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.
Bowman's Layer
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.
Descemet's Membrane
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.


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 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.

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:

  • 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.  See the separate page on dry eyes.
  • 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.

    At first, a  person with Fuchs' dystrophy will awaken with blurred vision that will  gradually clear during the day. This occurs because the cornea is  normally thicker in the morning; it retains fluids during sleep that  evaporate in the tear film while we are awake. As the disease worsens,  this swelling will remain constant and reduce vision throughout the day.

    When  treating the disease, doctors will try first to reduce the swelling  with drops, ointments, or soft contact lenses. They also may instruct a  person to use a hair dryer, held at arm's length or directed across the  face, to dry out the epithelial blisters. This can be done two or three  times a day.

    When the disease interferes with daily activities, a  person may need to consider having a corneal transplant to restore  sight. The short-term success rate of corneal transplantation is quite  good for people with Fuchs' dystrophy. However, some studies suggest  that the long-term survival of the new cornea can be a problem.

        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. These diseases share many traits:
    1. They are usually inherited.  
    2. They affect the right and left eyes equally.  
    3. They are not caused by outside factors, such as injury or diet.  
    4. Most progress gradually.  
    5. Most usually begin in one of the five corneal layers and may later spread to nearby layers.  
    6. Most do not affect other parts of the body, nor are they related to diseases affecting other parts of the eye or body.  
    7. Most can occur in otherwise totally healthy people, male or female. 
    8. Corneal dystrophies affect vision in  widely differing ways. Some cause severe visual impairment, while a few  cause no vision problems and are discovered during a routine eye  examination. Other dystrophies may cause repeated episodes of pain  without leading to permanent loss of vision.
      Some of the most common corneal  dystrophies include Fuchs' dystrophy, keratoconus, lattice dystrophy,  and map-dot-fingerprint dystrophy.


  • Herpes Zoster (Shingles). This infection is produced by the varicella-zoster virus, the same  virus that causes chickenpox. After an initial outbreak of chickenpox  (often during childhood), the virus remains inactive within the nerve  cells of the central nervous system. But in some people, the  varicella-zoster virus will reactivate at another time in their lives.  When this occurs, the virus travels down long nerve fibers and infects  some part of the body, producing a blistering rash (shingles), fever,  painful inflammations of the affected nerve fibers, and a general  feeling of sluggishness.

    Varicella-zoster virus may travel to the  head and neck, perhaps involving an eye, part of the nose, cheek, and  forehead. In about 40 percent of those with shingles in these areas, the  virus infects the cornea. Doctors will often prescribe oral anti-viral  treatment to reduce the risk of the virus infecting cells deep within  the tissue, which could inflame and scar the cornea. The disease may  also cause decreased corneal sensitivity, meaning that foreign matter,  such as eyelashes, in the eye are not felt as keenly. For many, this  decreased sensitivity will be permanent.

    Although shingles can  occur in anyone exposed to the varicella-zoster virus, research has  established two general risk factors for the disease: (1) Advanced age;  and (2) A weakened immune system. Studies show that people over age 80  have a five times greater chance of having shingles than adults between  the ages of 20 and 40. Unlike herpes simplex I, the varicella-zoster  virus does not usually flare up more than once in adults with normally  functioning immune systems.

    Be aware that corneal problems may  arise months after the shingles are gone. For this reason, it is  important that people who have had facial shingles schedule follow-up  eye examinations.   
  • Iridocorneal Endothelial Syndrome. More common in women and usually diagnosed between ages 30-50,  iridocorneal endothelial (ICE) syndrome has three main features: (1)  Visible changes in the iris, the colored part of the eye that regulates  the amount of light entering the eye; (2) Swelling of the cornea; and  (3) The development of glaucoma, a disease that can cause severe vision  loss when normal fluid inside the eye cannot drain properly. ICE is  usually present in only one eye.

    ICE syndrome is actually a  grouping of three closely linked conditions: iris nevus (or Cogan-Reese)  syndrome; Chandler's syndrome; and essential (progressive) iris atrophy  (hence the acronym ICE). The most common feature of this group of  diseases is the movement of endothelial cells off the cornea onto the  iris. This loss of cells from the cornea often leads to corneal  swelling, distortion of the iris, and variable degrees of distortion of  the pupil, the adjustable opening at the center of the iris that allows  varying amounts of light to enter the eye. This cell movement also plugs  the fluid outflow channels of the eye, causing glaucoma.

    The  cause of this disease is unknown. While we do not yet know how to keep  ICE syndrome from progressing, the glaucoma associated with the disease  can be treated with medication, and a corneal transplant can treat the  corneal swelling.

  • Keratoconus. This disorder--a progressive thinning of the cornea--is the most common  corneal dystrophy in the U.S., affecting one in every 2000 Americans.  It is more prevalent in teenagers and adults in their 20s. Keratoconus  arises when the middle of the cornea thins and gradually bulges outward,  forming a rounded cone shape. This abnormal curvature changes the  cornea's refractive power, producing moderate to severe distortion  (astigmatism) and blurriness (nearsightedness) of vision. Keratoconus  may also cause swelling and a sight-impairing scarring of the tissue.

    Studies indicate that keratoconus stems from one of several possible causes:

    1. An inherited corneal abnormality. About seven percent of those with the condition have a family history of keratoconus.  
    2. An eye injury, i.e., excessive eye rubbing or wearing hard contact lenses for many years.  
    3. Certain eye diseases, such as retinitis pigmentosa, retinopathy of prematurity, and vernal keratoconjunctivitis.  
    4. Systemic diseases, such as Leber's congenital amaurosis, Ehlers-Danlos syndrome, Down syndrome, and osteogenesis imperfecta. 

    5. Keratoconus  usually affects both eyes. At first, people can correct their vision  with eyeglasses. But as the astigmatism worsens, they must rely on  specially fitted contact lenses to reduce the distortion and provide  better vision. Although finding a comfortable contact lens can be an  extremely frustrating and difficult process, it is crucial because a  poorly fitting lens could further damage the cornea and make wearing a  contact lens intolerable.

      In most cases, the cornea will  stabilize after a few years without ever causing severe vision problems.  But in about 10 to 20 percent of people with keratoconus, the cornea  will eventually become too scarred or will not tolerate a contact lens.  If either of these problems occur, a corneal transplant may be needed.  This operation is successful in more than 90 percent of those with  advanced keratoconus. Several studies have also reported that 80 percent  or more of these patients have 20/40 vision or better after the  operation.

      The National Eye Institute is conducting a natural history study--called the Collaborative Longitudinal Evaluation of Keratoconus Study--to identify factors that influence the severity and progression of keratoconus.


  • Lattice Dystrophy. Lattice dystrophy gets its name from an accumulation of amyloid  deposits, or abnormal protein fibers, throughout the middle and anterior  stroma. During an eye examination, the doctor sees these deposits in  the stroma as clear, comma-shaped overlapping dots and branching  filaments, creating a lattice effect. Over time, the lattice lines will  grow opaque and involve more of the stroma. They will also gradually  converge, giving the cornea a cloudiness that may also reduce vision.

    In  some people, these abnormal protein fibers can accumulate under the  cornea's outer layer--the epithelium. This can cause erosion of the  epithelium. This condition is known as recurrent epithelial erosion.  These erosions: (1) Alter the cornea's normal curvature, resulting in  temporary vision problems; and (2) Expose the nerves that line the  cornea, causing severe pain. Even the involuntary act of blinking can be  painful.

    To ease this pain, a doctor may prescribe eye drops and  ointments to reduce the friction on the eroded cornea. In some cases,  an eye patch may be used to immobilize the eyelids. With effective care,  these erosions usually heal within three days, although occasional  sensations of pain may occur for the next six-to-eight weeks.

    By  about age 40, some people with lattice dystrophy will have scarring  under the epithelium, resulting in a haze on the cornea that can greatly  obscure vision. In this case, a corneal transplant may be needed.  Although people with lattice dystrophy have an excellent chance for a  successful transplant, the disease may also arise in the donor cornea in  as little as three years. In one study, about half of the transplant  patients with lattice dystrophy had a recurrence of the disease from  between two to 26 years after the operation. Of these, 15 percent  required a second corneal transplant. Early lattice and recurrent  lattice arising in the donor cornea responds well to treatment with the  excimer laser.

    Although lattice dystrophy can occur at any time  in life, the condition usually arises in children between the ages of  two and seven.

  • Map-Dot-Fingerprint Dystrophy. This dystrophy occurs when the epithelium's basement membrane develops  abnormally (the basement membrane serves as the foundation on which the  epithelial cells, which absorb nutrients from tears, anchor and organize  themselves). When the basement membrane develops abnormally, the  epithelial cells cannot properly adhere to it. This, in turn, causes  recurrent epithelial erosions, in which the epithelium's outermost layer  rises slightly, exposing a small gap between the outermost layer and  the rest of the cornea.

    Epithelial erosions can be a chronic  problem. They may alter the cornea's normal curvature, causing periodic  blurred vision. They may also expose the nerve endings that line the  tissue, resulting in moderate to severe pain lasting as long as several  days. Generally, the pain will be worse on awakening in the morning.  Other symptoms include sensitivity to light, excessive tearing, and  foreign body sensation in the eye.

    Map-dot-fingerprint dystrophy,  which tends to occur in both eyes, usually affects adults between the  ages of 40 and 70, although it can develop earlier in life. Also known  as epithelial basement membrane dystrophy, map-dot-fingerprint dystrophy  gets its name from the unusual appearance of the cornea during an eye  examination. Most often, the affected epithelium will have a map-like  appearance, i.e., large, slightly gray outlines that look like a  continent on a map. There may also be clusters of opaque dots underneath  or close to the map-like patches. Less frequently, the irregular  basement membrane will form concentric lines in the central cornea that  resemble small fingerprints.

    Typically, map-dot-fingerprint  dystrophy will flare up occasionally for a few years and then go away on  its own, with no lasting loss of vision. Most people never know that  they have map-dot-fingerprint dystrophy, since they do not have any pain  or vision loss. However, if treatment is needed, doctors will try to  control the pain associated with the epithelial erosions. They may patch  the eye to immobilize it, or prescribe lubricating eye drops and  ointments. With treatment, these erosions usually heal within three  days, although periodic flashes of pain may occur for several weeks  thereafter. Other treatments include anterior corneal punctures to allow  better adherence of cells; corneal scraping to remove eroded areas of  the cornea and allow regeneration of healthy epithelial tissue; and use  of the excimer laser to remove surface irregularities.

  • Ocular Herpes. Herpes of the eye, or ocular herpes, is a recurrent viral infection  that is caused by the herpes simplex virus and is the most common  infectious cause of corneal blindness in the U.S. Previous studies show  that once people develop ocular herpes, they have up to a 50 percent  chance of having a recurrence. This second flare-up could come weeks or  even years after the initial occurrence.

    Ocular herpes can  produce a painful sore on the eyelid or surface of the eye and cause  inflammation of the cornea. Prompt treatment with anti-viral drugs helps  to stop the herpes virus from multiplying and destroying epithelial  cells. However, the infection may spread deeper into the cornea and  develop into a more severe infection called stromal keratitis, which  causes the body's immune system to attack and destroy stromal cells.  Stromal keratitis is more difficult to treat than less severe ocular  herpes infections. Recurrent episodes of stromal keratitis can cause  scarring of the cornea, which can lead to loss of vision and possibly  blindness.

    Like other herpetic infections, herpes of the eye can  be controlled. An estimated 400,000 Americans have had some form of  ocular herpes. Each year, nearly 50,000 new and recurring cases are  diagnosed in the United States, with the more serious stromal keratitis  accounting for about 25 percent. In one large study, researchers found  that recurrence rate of ocular herpes was 10 percent within one year, 23  percent within two years, and 63 percent within 20 years. Some factors  believed to be associated with recurrence include fever, stress,  sunlight, and eye injury.

    The National Eye Institute supported the Herpetic Eye Disease Study, a group of clinical trials that studied various treatments for severe ocular herpes.

  • Pterygium. A pterygium is a pinkish, triangular-shaped tissue growth on the  cornea. 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.

    Pterygia are  more common in sunny climates and in the 20-40 age group. Scientists do  not know what causes pterygia to develop. However, since people who have  pterygia usually have spent a significant time outdoors, many doctors  believe ultraviolet (UV) light from the sun may be a factor. In areas  where sunlight is strong, wearing protective eyeglasses, sunglasses,  and/or hats with brims are suggested. While some studies report a higher  prevalence of pterygia in men than in women, this may reflect different  rates of exposure to UV light.

    Because a pterygium is visible,  many people want to have it removed for cosmetic reasons. It is usually  not too noticeable unless it becomes red and swollen from dust or air  pollutants. Surgery to remove a pterygium is not recommended unless it  affects vision. If a pterygium is surgically removed, it may grow back,  particularly if the patient is less than 40 years of age. Lubricants can  reduce the redness and provide relief from the chronic irritation.

  • Stevens-Johnson Syndrome. Stevens-Johnson Syndrome (SJS), also called erythema multiforme major,  is a disorder of the skin that can also affect the eyes. SJS is  characterized by painful, blistery lesions on the skin and the mucous  membranes (the thin, moist tissues that line body cavities) of the  mouth, throat, genital region, and eyelids. SJS can cause serious eye  problems, such as severe conjunctivitis; iritis, an inflammation inside  the eye; corneal blisters and erosions; and corneal holes. In some  cases, the ocular complications from SJS can be disabling and lead to  severe vision loss.

    Scientists are not certain why SJS develops.  The most commonly cited cause of SJS is an adverse allergic drug  reaction. Almost any drug--but most particularly sulfa drugs--can cause  SJS. The allergic reaction to the drug may not occur until 7-14 days  after first using it. SJS can also be preceded by a viral infection,  such as herpes or the mumps, and its accompanying fever, sore throat,  and sluggishness. Treatment for the eye may include artificial tears,  antibiotics, or corticosteroids. About one-third of all patients  diagnosed with SJS have recurrences of the disease.

    SJS occurs  twice as often in men as women, and most cases appear in children and  young adults under 30, although it can develop in people at any age. 

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

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:

National Eye Institute
Building 31, Room 6A32
31 Center Drive, MSC 2510
Bethesda, MD 20892-2510
Telephone: (301) 496-5248

Other Information Sources  

Cornea Research Foundation of America
9002 N. Meridian Street, Suite 212
Indianapolis, IN 46260
(317) 844-5610

National Keratoconus Foundation
8733 Beverly Blvd., Suite 201
Los Angeles, CA 90048
(310) 423-6455

Sjögren's Syndrome Foundation (SSF)
8120 Woodmont Avenue, Suite 530
Bethesda, MD 20814
(301) 718-0300

Stevens Johnson Syndrome Foundation
P.O. Box 350333
Westminster, CO 80030
(303) 635-1241