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.

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:

1. 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.

2. 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.

Refractive Errors
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?

Allergies
Allergies affecting the eye are fairly common. The most common allergies are those related to pollen, particularly when the weather is warm and dry. Symptoms can include redness, itching, tearing, burning, stinging, and watery discharge, although they are not usually severe enough to require medical attention. Antihistamine decongestant eyedrops can effectively reduce these symptoms, as does rain and cooler weather, which decreases the amount of pollen in the air.

An increasing number of eye allergy cases are related to medications and contact lens wear. Also, animal hair and certain cosmetics, such as mascara, face creams, and eyebrow pencil, can cause allergies that affect the eye. Touching or rubbing eyes after handling nail polish, soaps, or chemicals may cause an allergic reaction. Some people have sensitivity to lip gloss and eye makeup. Allergy symptoms are temporary and can eliminated by not having contact with the offending cosmetic or detergent.

Conjunctivitis (Pink Eye)
This term describes a group of diseases that cause swelling, itching, burning, and redness of the conjunctiva, the protective membrane that lines the eyelids and covers exposed areas of the sclera, or white of the eye. Conjunctivitis can spread from one person to another and affects millions of Americans at any given time. Conjunctivitis can be caused by a bacterial or viral infection, allergy, environmental irritants, a contact lens product, eyedrops, or eye ointments.
At its onset, conjunctivitis is usually painless and does not adversely affect vision. The infection will clear in most cases without requiring medical care. But for some forms of conjunctivitis, treatment will be needed. If treatment is delayed, the infection may worsen and cause corneal inflammation and a loss of vision.

Corneal Infections
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.

Dry Eye
The continuous production and drainage of tears is important to the eye's health. Tears keep the eye moist, help wounds heal, and protect against eye infection. In people with dry eye, the eye produces fewer or less quality tears and is unable to keep its surface lubricated and comfortable.

The tear film consists of three layers--an outer, oily (lipid) layer that keeps tears from evaporating too quickly and helps tears remain on the eye; a middle (aqueous) layer that nourishes the cornea and conjunctiva; and a bottom (mucin) layer that helps to spread the aqueous layer across the eye to ensure that the eye remains wet. As we age, the eyes usually produce fewer tears. Also, in some cases, the lipid and mucin layers produced by the eye are of such poor quality that tears cannot remain in the eye long enough to keep the eye sufficiently lubricated.

The main symptom of dry eye is usually a scratchy or sandy feeling as if something is in the eye. Other symptoms may include stinging or burning of the eye; episodes of excess tearing that follow periods of very dry sensation; a stringy discharge from the eye; and pain and redness of the eye. Sometimes people with dry eye experience heaviness of the eyelids or blurred, changing, or decreased vision, although loss of vision is uncommon.

Dry eye is more common in women, especially after menopause. Surprisingly, some people with dry eye may have tears that run down their cheeks. This is because the eye may be producing less of the lipid and mucin layers of the tear film, which help keep tears in the eye. When this happens, tears do not stay in the eye long enough to thoroughly moisten it.

Dry eye can occur in climates with dry air, as well as with the use of some drugs, including antihistamines, nasal decongestants, tranquilizers, and anti-depressant drugs. People with dry eye should let their health care providers know all the medications they are taking, since some of them may intensify dry eye symptoms.

People with connective tissue diseases, such as rheumatoid arthritis, can also develop dry eye. It is important to note that dry eye is sometimes a symptom of Sjögren's syndrome, a disease that attacks the body's lubricating glands, such as the tear and salivary glands. A complete physical examination may diagnose any underlying diseases.

Artificial tears, which lubricate the eye, are the principal treatment for dry eye. They are available over-the-counter as eye drops. Sterile ointments are sometimes used at night to help prevent the eye from drying. Using humidifiers, wearing wrap-around glasses when outside, and avoiding outside windy and dry conditions may bring relief. For people with severe cases of dry eye, temporary or permanent closure of the tear drain (small openings at the inner corner of the eyelids where tears drain from the eye) may be helpful.

Fuchs' Dystrophy
Fuchs' dystrophy is a slowly progressing disease that usually affects both eyes and is slightly more common in women than in men. Although doctors can often see early signs of Fuchs' dystrophy in people in their 30s and 40s, the disease rarely affects vision until people reach their 50s and 60s.

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

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.