Corneal Disease, Facts About

What is the cornea?

The cornea is the eye's outermost layer. It is a clear, dome-shaped surface that covers the front of the eye. It has two main functions:

  • Protection – The cornea helps to shield the rest of the eye from germs, dust, and other harmful objects. The cornea also serves as a filter, screening out some of the most damaging ultraviolet (UV) wavelengths in sunlight. Without this protection, other parts of the eye might be harmed by UV radiation.
  • Vision - 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. When light strikes the cornea, it bends--or refracts--the incoming light onto the lens. The lens refocuses that light onto the retina, a layer of light-sensitive cells at the back of the eye. The retina then converts the light into electrical signals that it sends to the brain. The brain translates these signals into images.

What is the structure of the cornea?

The cornea is a highly organized group of cells and proteins. The tissue of the cornea has five main layers, and each has an important function. In order for a person to see well, all layers of the cornea must be free of any cloudy or opaque areas.

The five layers of the cornea are:

  • Epithelium – This is the outermost layer of the cornea that blocks foreign material, such as dust, water, and bacteria, from entering the eye and provides a smooth surface that allows the cornea to get nutrients from tears. The epithelium is filled with thousands of tiny nerve endings that make the cornea very sensitive to pain.
  • Bowman's Layer – Bowman's layer is a transparent sheet of tissue that lies beneath the epithelium. It is made of strong layered protein fibers called collagen. If it is injured, the tissue in Bowman's layer forms a scar as it heals. If these scars are large and centrally located, they can interfere with vision.
  • Stroma - Beneath Bowman's layer is the stroma, which accounts for about 90 percent of the cornea's thickness. It is mostly made of water and collagen. The collagen's unique shape, arrangement, and spacing are important in allowing light to pass through the cornea. The stromal layer is is the part of the cornea that is reshaped in laser vision correction, such as PRK and LASIK procedures.
  • Descemet's Membrane – This is a thin but strong sheet of tissue that protects the eye from infection and injuries. It also holds a part of the cornea called the corneal endothelium. Descemet's membrane heals quickly after injury.
  • Endothelium - The endothelium is the extremely thin, innermost layer of the cornea. Endothelial cells are essential to keep the cornea clear. Normally, fluid leaks slowly from inside the eye into the 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 eventually opaque.

    Once endothelium cells are destroyed by disease or injury, they are lost forever. If a person loses too many endothelial cells, he or she might need a corneal transplantation.

What are refractive errors?

Refractive errors include nearsightedness, farsightedness, and astigmatism. During the refraction process, the cornea and lens bend light to focus it on the retina. When light hits the retina in the correct place, it produces good vision. In people with refractive errors, the light is not bent correctly and hits the retina in the wrong place. This results in imperfect vision.

Can the cornea get injured?

The cornea can recover from minor injuries on its own. If it is scratched, healthy cells slide over quickly and patch the injury before it causes infection or affects vision. But if a scratch causes a deep injury to the cornea, it will take longer to heal. It might also cause pain, blurred vision, tearing, redness, and extreme sensitivity to light. People with these symptoms need professional treatment.

Deeper scratches can also cause corneal scarring. This can cause a haze on the cornea that can impair vision. In this case, a corneal transplant might be needed.

What diseases and disorders affect the cornea?

Diseases and disorders that affect the cornea include:

  • Corneal infections – Damage to the cornea from injury or bacteria can cause painful inflammation and corneal infections. Another name for a corneal infection is keratitis. Corneal infections can be a rare but serious complication of contact lens wear. They can reduce clarity of vision, cause corneal discharges, and even erode the cornea. They can also lead to corneal scarring, which can harm vision and might require a corneal transplant.

    Minor corneal infections are usually treated with anti-microbial eye drops. Severe infections might require stronger antibiotic or antifungal or antiviral treatments. It might be necessary to make frequent visits to an eye doctor for several months. Once the infection is under control, topical steroid treatments might be used to help with corneal scarring.

  • Dry eye - In people with dry eye, the eye produces fewer or lower quality tears and is unable to keep itself moist and comfortable. The main symptom of dry eye is a scratchy or sandy feeling as if something is in the eye. Other symptoms might include stinging or burning of the eye; periods of excess tearing that follow periods of dryness; a stringy discharge from the eye; and pain and redness of the eye. Sometimes people with dry eye feel heaviness in their eyelids or have vision problems.

    Dry eye can be more likely in people who live in dry air, as well as those who use certain medications, including antihistamines, nasal decongestants, tranquilizers, and anti-depressants. People with dry eye should let their health care providers know all the medications they are taking, since some medications might make dry eye symptoms worse. Dry eye is also more common in women, especially after menopause.

    People with connective tissue diseases, such as rheumatoid arthritis, can be prone to dry eye. 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 can help diagnose any underlying diseases. An exam called a Schrimer's test might be used to measure tear production.

    The main treatment for dry eye is artificial tears, which can moisten the eye. They are available over-the-counter as eye drops. 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 might help as well. For people with severe cases of dry eye, temporarily or permanently blocking the eyelid puncta (small openings at the inner corner of the eyelids where tears drain from the eye) with punctual plugs is another treatment option. A treatment called topical cyclosporine therapy (Restasis), might also be used to increase tear production. Most people who have this treatment need to use it continuously for at least three months before their tear production improves.

  • Corneal dystrophies - A corneal dystrophy is a condition in which one or more parts of the cornea lose their normal clarity due to a buildup of cloudy material. There are over 20 corneal dystrophies that affect all parts of the cornea. Many of them are inherited and not the result of other health or lifestyle factors.

    Corneal dystrophies affect vision in very different ways. Some cause severe vision problems, while a few cause no vision problems and are only noticed during a routine eye examination. Other dystrophies might cause severe pain without leading to permanent vision loss.

    Some of the most common corneal dystrophies include:

    • Fuchs' dystrophy - Fuchs' dystrophy is a slowly progressing condition in which cells in the endothelium deteriorate for no apparent reason. As of these cells are lost, the endothelium cannot pump water out of the stroma as it should, and the cornea swells. This distorts vision. Eventually, the epithelium also takes on water, resulting in pain and severe vision problems. 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.

      At first, a person with Fuchs' dystrophy will awaken with blurred vision that will gradually get better during the day. As the disease worsens, this blurred vision will stay constant throughout the day.

      To treat the disease, doctors will try first to lower the swelling with saline eyedrops, ointments, or soft contact lenses. Once the disease interferes with daily activities, the person might need to have a corneal transplant.

    • Keratoconus - This is the most common corneal dystrophy in the U.S, and is most prevalent in teenagers and adults in their 20s. It happens when the middle of the cornea thins and gradually bulges outward, forming a cone shape. This bulging changes the how the cornea refracts (bends) light, and can cause refractive errors such as nearsightedness and astigmatism. It can also cause swelling and scarring of the cornea, which can harm vision. Keratoconus usually affects both eyes.

      At first, vision problems from keratoconus can be treated with eyeglasses. But as the astigmatism worsens, specially-fitted contact lenses might be needed to reduce the distortion and provide better vision. It is important that the contact lenses fit properly, because a poorly fitting lens can cause further damage the cornea. This might prevent the person from being able to wear contact lenses.

      Another treatment, called corneal collagen crosslinking, has recently been shown to stop the progression of keratoconus.

      However, not all people with keratoconus can be helped by these treatments. In about 10 to 20 percent of people with this condition, the cornea will become too scarred or will not tolerate a contact lens. If either of these problems happen, a corneal transplant might be necessary to treat the keratoconus.

    • Granular dystrophy – Granular dystrophy causes "crumb-shaped" lesions to form on the inside of the eye. Over time, these lesions can grow and affect vision. In some people, the condition also causes eye pain or discomfort.

      There are two types of granular dystrophy: type I and type II. Usually, people with type I granular dystrophy will begin to have symptoms in childhood or early adulthood, while people with type II will begin to have symptoms in their 20s and 30s.

    • Lattice dystrophy - Lattice dystrophy causes abnormal protein fibers (called amyloid deposits) to collect in the stroma. Over time, these groups of fibers converge and take up more of the stroma. This causes cloudiness in the cornea and can also harm vision. Although lattice dystrophy can occur at any time in life, the condition usually happens in children between the ages of two and seven.

      Some people with lattice dystrophy also develop recurrent epithelial erosions, which is when the part of the cornea called the epithelium is damaged, or eroded. These erosions change the curvature of the cornea, causing vision problems, and also expose the nerves that line the cornea, causing severe pain. For people with recurrent epithelial erosions, blinking might even be painful.

      To help with pain from epithelial erosion, the doctor might give eye drops and ointments to reduce friction on the damaged area. In some cases, an eye patch may be used to keep the eyelids still. With good treatment, these erosions usually heal within three days. But the person might have occasional pain for the next six to eight weeks.

      By about age 40, some people with lattice dystrophy will have scarring under the epithelium. This causes in a haze on the cornea that can greatly harm vision. In this case, a corneal transplant might be needed. While transplant is a good option for people with this condition, the condition might happen in the donor cornea in as little as three years. If this happens, early treatment with the excimer laser can help. This treatment is called phototherapeutic keratectomy (PTK). PTK can remove the diseased layers of the cornea.

    • Map-dot-fingerprint dystrophy - This dystrophy happens when part of the epithelium does not develop normally, and epithelial cells cannot properly adhere to it. This causes the erosion of the epithelium. This condition is also called "epithelial basement membrane dystrophy," since it affects the area of the epithelium called the basement membrane.

      For people with this condition, erosion of the epithelium leads to changes in the curve of the cornea, causing periodic blurred vision. Erosion might also expose the nerve endings that line the tissue and cause moderate to severe pain. This can last for several days. Generally, the pain will be worse when the person wakes in the morning. Other symptoms include sensitivity to light, excessive tearing, and foreign body sensation in the eye. Usually the condition will flare up for a few years and then go away on its own, with no lasting loss of vision. Most people do not have any pain or vision loss, and might not even know they have the condition.

      If treatment is needed, doctors will try to control the pain from epithelial erosion. They might patch the eye to keep it still or give lubricating eye drops and ointments. With treatment, these erosions usually heal within three days, although periodic flashes of pain might occur for several weeks afterward. Other treatments include anterior corneal punctures to allow cells to adhere to the epithelium better, corneal scraping to remove damaged areas of the cornea and allow healthy epithelial tissue to grow, and excimer laser treatment (PTK) to remove areas on the surface of the cornea.

  • Herpes zoster (shingles) - This is an infection that is caused by the varicella-zoster virus. This is the same virus that causes chickenpox. In most cases, once a person recovers from chickenpox, the virus remains inactive in his or her body. But in some people, the virus becomes active again later in life. When this happens, the virus travels down nerve fibers and can infect any part of the body. The infection causes a blistering rash (called shingles), fever, painful inflammations of the affected nerve fibers, and a general feeling of sluggishness. In many people who have a varicella-zoster infection in their head or neck, the virus also affects the cornea.

    To treat an infection in the cornea, doctors will often prescribe oral anti-viral medication. This can lower the risk of inflammation and scarring in the cornea. The virus can also decrease corneal sensitivity. This means that the cornea will not be as sensitive to foreign objects, such as stray eyelashes. In many cases, this decreased sensitivity is permanent.

    Corneal problems can happen arise after the virus is gone. For this reason, it is important that people who have had shingles on their face have regular follow-up eye examinations.

  • Iridocorneal endothelial syndrome (ICE) – This condition involves changes in the iris (the colored part of the eye), swelling of the cornea, and the development of glaucoma. Glaucoma is a disease in which normal fluid inside the eye cannot drain properly. Glaucoma can cause severe vision loss. ICE is most common in women and usually diagnosed between ages 30 to 50. It usually only happens in one eye.

    ICE syndrome is actually a group of three closely linked conditions: iris nevus (or Cogan-Reese) syndrome; Chandler's syndrome; and essential (progressive) iris atrophy.

    There are no treatments that can keep ICE syndrome from progressing, but doctors can give medications to treat the glaucoma associated with ICE. A corneal transplant can also treat the corneal swelling.

  • Ocular herpes – This condition is caused by recurrent viral infections from the herpes simplex virus. It is the most common infectious cause of corneal blindness in the U.S. It causes a painful sore on the eyelid or surface of the eye and inflammation of the cornea.

    Prompt treatment with anti-viral medications can help stop the herpes virus from multiplying and destroying cells in the cornea.

    Research has shown that once a person develops ocular herpes, he or she has up to a 50 percent chance of having a recurrence. The next flare-up could come weeks or even years after the last occurrence. Some factors believed to be associated with recurrence include fever, stress, sunlight, and eye injury.

    In some cases, the infection spreads and becomes a more severe infection called stromal keratitis. This causes the body's immune system to attack and destroy cells in the stroma. Stromal keratitis is more difficult to treat than other ocular herpes infections. Repeated episodes of stromal keratitis can cause scarring of the cornea. This can lead to loss of vision and even blindness.

  • Pterygium - A pterygium is a pinkish, triangular-shaped growth of tissue on the cornea. Pterygia are more common in sunny climates and in people aged 20 to 40. Some pterygia grow slowly throughout a person's life, while others stop growing after a certain point. A pterygium rarely grows so large that it begins to cover the pupil of the eye.

    Because a pterygium is easy to notice on the face, many people with the condition want to have it removed for cosmetic reasons. However, surgery to remove a pterygium is not recommended unless the pterygium affects vision or causes discomfort. If a pterygium is surgically removed, it might grow back, particularly if the person is younger than 40. There are many treatments eye doctors can use to make it less likely that the pterygium will grow back. These include the use of conjunctival autografts, amniotic membrane, and a medication called mitomycin-C.

    The pterygium should not be very noticeable unless it becomes red and swollen from dust or air pollutants. Lubricants can reduce the redness and provide relief from the chronic irritation.

    Scientists do not know what causes pterygia. However, people who have pterygia usually have spent a lot of time outdoors, and many doctors believe ultraviolet (UV) light from the sun might be a factor. In areas where sunlight is strong, it is important to wear protective eyeglasses, sunglasses, and/or hats with brims.

What kinds of corneal surgery might I have?

If you need surgery to treat a disease or disorder in your cornea, you might have:

  • Descemet's stripping endothelial keratoplasty (DSEK) – DSEK is another option for people who need a cornea transplant for cornea swelling. DSEK is also called the "sutureless corneal transplant," since it does not require stitches. DSEK has some advantages over traditional corneal transplant. These include faster vision recovery, a stronger eye, lower risk of rejection, and lower risk of conditions that can be caused by stiches, such as astigmatism and infections.

    For DSEK, the eye doctor removes or "strips" damaged endothelial cells and places a partial corneal transplant in the eye. As with traditional corneal transplant, this cornea is usually donated from an eye bank. The eye doctor then injects an air bubble into the eye to attach the partial corneal transplant to the surface of the person's original cornea. There are no stitches needed to attach the partial transplant. The natural pumping action of the endothelial cells helps create a suction that keeps both the original and transplanted layers of cornea together. If you have this surgery, it is very important that you use an eye shield and stay in a face-up position for one to two days afterward. This will help the transplant attach to your cornea.

  • Phototherapeutic keratectomy (PTK) - PTK can treat superficial 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.

    For this treatment, doctors can use an excimer laser to eliminate the outermost thin layers of diseased corneal tissue and etch away the surface problems associated with many corneal dystrophies and scars. Healthy tissue can then grow over the new, smooth surface. The laser is controlled with a computer, and damage to surrounding areas is minimal or non-existent. Sometimes the doctor will also use a medicine called mitomycin-C after the PTK treatment to reduce the risk of a side effect called corneal haze.

    While recovery from a corneal transplant takes months, recovery from PTK takes only days. Vision can come back very quickly, especially if the problem was in the top layer of the cornea. The PTK procedure has a good success rate, and is especially useful for people with inherited disorders, whose scars or other corneal problems harm vision by blocking the way light hits the retina. Because PTK reshapes the stroma of the cornea, a person might need a different glasses or contact lens prescription after this procedure. This is because PTK can change a person's refractive error.