Corneal transplantation
In corneal transplant, also known as keratoplasty, a patient's damaged corneais replaced by the cornea from the eye of a human cadaver. This is the single most common type of human transplant surgery and has the highest success rate. Eye banks acquire and store eyes from donor individuals largely to supplythe need for transplant corneas.
Corneal transplant is used when vision is lost in an eye because the cornea has been damaged by disease or traumatic injury. Some of the disease conditions that might require corneal transplant include the bulging outward of the cornea (keratoconus), a malfunction of the inner layer of the cornea (Fuchs' dystrophy), and painful swelling of the cornea (pseudophakic bullous keratopathy). Some of these conditions cause cloudiness of the cornea; others alter itsnatural curvature, which can also reduce the quality of vision.
Injury to the cornea can occur because of chemical burns, mechanical trauma,or infection by viruses, bacteria, fungi, or protozoa. Herpes virus is one ofthe more common infections leading to corneal transplant.
Surgery would only be used when damage to the cornea is too severe to be treated with corrective lenses. Occasionally, corneal transplant is combined withother types of eye surgery (such as cataract surgery) to solve multiple eyeproblems in one procedure.
Corneal transplant is a very safe procedure that can be performed on almost any patient who would benefit from it. Any active infection or inflammation ofthe eye usually needs to be brought under control before surgery can be performed.
The cornea is the transparent layer of tissue at the very front of the eye. It is composed almost entirely of a special type of collagen. It normally contains no blood vessels, but because it contains nerve endings, damage to the cornea can be very painful.
In a corneal transplant, a disc of tissue is removed from the center of the eye and replaced by a corresponding disc from a donor eye. The circular incision is made using an instrument called a trephine. In one form of corneal transplant (penetrating keratoplasty), the disc removed is the entire thickness of the cornea and so is the replacement disc. Over 90% of all corneal transplants in the United States are of this type. In lamellar keratoplasty, on the other hand, only the outer layer of the cornea is removed and replaced.
The donor cornea is attached with extremely fine sutures. Surgery can be performed under anesthesia that is confined to one area of the body while the patient is awake (local anesthesia) or under anesthesia that places the entire body of the patient in a state of unconsciousness (general anesthesia.) Surgery requires 30-90 minutes.
Over 40,000 corneal transplants are performed in the United States each year.
A less common but related procedure called epikeratophakia involves suturingthe donor cornea directly onto the surface of the existing host cornea. The only tissue removed from the host is the extremely thin epithelial cell layeron the outside of the host cornea. There is no permanent damage to the host cornea, and this procedure can be reversed. It is usually employed in children. In adults, the use of contact lenses can usually achieve the same goals.
Corneal transplants are highly successful, with over 90% of operations in United States achieving restoration of sight. However, there is always some riskassociated with any surgery. Complications that can occur include infection,glaucoma, retinal detachment, cataract formation, and rejection of the donorcornea.
Graft rejection occurs in 5-30% of patients, a complication possible with anyprocedure involving tissue transplantation from another person (allograft).Allograft rejection results from a reaction of the patient's immune system tothe donor tissue. Cell surface proteins called histocompatibility antigens trigger this reaction. These antigens are often associated with vascular tissue (blood vessels) within the graft tissue. Since the cornea normally containsno blood vessels, it experiences a very low rate of rejection. Generally, blood typing and tissue typing are not needed in corneal transplants, and no close match between donor and recipient is required. Symptoms of rejection include persistent discomfort, sensitivity to light, redness, or a change in vision.
If a rejection reaction does occur, it can usually be blocked by steroid treatment. Rejection reactions may become noticeable within weeks after surgery,but may not occur until 10 or even 20 years after the transplant. When full rejection does occur, the surgery will usually need to be repeated.
Although the cornea is not normally vascular, some corneal diseases cause vascularization (the growth of blood vessels) into the cornea. In patients withthese conditions, careful testing of both donor and recipient is performed just as in transplantation of other organs and tissues such as hearts, kidneys,and bone marrow. In such patients, repeated surgery is sometimes necessary in order to achieve a successful transplant.
Cornea donors are carefully screened. Individuals with infectious diseases are not accepted as donors.
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