corneal transplantation

Corneal Transplantation 

Definition

In corneal transplant, also known as keratoplasty, a patient's damaged cornea is 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 supply the need for transplant corneas.

Purpose

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 its natural 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. The herpes virus produces one of the 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 with other types of eye surgery (such as cataract surgery) to solve multiple eye problems in one procedure.

Precautions

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 of the eye usually needs to be brought under control before surgery can be performed.

Description

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. Medicare reimbursement for a corneal transplant in one eye was about $1,200 in 1997.

A less common but related procedure called epikeratophakia involves suturing the donor cornea directly onto the surface of the existing host cornea. The only tissue removed from the host is the extremely thin epithelial cell layer on 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.

Preparation

No special preparation for corneal transplant is needed. Some eye surgeons may request the patient have a complete physical examination before surgery. The patient may also be asked to skip breakfast on the day of surgery.

Aftercare

Corneal transplant is often performed on an outpatient basis, although some patients need brief hospitalization after surgery. The patient will wear an eye patch at least overnight. An eye shield or glasses must be worn to protect the eye until the surgical wound has healed. Eye drops will be prescribed for the patient to use for several weeks after surgery. These drops include antibiotics to prevent infection as well as corticosteroids to reduce inflammation and prevent graft rejection.

For the first few days after surgery, the eye may feel scratchy and irritated. Vision will be somewhat blurry for as long as several months.

Sutures are often left in place for six months, and occasionally for as long as two years.

Risks

Corneal transplants are highly successful, with over 90% of operations in United States achieving restoration of sight. However, there is always some risk associated with any surgery. Complications that can occur include infection, glaucoma, retinal detachment, cataract formation, and rejection of the donor cornea.

Graft rejection occurs in 5-30% of patients, a complication possible with any procedure involving tissue transplantation from another person (allograft). Allograft rejection results from a reaction of the patient's immune system to the 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 contains no 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 with these 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.

Key terms

Cadaver — The human body after death.

Cataract — A condition of cloudiness of the lens of the eye.

Cornea — The transparent layer of tissue at the very front of the eye.

Corticosteroids — Synthetic hormones widely used to fight inflammation.

Epikeratophakia — A procedure in which the donor cornea is attached directly onto the host cornea.

Epithelial cells — Cells that form a thin surface coating on the outside of a body structure.

Fibrous connective tissue — Dense tissue found in various parts of the body containing very few living cells.

Fuchs' dystrophy — A hereditary disease of the inner layer of the cornea. Treatment requires penetrating keratoplasty. The lens of the eye may also be affected and require surgical replacement at the same time as the cornea.

Glaucoma — A vision defect caused when excessive fluid pressure within the eye damages the optic nerve.

Histocompatibility antigens — Proteins scattered throughout body tissues that are unique for almost every individual.

Keratoconus — An eye condition in which the cornea bulges outward, interfering with normal vision. Usually both eyes are affected.

Pseudophakic bullous keratopathy — Painful swelling of the cornea occasionally occurring after surgery to implant an artificial lens in place of a lens affected by cataract.

Retinal detachment — A serious vision disorder in which the light-detecting layer of cells inside the eye (retina) is separated from its normal support tissue and no longer functions properly.

Trephine — A small surgical instrument that is rotated to cut a circular incision.

Resources

Organizations

American Academy of Ophthalmology. 655 Beach Street, P.O. Box 7424, San Francisco, CA 94120-7424. http://www.eyenet.org.

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corneal transplantation,

See corneal grafting.

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corneal

pertaining to the cornea. See also keratitis, keratopathy.

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corneal anomaly

includes microcornea, coloboma, megalocornea, dermoid, congenital opacity.

corneal black body

see corneal sequestrum (below).

corneal coloboma

an uncommon congenital defect in the continuity of the cornea; may have concurrent herniation of the uveal tract. See also coloboma.

corneal dystrophy

a developmental condition, inherited in some breeds of dogs and cats. May cause corneal edema and ulceration. See also keratopathy.

corneal ectasia

see kerectasia.

corneal edema

occurs when fluid accumulates in the corneal stroma, disrupting the normal lamellar structure and causing a loss of transparency. Commonly called blue eye.

corneal erosion syndrome

see refractory ulcer.

feline focal corneal necrosis

see corneal sequestrum (below).

corneal hyaline membrane

an abnormal, semitransparent membrane on the posterior surface of the cornea, attached to the endothelium. Can be associated with persistent pupillary membrane. Caused by inflammation or a developmental defect.

corneal inflammation

see keratitis.

inherited corneal opacity

congenital opacity of the cornea occurs in cattle. The animals are not completely blind and the rest of the eye is normal. Both eyes are affected. The lesion is an edema of the corneal lamellae.

corneal laminae

the limiting membranes that separate the bulk of the cornea from the covering epithelia; the anterior is Bowman's, the posterior is descemet's membrane.

corneal lipidosis

cholesterol crystals and lipid vacuoles may be found in the corneal stroma as a result of persistent hypercholesterolemia or chronic stromal inflammation.

melting corneal

see collagenase ulcer.

corneal mummification

see corneal sequestrum (below).

corneal opacity

see leukoma, macula, nebula.

corneal pigmentation

results from chronic irritation. The melanin is in the superficial stroma and the basal layer of the corneal epithelium. See also superficial pigmentary keratitis.

corneal reflex

a reflex action of the eye resulting in automatic closing of the eyelids when the cornea is stimulated. The corneal reflex can be elicited in a normal animal by gently touching the cornea with a wisp of cotton. Absence of the corneal reflex indicates deep coma or injury of one of the nerves carrying the reflex arc.

corneal ring abscess

an infected corneal ulcer in which there is a surrounding zone of liquefaction encircled by a zone of neutrophils.

corneal scar

corneal opacity.

corneal sequestrum

a central, focal, dark necrotic plaque on the cornea of cats, especially Persians, associated with chronic ulcerative or inflammatory disease of the cornea. Called also focal superficial necrosis, corneal mummification, keratitis nigrum.

corneal shield

protection used in the treatment of corneal ulcers or wounds; commercial products consisting of collagen which is dissolved in the tear film are claimed to enhance healing.

corneal stromal depositions

minerals, lipids or pigment deposited in the stroma following injury.

superficial corneal erosion

see refractory ulcer.

corneal tattooing

done mainly in horses to obscure unsightly scarring of the cornea.

corneal transparency

the quality of being able to see objects through the cornea; partly the result of the strict horizontal lamellal distribution of its collagen fibers, parallel to the corneal surface.

corneal transplantation

see keratoplasty.

corneal ulcer

a defect in the corneal epithelium and some amount of stroma; may be caused by trauma, chronic irritation as from distichiasis, entropion or keratitis sicca, or infectious agents. Deep ulcers can lead to rupture of the cornea, the escape of aqueous humor and often prolapse of the iris with a secondary uveitis and endophthalmitis. See also ulcer.

Corneal ulcer in a horse. By permission from Knottenbelt DC, Pascoe RR, Diseases and Disorders of the Horse, Saunders, 2003

corneal vascularization

results from inflammation of the cornea, the vessels growing in from the limbus. It is a necessary repair process but it reduces visual acuity.

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transplantation

the transfer of living organs and tissue from one part of the body to another or from one individual to another. Transplantation and grafting mean the same thing, though the term grafting is more commonly used to refer to the transfer of skin. See grafting (1).

Occasionally an organ is transplanted from one place to another within the body (autotransplants). Kidneys, for example, have been relocated to enable them to continue functioning after the ureters have been damaged. See also graft rejection.

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transplantation antigen

see histocompatibility antigen.

bone marrow transplantation

has been used in the treatment of a variety of hematopoietic and immunological disorders, e.g. in dogs with aplastic anemia.

corneal transplantation

full-depth and part-depth (lamellar) transplants are performed in animals when there is scarring of the cornea in the visual axis but the operation is difficult, the aftercare intensive and the failure rate high.

transplantation immunology

the study of immune responses that distinguish between self and nonself and the rejection of transplanted tissue or organs.

tendon transplantation

the procedure is not favored in horses where it was at one time used as a treatment for tendonitis. The success rate for return to racing performance is poor.

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Patient discussion about corneal transplantation.

Q. Has anyone had experience with a corneal transplant because of keratoconus?

A. my uncle had to do a transplant- it took 5 weeks until he could see anything , another year to get his vision straightened up. but now he is fine! i know that he looked for information in the "National Keratoconus Foundation". they were very helpful (and nice!), they have a website with information on all forms of treatment:
http://www.nkcf.org/

good luck :)