IT IS OUR WINDOW ON THE WORLD. But when the cornea is scratched or clouded, the entire amazing human visual apparatus is useless: the finely machined iris adjusting the amount of light entering the eye; the sensitive retina with its array of specialized cells; the optic nerve carrying the retinal message; the highly engineered brain complex that interprets the retinal message and interprets a comprehensible image.
It happened to Mauricio Nungaray. When he was 4, an infection permanently scarred the cornea in his right eye, costing the little boy his vision.
Three years later, Mauricio had a rendezvous with a small glass vial in an operating room at the Keck School of Medicines Doheny Eye Institute. A label on the vial identified its contents as a human cornea packaged by the Doheny Eye and Tissue Transplant Bank. The label, with its unique identification number, also contained information on the many tests certifying that the tissue was free of infectious disease. The vial was refrigerator-cold, as it had been since it was sealed hours earlier.
Ophthalmologic surgeon John Irvine carefully sliced through the outer layer of Mauricios eye with a trephine, a circle-cutting surgical instrument, and gently pulled off the clouded disk that kept Mauricio from seeing the world.
Working with careful speed, Irvine positioned the replacement cornea and rapidly stitched it into place with #10 monofilament nylon thread so fine its almost invisible.
Within an hour, Mauricio was out of surgery, his eye bandaged. When the gauze came off the next day, the 7-year-old boy saw out of his right eye for the first time in three years. Over the succeeding months, the stitches were carefully removed. His vision and depth perception restored, Mauricio now plays baseball.
This operation, the replacement of a damaged cornea, is by far the most common and most successful tissue transplant performed in medicine. USC physicians are leaders in corneal transplants both in the surgical procedure and in the acquisition and distribution of donor corneas.
RONALD SMITH KNOWS as much about the delicate bit of tissue as anyone in the world. In the past 25 years, the chairman of the Doheny Eye Institutes ophthalmology department has transplanted an estimated 2,000 corneas. His voice warms as he describes the light-giving structure, which he compares to the crystal on a watch.
The cornea seems to be a simple disk of transparent film, but its alive and must stay alive to function. Consisting of three layers of cells, it is nourished not by blood, but in a manner unique to the eye by tears.
This uniqueness is a great boon to ophthalmologic surgeons. It means that the cornea is substantially not wholly, but largely insulated from the effects of the bodys immune system, which means a transplant can be accomplished without the heavy doses of anti-immune drugs usually required for tissue grafts.
Transplanted corneas, however, dont always thrive. The same osmotic pressure that pushes nutrients into the cell produces pressure that must be relieved by a continual pumping action. This task is performed by the innermost of the three corneal layers, called the endothelium. Key to the transplant, the endothelium must be kept alive or the new cornea will swell, lose transparency, and the patient will again lose sight.
Only living tissue will do. Teflon, plastics and other materials have been tried, but the stitches that must anchor them eventually pull loose.
Nor will non-human corneas do. Those of other species have been tried. Insulation from the immune system isnt sufficient to overcome such drastic incompatibility.
Even human corneas dont work without effort. A strong dose of cortisone eye-drops is always required after the transplant to overcome inflammation and prevent rejection.
Development of steroids, in fact, was what made corneal transplants possible, according to Irvine. Physicians suggested corneal transplants as early as the end of the 19th century, and actually attempted the operation in the 1920s. But consistent success was not possible until the 1950s, when powerful steroid anti-inflammatory agents and ultrafine nylon sutures became available. Today, according to Smith, a success rate of more than 90 percent is attainable in uncomplicated cases.
PEOPLE SUFFER PROBLEMS with their corneas for a wide variety of reasons. The most common is edema, excess fluid entering the cornea and destroying its light transmitting properties, caused by a variety of problems including complications after cataract surgery. Traumatic eye injury, from a rock thrown by a lawn mower blade or caustic chemicals, is another cause.
Improper contact lens care negligence about washing hands and disinfecting lenses, or wearing disposable lenses too long can create conditions ripe for infections that will injure the cornea.
As a specialist, Smith tends to see difficult, refractory cases, where one or two previous transplant attempts have failed. Often, in these cases, blood vessels have invaded the scarred, cloudy cornea. This complicates surgery and makes severe immune reactions more common.
Doheny surgeons have conducted intensive research on ways to improve the prognosis in problem repeat transplants. Smith and his colleagues believe that a better procedure transplanting only the crucial endothelium layer, which protects the tissue from edema may offer new hope in difficult and even routine cases.
The youngest cornea patients pose a whole other set of problems, both surgical and medical. The Keck School has a unique specialist to deal with them: Jonathan N. Song, assistant professor of ophthalmology and pediatrics at Childrens Hospital Los Angeles.
The stakes with children are especially high for a corneal transplant, Song says. If a newborns vision is not cleared before two months of life, she will likely never learn to use both eyes together to form stereo images. Indeed, the entire first eight years of life are critical for vision. It is during this period that the nerve links between eye and brain are formed and perfected, says Song. If you dont clear up their vision, the links wont develop.
Surgery on newborns is a special challenge. Their eyes are different from adults, or even older children. The tissue is softer, he says. Unless a surgeon is very careful, the infants eye can collapse under his hands. At the same time, because the newborns immune system is revved up, as Song puts it, the follow-up to avoid rejection must be rigorous. Even with the best care, the failure rate in such operations is high, and repeat operations are common.
Whether a transplant ends in success or failure, Song offers his young patients and their parents encouragement and support. In many places, he notes, a surgeon sees the patient once, then hands the case over to a pediatric ophthalmologist for follow-up. Here, we do it all together, to give children the best vision possible. I perform the operation, and I follow the patients myself.