The Cornea and Corneal DiseaseWhat is the cornea?
The cornea is the transparent tissue that covers the front
of the eye (see diagram). An easy way to locate the cornea is simply to look at
your eye in the mirror. You will notice a clear surface covering the iris (the
colored part of the eye) and pupil. This is the cornea.
What is the function of the cornea?
Because the cornea is as smooth and clear as glass but as
strong and durable as plastic, it helps the eye in two ways:
The cornea provides a physical barrier that shields the
inside of the eye from germs, dust, and other harmful matter. It shares this
protective task with the sclera (the white of the eye).
It acts as the eye's outermost lens. When light strikes the
cornea, it bends--or refracts--the incoming light onto the crystalline lens. The
lens then focuses the light onto the retina, the paper-thin tissue at the back
of the eye that starts the translation of light into vision.
Although much thinner than the lens, the cornea provides
about 65 percent of the eye's power to bend light. Most of this power resides in
the center of the cornea, which is rounder and thinner than the outer part of
the tissue and is thus better suited to bend lightwaves.
How important is the cornea to good vision?
The cornea is essential to good vision. As the eye's
outermost tissue, the cornea functions like a window that controls the entry of
light into the eye. For example, the cornea filters out some of the most
damaging ultraviolet (UV) wavelengths in sunlight. Without this protection, the
crystalline lens and the retina would be highly susceptible to injury from UV
If this "window" is curved too much, as is the
case in some nearsighted people, faraway objects will appear blurry because
distant light waves will refract imperfectly on the retina. If this "window"
has imperfections or irregularities, as is the case in people with an
astigmatism, light will refract unequally, causing a slight distortion of the
visual image. But, if this "window" is of normal shape and curvature,
light will refract with exquisite precision to the crystalline lens.
STRUCTURE OF THE CORNEA
Although the cornea is clear and seems to lack substance,
it is actually a highly organized group of cells and protein. The cornea
receives its nourishment from the tears and aqueous humor that fills the chamber
behind it. Unlike most tissues in the body, the cornea contains no blood vessels
to nourish or protect it against infection. It must remain transparent to
refract light properly, and the presence of even the tiniest capillaries would
interfere with this process.
The tissue is arranged in three main regions, or layers:
EPITHELIUM As 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
or water--into the eye and other layers of the cornea, and (2) provide a smooth
surface that absorbs oxygen and other needed cell nutrients that are contained
in tears. This layer, which is about five cells deep, is filled with thousands
of tiny nerve endings that make the cornea extremely sensitive to pain when
rubbed or scratched.
STROMA Located behind the epithelium, the stroma
comprises about 90 percent of the cornea. It consists primarily of water (78
percent); layered protein fibers (16 percent) that give the cornea its strength,
elasticity, and form; and cells that nourish it. The unique shape, arrangement,
and spacing of the protein fibers are essential in producing the cornea's
ENDOTHELIUM This single layer of cells is located
between the stroma and the aqueous humor (see diagram). Because the stroma tends
to absorb water, the endothelium's primary task is to pump excess water out of
the stroma. Without this pumping action, the stroma would swell with water,
become hazy, and ultimately opaque.
What problems may affect the cornea?
The cornea copes very well with minor injuries or
abrasions. If dirt scratches the highly sensitive cornea, epithelial cells slide
over quickly and patch the injury before infection occurs and vision is
But if the scratch penetrates the cornea more deeply, the
healing process will take longer, resulting in greater pain, blurred vision,
tearing, redness, and extreme sensitivity to light. These symptoms require
professional treatment. Some of the more serious problems that affect the cornea
Microbial Infections (keratitis)
When the cornea is damaged, such as after a foreign object
has penetrated the tissue, bacteria or fungi can pass into the cornea, causing a
deep infection and inflammation. This condition may cause severe pain, reduce
visual clarity, produce a corneal discharge, and perhaps erode the cornea.
As a general rule, the deeper the corneal infection, the
more severe the symptoms and complications. It should be noted that microbial
infections, although relatively infrequent, are the most serious complication of
contact lens wear.
Minor corneal infections are commonly treated with
anti-bacterial or anti-fungal eye drops. If the problem is more severe, a person
may receive more intensive antibiotic treatment to eliminate the infection and
may need to take steroid eye drops to reduce inflammation. Frequent visits to an
eye care professional may be necessary for several months to eliminate the
Conjunctivitis ("pink eye")
This term describes a group of inflammatory and often
contagious diseases of the conjunctiva (the protective membrane that lines the
eyelids and covers exposed areas of the sclera, or white of the eye). These
diseases can be caused by a bacterial or viral infection, drug allergy,
environmental irritants, or a contact lens product.
At its onset, pink eye is usually painless and does not
adversely affect vision. The infection will come and go in most cases without
requiring medical care. But for some forms of pink eye, such as epidemic
keratoconjunctivitis, treatment will be needed. If treatment is delayed, the
infection may worsen and cause corneal inflammation and a loss of vision.
Depending on the type of pink eye that a person develops, treatment often
consists of antibiotics and steroids.
Herpes of the eye is a recurrent viral infection that
affects an estimated 400,000 Americans with herpes. Although ocular herpes can
result from the sexually transmitted herpes simplex II virus, it is usually
caused by herpes simplex virus I (HSV I), the virus responsible for cold sores.
In about 12 percent of those with ocular herpes, both eyes are involved.
Ocular herpes produces a relatively painful sore on the
surface of the cornea. Prompt treatment with anti-viral drugs helps to stop the
herpes virus from multiplying and destroying epithelial cells. In time, the
infection may also spread into the stroma, causing the body's immune system to
attack and destroy stromal cells. This more severe infection, called herpes
simplex stromal keratitis, is harder to treat and can scar the cornea, causing
vision loss. It may also produce an infection of the inside of the eye.
Like other herpetic infections, herpes of the eye remains a
controllable, but incurable, problem. For those who lose vision to ocular
herpes, it usually results from recurrent attacks that lead to severe stromal
keratitis. Studies indicate that after a person has had an initial outbreak of
ocular herpes, he or she has better than a 50 percent chance of having a
recurrence of the disease. This second outbreak could come weeks or decades
after the initial attack. In one large study, researchers found that recurrence
rates were 10 percent after one year, 23 percent at two years, and 63 percent at
20 years. Some factors associated with recurrence include fever, stress,
sunlight, and trauma. Anyone with ocular herpes should avoid using
over-the-counter steroid eye drops. Steroids cause the virus to multiply and the
infection to worsen.
Herpes Zoster (shingles)
This infection is produced by the varicella-zoster virus,
the same virus that causes chicken pox. After an initial outbreak of chicken pox
(often during childhood), the virus remains dormant within the nerve cells of
the central nervous system. But in some people, the varicella-zoster virus will
reactivate at some time during 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 malaise.
Varicella-zoster virus may travel to the head and neck,
perhaps involving an eye, part of the nose, mouth, cheek, and forehead. In about
40 percent of those with shingles in this area, the virus infects the cornea.
These zoster-related corneal lesions will usually clear up on their own. But
without early anti-viral treatment, a person runs the risk of the virus
infecting cells deep within the tissue, causing inflammation and scarring of the
cornea. The disease may also cause decreased corneal sensitivity. For many, this
decreased sensitivity will be permanent.
Although shingles can occur in anyone exposed to the
varicella-zoster virus, several studies have 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 complications may arise months after
the shingles are gone. For this reason, it is important that patients schedule
followup eye examinations.
There are over 20 corneal dystrophies that affect all parts
of the cornea. Some of the most common are:
The disorder 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 (near- and farsightedness) of vision. These changes
may also disrupt the normal, light-conducting arrangement of corneal protein,
causing swelling and a sight-impairing scarring of the tissue.
Studies indicate that keratoconus stems from one of several
causes: (1) an inherited corneal abnormality. About 7 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, vernal
keratoconjunctivitis; or (4) systemic diseases, such as Leber's congenital
amaurosis, Ehlers-Danlos Syndrome, Down's syndrome, osteogenesis imperfecta, and
Keratoconus usually occurs during puberty, or shortly
thereafter. 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. Finding a comfortable contact
lens can be an extremely frustrating and difficult process. However, 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 person will
probably need to replace the diseased tissue with a donor cornea, called a
This operation is successful in about 9 out of 10 people
with advanced keratoconus. Several studies have also reported that about 80
percent of these patients have 20/40 vision or better with contact lenses after
the operation. In fact, about 60 percent of transplant recipients will need to
wear contact lenses after the surgery to correct astigmatism and
For those with no scarring near the center of the cornea
and 20/40 vision or better with contact lenses, another option is surgically
grafting a layer of epithelial cells to flatten the cone-shaped cornea. This
process is called epikeratophakia. It has comparable results to corneal
transplantation and, if unsuccessful, it can be followed with corneal
The epithelium is separated from the stroma, in part, by a
membrane. It serves as the foundation on which the epithelial cells anchor and
organize themselves. In map-dot-fingerprint dystrophy, the membrane develops
abnormally. Like building a house on a damaged foundation, the epithelial cells
anchor to an irregular membrane that, in turn, may make the epithelium slightly
The condition, which tends to occur in both eyes, usually
affects adults between the ages of 40 and 70. It is also occasionally
inherited--in which case it arises at about age 6.
Map-dot-fingerprint dystrophy gets its name from the
unusual appearance of the cornea during an eye examination. Most often, the
epithelium will have a map-like appearance, i.e., large, slightly cloudy bodies
that look like a continent on a map. This configuration is actually the
irregular pattern of the membrane extending into the epithelium. There may also
be a sequence of opaque dots--formed from cellular debris--underneath or close
to the map-like patches. Less frequently, the irregular membrane will form
concentric lines in the central cornea that resemble small fingerprints.
Most people will never know that they have this corneal
dystrophy, since they will never have any pain and vision loss.
But for others, the disease will cause recurrent epithelial
erosions--possibly from poor epithelial adhesion to the membrane.
These erosions can be a chronic problem. They alter the
cornea's normal curvature, causing periodic blurred vision. They may also expose
the nerve endings that line the tissue, causing moderate to severe pain for
several days. Generally, the pain will be worse in the morning. Other symptoms
include: sensitivity to light, excessive tearing, and foreign body sensation in
Typically, the problem will flair up occasionally for a few
years in adults and then go away on its own, with no lasting loss of vision.
However, if treatment is needed, the doctor will try to
control the pain associated with the corneal erosion. He or she may do this by
patching the eye to immobilizeit or by prescribing lubricating eye drops and
ointments. With effective care, the pain will subside in about 10 days, although
periodic flashes of pain may occur for several weeks thereafter.
Fuch's 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 Fuch's dystrophy in people in
their 30s and 40s, the disease rarely affects vision until a person reaches
their 50s and 60s.
Fuch's dystrophy occurs when endothelial cells gradually
deteriorate without any apparent reason, such as trauma or inflammation. As more
endothelial cells are lost over the years, the cornea becomes less efficient at
pumping water out of the stroma. This causes the cornea to swell and to distort
vision. Eventually, the epithelium also takes on water, resulting in great pain
and severe visual impairment.
Epithelial swelling damages vision in two ways: (1)
changing the cornea's normal curvature, and (2) causing a sight-impairing haze
to appear in the tissue. Epithelial swelling will also produce tiny blisters on
the corneal surface. When the blisters burst, they are extremely painful.
At first, a person with Fuch's dystrophy will awaken with
blurred vision that will gradually clear during the day. This occurs because the
cornea is normally thicker in the morning, and it retains fluids during sleep
that evaporate in the tear film while we are awake. But 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 ointments or soft contact lenses. They may also 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 per day.
But when the disease makes even the most simple tasks hard
to complete, 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 Fuch's dystrophy. But, some studies do suggest that the
long-term survival of the donor cornea can be a problem.
Although lattice dystrophy can occur at any time in life,
the condition usually arises in children between the ages of 2 and 7. It is
characterized by an accumulation of abnormal protein fibers (amyloid) throughout
the middle and anterior stroma. However, the disease is NOT related to
amyloidosis, a serious systemic disease.
Lattice dystrophy gets its name from the amyloid deposits,
which during an eye examination can appear as clear, comma-shaped dots and
branching filaments that overlap each other in the stroma, creating a lattice
effect. Over time, the lattice lines will grow opaque and involve more of the
stroma. They will also gradually coalesce, giving the cornea a slight cloudiness
that may also reduce vision somewhat.
In some people, abnormal protein also accumulates under the
epithelium. This may result in poor adhesion between the stroma and epithelium,
causing periodic epithelial erosions. The erosion will: (1) alter the cornea's
normal curvature, resulting in temporary vision problems such as astigmatism and
nearsightedness, and (2) expose the nerves that line the cornea, causing severe
pain. In fact, 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 eye. With effective care, the pain will
subside in about 10 days, although occasional sensations of pain may occur for
about the next 6 to 8 weeks.
By about age 40, some people will have scarring under the
epithelium. As a result, a haze will develop on the cornea that can greatly
obscure a person's vision. In this case, a corneal transplant may be needed.
Patients with lattice dystrophy have an excellent chance for a successful
transplant with good vision. However, in as little as three years, lattice
dystrophy may also arise in the donor cornea. In one study, in fact, about half
of the transplant patients with lattice dystrophy had a recurrence of the
disease from 2 to 26 years after the operation. Of these, 15 percent required a
second corneal transplant.
CURRENT CORNEAL RESEARCH
Although vision researchers have learned much about the
structure and function of the cornea in health and disease, many important
scientific questions remain to be answered. For example, vision researchers
supported by the National Eye Institute, one of the Federal government's
National Institutes of Health, are beginning to identify the specific genes that
are activated, or switched on, in corneal cells. By understanding more about
these genes and how they produce and maintain a healthy cornea, it will help
immensely in understanding and treating corneal disease.
Although about 90 percent of all corneal transplant
operations are successful, it is preferable for people to retain their natural
corneas. For this reason, vision researchers continue to investigate ways to
enhance corneal healing and eliminate the sight-threatening scarring that can
complicate this process. NEI-supported scientists took an important step forward
in describing this process when they recently developed a method of culturing
rabbit cornea tissue that allows the researchers to measure cell movement
rapidly during healing. The culture is also being used to study drugs and
biochemicals that may promote or slow corneal healing and identify those agents
that promote better wound healing.
Another area of research interest is laser therapy. Several
small-scale studies show that lasers may be effective in sculpting the cornea to
improve its ability to refract light in people with myopia and astigmatism.
While this news is hopeful, many questions need to be answered about its short
and long-term benefits before laser therapy can be recommended as a safe and
The NEI also supports the Herpetic Eye Disease Study
(HEDS), a group of clinical trials that is designed to study various treatments
for severe ocular herpes. Recently, HEDS researchers reported that orally
administered acyclovir, a drug widely used for genital herpes, had no
significant effect in treating herpes simplex stromal keratitis. The HEDS should
provide eye care professionals with valuable information about when to use and
not to use specific antiviral drugs for this condition. But, more importantly,
these clinical trials will improve care for people with advanced ocular herpes.
Revised May 1994