the blood type, but not tissue type, of the recipient with that of the cornea donor may improve the success rate of corneal transplants in people at high risk for graft failure. Approximately 20 percent of corneal transplant patients--between 6000-8000 a year--reject their donor corneas. The NEI-supported study, called the Collaborative Corneal Transplantation Study, found that high-risk patients may reduce the likelihood of corneal rejection if their blood types match those of the cornea donors. The study also concluded that intensive steroid treatment after transplant surgery improves the chances for a successful transplant.
Are there alternatives to a corneal transplant?
Phototherapeutic keratectomy (PTK) is one of the latest advances in eye care for the treatment of corneal dystrophies, corneal scars, and certain corneal infections. Only a short time ago, people with these disorders would most likely have needed a corneal transplant. By combining the precision of the excimer laser with the control of a computer, doctors can vaporize microscopically thin layers of diseased corneal tissue and etch away the surface irregularities associated with many corneal dystrophies and scars. Surrounding areas suffer relatively little trauma. New tissue can then grow over the now-smooth surface. Recovery from the procedure takes a matter of days, rather than months as with a transplant. The return of vision can occur rapidly, especially if the cause of the problem is confined to the top layer of the cornea. Studies have shown close to an 85 percent success rate in corneal repair using PTK for well-selected patients.
The Excimer Laser
One of the technologies developed to treat corneal disease is the excimer laser. This device emits pulses of ultraviolet light--a laser beam--to etch away surface irregularities of corneal tissue. Because of the laser's precision, damage to healthy, adjoining tissue is reduced or eliminated.
The PTK procedure is especially useful for people with inherited disorders, whose scars or other corneal opacities limit vision by blocking the way images form on the retina. PTK has been approved by the U.S. Food and Drug Administration.
Current Corneal Research
Vision research funded by the National Eye Institute (NEI) is leading to progress in understanding and treating corneal disease.
For example, scientists are learning how transplanting corneal cells from a patient's healthy eye to the diseased eye can treat certain conditions that previously caused blindness. Vision researchers continue to investigate ways to enhance corneal healing and eliminate the corneal scarring that can threaten sight. Also, understanding how genes produce and maintain a healthy cornea will help in treating corneal disease.
Genetic studies in families afflicted with corneal dystrophies have yielded new insight into 13 different corneal dystrophies, including keratoconus. To identify factors that influence the severity and progression of keratoconus, the NEI is conducting a natural history study--called the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study--that is following more than 1200 patients with the disease. Scientists are looking for answers to how rapidly their keratoconus will progress, how bad their vision will become, and whether they will need cornealsurgery to treat it. Results from the CLEK Study will enable eye care practitioners to better manage this complex disease.
The NEI also supported the Herpetic Eye Disease Study (HEDS), a group of clinical trials that studied various treatments for severe ocular herpes. HEDS researchers reported that oral acyclovir reduced by 41 percent the chance that ocular herpes, a recurrent disease, would return. The study clearly showed that acyclovir therapy can benefit people with all forms of ocular herpes. Current HEDS research is examining the role of psychological stress and other factors as triggers of ocular herpes recurrences.
The information in this section was obtained from the NEI website www.nei.nih.gov.
About the National Eye Institute
The National Eye Institute (NEI) is one of the Federal government's National Institutes of Health. It was established by Congress in 1968 to discover safe and effective ways of preventing, diagnosing, and treating eye diseases and disorders. The NEI is the major sponsor of vision research in the U.S. This research is conducted at about 250 medical centers, hospitals, and universities across the country. Other clinical trials are conducted by NEI researchers at the National Institutes of Health campus in Bethesda, Maryland.
For more information about the NEI or NEI-sponsored clinical trials, contact the:
Other Information Sources
Cornea Research Foundation of America
9002 N. Meridian Street, Suite 212
Indianapolis, IN 46260
National Keratoconus Foundation
8733 Beverly Blvd., Suite 201
Los Angeles, CA 90048
Sjögren's Syndrome Foundation (SSF)
8120 Woodmont Avenue, Suite 530
Bethesda, MD 20814
Stevens Johnson Syndrome Foundation
P.O. Box 350333
Westminster, CO 80030