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The
cornea is the transparent tissue that covers the front of the eye. 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? |
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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:
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.
Corneal Thickness Corneal 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. 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. 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 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.
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 affected.
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 problem. 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 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: Keratoconus 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 Addison's
disease. 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 corneal transplant. 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 nearsightedness.
Map-Dot-Fingerprint Dystrophy A membrane separates
the epithelium from the stroma, in part. 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 irregular. 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 the eye. 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
immobilize it 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. Lattice Dystrophy 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.
The procedure
usually takes approximately one hour and is performed with local
anesthesia on an outpatient basis. After the surgery, a plastic
shield or glasses should be worn at all times to avoid accidentally
rubbing, bumping or hitting the eye. Drops are prescribed to
prevent rejection of the donor cornea. During the postoperative period, the surgeon monitors the cornea’s healing with special computer mapping called corneal topography. This allows the doctor to evaluate the shape of the new cornea and remove sutures as needed to control astigmatism. |
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.
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.
The
cornea is normally a clear layer of tissue covering the front of the
eye, similar to a watch crystal. Its purpose is to refract or bend
light rays as they enter the eye, allowing them to focus on the
retina. In cases where the cornea has become clouded as a result of
disease, swelling, scarring, infection, or chemical burns, a corneal
transplant (also called keratoplasty) is sometimes necessary to
restore functional vision. For this procedure, the surgeon
carefully removes the central corneal tissue and replaces it with a
precisely shaped replica of donor tissue. |
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