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Defination
The transparent
cornea sits front and center on the outside of the eye, directly over
the iris (colored part) and the pupil. This tissue is one of the eye's
protectors, guarding against infection, injury, and other potential dangers.
The cornea is also a critical part of the visual process that, with the
lens, focuses images on the light-sensitive retina at the eye's back wall.
Because of these important functions, a strong, smooth, clear cornea is
vital to the health of the eye and to good vision.
Unfortunately,
because of its location, the cornea is susceptible to wounding and disease.
Corneal wounds and subsequent infections that lead to the formation of
corneal ulcers can cause blindness. In fact, corneal diseases caused by
infection and/or nutritional deficiencies are the leading cause of blindness
worldwide. In the United States alone, corneal injury and disease account
for more that 60% of all reported eye afflictions.
The cornea
has three layers. The epithelium is the outermost layer, exposed directly
to the air, and is the first line of defense against injury and disease,
healing most easily. The middle corneal layer, the stroma, governs the
clarity of the cornea and accounts for 90% of its tissue. The third corneal
layer, the endothelium, is the thin, innermost layer. The endothelium
serves as a barrier between the fluid in the eye's front chamber and the
stroma and acts as a "pump" that regulates the fluid level in the stroma.
The delicate layer is only one cell thick and cannot grow back once it
is damaged.
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Causes,
Incidence and Risk Factors
Corneal
disease is very easy to come -- even a faint scratch caused by a contact
lens can damage the corneal surface. Other common causes of corneal wounds
are injury caused by foreign objects, such as a splinter; burns caused
by harmful chemicals; and infection by ulcer-causing bacteria or viruses.
Abnormal change in the curvature of the cornea (keratoconus), degeneration
that may follow cataract surgery (corneal edema or swelling), and some
aging processes can also affect the clarity and health of the cornea.
The cornea heals itself when wounded; sometimes, however, wound healing
is less than perfect. Damage can include small breaks in the cornea's
outer surface; and ulceration, an erosion caused by over-activity of enzymes
needed for wound healing. Damage to the outer layer makes the stroma vulnerable
to infection and scarring. Viral infections, such as ocular herpes, can
also damage the cornea.
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Treatment
Antibiotics
and antiviral drugs can combat some kinds of infection. If the cornea
becomes permanently scarred or cloudy, the only way to restore sight is
to replace it surgically by corneal transplantation, using donor tissue
from an eye bank. However, the success rate depends on the cause of the
clouding. While the rate of success for transplants for degeneration following
cataract surgery and those for keratoconus both are high, transplants
for chemical burns may be less successful, although research at The Schepens
Eye Research Institute has improved the prognosis for the latter condition.
Further information about this tissue and the remarkable biochemistry
of wound healing is essential if we are to develop better methods of treating
corneal wounds and disease.
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