Indications and Procedures
The cornea, which has four distinct layers, is the transparent outer coating of the eye. It serves both to protect the eye and to provide the main refracting surface as light reaches the eye and is transmitted to the lens and retina. Its total thickness is approximately 0.52 millimeter. The layers of specialized tissue are the epithelium, the stroma, Descemet’s membrane, and the endothelium, or inner surface of the cornea.
Several types of corneal disorders may lead to a decision to perform partial or total keratoplasty. Most of these fall under the general term “corneal dystrophy.” The most common, or classical, cases of corneal dystrophy involve the deposit of abnormal material in the cornea, resulting in irritation and eventually damage. Frequently, such disorders stem from genetic factors, making it possible to diagnose the dystrophy during the patient’s childhood and perform a lamellar keratoplasty. Other dystrophies include granular dystrophy and macular dystrophy. The former involves lesions in the center of the cornea, which may multiply and coalesce. At that stage, they may extend into the deeper layers of the stroma, the second layer of the cornea. Macular dystrophy actually begins in the stroma, causing all layers to become opaque.
Entirely different types of disorders that may call for corneal transplantation are interstitial
keratitis (a type of inflammation) and trachoma
. The latter condition can reach near epidemic levels in underdeveloped areas of the world, where low levels of hygiene allow the implantation and rapid multiplication of bacteria in the cornea. The effect is a breakdown of tissue accompanied by the discharge of mucus.
Whether the cornea has been affected by disease or injury, the goal of corneal transplantation is to eliminate any opacity that can hamper vision. The graft operation itself may be described in only a few stages, each marked by the need for a high degree of technical skill to increase the likelihood of success. First, the surgeon must calculate the exact size of the graft in question. This is done through the use of a special tool called a trephine, which will make the cuts to remove both the donor and the host eye corneas. Some trephines are equipped with transparent lenses to give the surgeon maximum levels of accuracy. When the two vital incisions are made, great care is taken to obtain a perfectly vertical cut.
Beginning with this initial stage, the surgeon may add a bubble of air through the incision to protect the endothelium and reduce the likelihood of an immune system reaction once the donor cornea has been transplanted. As the transfer occurs, another air bubble is introduced. After suturing, this bubble will be replaced by a balanced salt solution called acetylcholine.
This suturing, which must be very precise, almost always begins with four sutures at the cardinal points to ensure even tension. The last stage of the operation involves checking the wound for leakage of acetylcholine. This step is necessary not only to avoid infection but also to guard against rejection of the cornea by the host organ.
Uses and Complications
Significant differences in the healing process following corneal transplantation occur according to the method of suturing. A choice is made between a continuing or an interrupted series of sutures around the circumference of the cornea. Interrupted sutures may be preferred if there is a chance of uneven healing of the wound, something the physician may judge following examination of the degree of vascularization in particular corneal graft beds. In some cases, surgeons may opt for double suturing.
The chief complication that can follow corneal transplantation is rejection by the immune system. Surgeons try to obviate this risk by close study of the factors that can affect the receptivity of the eye to a new cornea. Earlier literature on corneal transplantation tended to assume that there was a lack of antigenicity—the production of disease-fighting antigens, or antibodies, as a defense system against viruses, bacteria, or foreign tissues—in the cornea. As ophthalmologists developed a fuller understanding of the immunological role of blood vessels and the lymphatic system, however, the need to give considerable attention to the degree of vascularization of the graft bed zone became more obvious. One method that surgeons can use to reduce antigen activity and enhance host acceptance is part of the transplantation operation itself: constant maintenance of a liquid layer between the host tissue and the new cornea tissue being transplanted. In the late 1990s, researchers at the University of Texas Southwestern Medical Center at Dallas announced the creation of an oral vaccine that may prevent rejection. Processed corneal cells in liquid form fed to laboratory mice produced a marked reduction in rejection rates. Current studies focus on the success of the vaccine with humans.
Although the period for healing and suture removal varies from patient to patient, the surgeon looks for the normal development of a gray-tinged scar tissue in the incision area as a sign of success. Failure, if discovered in time, may lead to a second transplantation attempt.
Perspective and Prospects
The first attempts to perform corneal transplantation—all unsuccessful—date from the nineteenth century. In the 1820s, German doctor F. Reisinger experimented with corneal grafts using rabbits and chickens. In the 1830s, Samuel Bigger of Ireland and R. S. Kissam of the United States tried to pioneer surgical grafts on humans, but both made the error of trying to replace human corneas with animal corneas. Success with living tissue (as opposed to the application of a glass product) finally came in 1905 when Moravian doctor Edward Zirm transplanted a child donor’s cornea to the eye of a chemical burn victim. Zirm’s success was based on cumulative medical knowledge of antiseptics, anesthesia, and technical aids such as the ophthalmoscope and the trephine. After a long period without major changes, in 1935 a Russian scientist named Filatov experimented with two innovations that were copied in other countries: the use of egg membrane to enhance a firm fix and the insertion of a delicate spatula between the cornea and lens to protect the intraocular tissues.
The greatest advances were made soon after antibiotics and steroids were introduced in the 1940s. By the 1950s, the use of extremely delicate surgical needles helped reduce postsurgical rejection rates. Major contributions to the development of delicate surgical instruments were made by the Spanish ophthalmologist Ramón Castroviejo, who performed many operations in the United States. By the 1980s, Castroviejo was urging others to follow the example of Townley Paton, who founded New York’s first eye bank some two decades earlier.
By the turn of the twenty-first century, forty thousand people in the United States had received corneal transplants using cells taken from the eyes of donors who had died. However, many patients with severe corneal damage cannot be helped by conventional cornea transplants. Two research teams, one in Taiwan, at the University of Taoyuan, and one at the University of California at Davis School of Medicine, used stem cells
to continually produce new corneal cells within the eye. In Taiwan, stem cells were placed on amniotic membrane, taken from placentas, to grow the tissue. In California, cells were first grown in laboratory dishes and then placed on the amniotic membrane to produce the tissue, which was transplanted to the damaged corneas. The use of this kind of tissue showed improved or restored vision for patients with corneal damage. While these procedures hold great potential for worldwide application, they have been called “investigational” and by no means eliminate the need for cornea donors.
The prospects for increasingly higher success rates in the field of corneal transplantation are linked to technical progress in donor organ conservation and the level of precision that can be achieved in carrying out transplantation operations.
Bibliography
Brightbill, Frederick S., ed. Corneal Surgery: Theory, Technique and Tissue. 4th ed. St. Louis, Mo.: Mosby, 2009.
De la Rocha, Kelly. "Corneal Transplant." Health Library, February 28, 2012.
Foster, C. Stephen, Dimitri T. Azar, and Claes H. Dohlman, eds. Smolin and Thoft’s The Cornea: Scientific Foundations and Clinical Practice. 4th ed. Philadelphia: Lippincott Williams & Wilkins, 2005.
Parker, James N., and Philip M. Parker, eds. The Official Patient’s Sourcebook on Corneal Transplant Surgery. San Diego, Calif.: Icon Health, 2002.
Spaeth, George L., ed. Ophthalmic Surgery: Principles and Practice. 4th ed. Philadelphia: W. B. Saunders, 2012.
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