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The cornea is the eye's outermost layer. It is the clear, dome-shaped
surface that covers the front of the eye.
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
The corneal tissue is arranged in five basic layers, each having an
important function. These five layers are:
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
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
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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
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.
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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
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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
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
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.
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.
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:
- They are usually inherited.
- They affect the right and left eyes equally.
- They are not caused by outside factors, such as injury or
- Most progress gradually.
- Most usually begin in one of the five corneal layers and may
later spread to nearby layers.
- Most do not affect other parts of the body, nor are they related
to diseases affecting other parts of the eye or body.
- Most can occur in otherwise totally healthy people, male or
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
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
- 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:
- An inherited corneal abnormality. About seven percent of those with
the condition have a family history of keratoconus.
- An eye injury, i.e., excessive eye rubbing or wearing hard contact
lenses for many years.
- Certain eye diseases, such as retinitis pigmentosa, retinopathy of
prematurity, and vernal keratoconjunctivitis.
- Systemic diseases, such as Leber's congenital amaurosis, Ehlers-Danlos
syndrome, Down syndrome, and osteogenesis imperfecta.
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
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
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.
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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.
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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
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.
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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
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
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
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.
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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
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Cornea Research Foundation of
Syndrome Foundation (SSF)
Johnson Syndrome Foundation
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