The Cornea The cornea is the clear, transparent covering over the eye. It acts like a window on the eye. When light enters the eye, the cornea bends or refracts the image onto the lens, which focuses the image onto the retina. The cornea also acts like a protective shield for the eye by blocking the passage of germs, dust and other debris.

There are five layers of the cornea. The outermost layer, or epithelium, protects the eye and provides a surface for the absorption of oxygen and nutrients for the rest of the cornea. The next layer is called Bowman's layer. It's made of strong collagen fibers. If the cells in this layer are injured, scar tissue can form that may interfere with vision. The third layer is called the stroma. It's the thickest of the layers, comprising about 90 percent of the cornea, and is made of collagen and water. The fourth layer is Descemet's membrane. It is a very thin, but strong layer that protects the eye from infection and injury. The innermost layer of the cornea is the endothelium. Specialized cells in the endothelium pump excess fluid from the stroma, which is slowly and constantly replenished with water.

Fuchs' Dystrophy Several different conditions can affect the cornea and interfere with clear vision. One problem is Fuchs' dystrophy. The pumping cells in the endothelium gradually deteriorate. As more cells are lost, the eye loses its ability to efficiently remove excess water from the stroma. The cornea swells and vision becomes distorted. The more pumping cells that are lost, the blurrier vision becomes. Eventually, the epithelium (the outermost layer) also fills with water, causing pain and severe vision problems. Tiny blisters may develop on the surface of the cornea. When the blisters pop, they cause extreme pain.

Fuchs dystrophy is more common in women than in men and usually affects both eyes. The condition is genetically acquired. When a parent has the disease, a child has a 50 percent chance of acquiring the gene and also developing the condition. Signs of Fuchs' dystrophy often occur between 30 and 40, but the condition progresses very slowly. Patients may not notice any significant vision problems until many years later.

Cornea Transplant Once the endothelial cells in the cornea are destroyed, they can't be replaced by the body. Eventually, vision will deteriorate to blindness. The only "cure" for the condition is a cornea transplant.

The traditional cornea transplantation is called penetrating keratoplasty. A circular surgical instrument is placed over the eye like a cookie cutter to make an incision into the cornea. The diseased section of the cornea is removed. Then, the same technique is used to cut a section of donor cornea. The new corneal "button" is placed into the recipient's eye and sewn in place with fine sutures.

After surgery, the eye is patched for a few days until the surface of the cornea is healed. Steroid eye drops are used for several months to prevent infection and reduce the risk of rejection of the donor cornea. The sutures remain in place until the eye heals completely (which can take months to years after the transplant).

According the National Eye Institute, about 40,000 cornea transplants are done each year. Research shows up to 80 percent of the transplants are still in place after 20 years. About 20 percent of corneal transplants lead to rejection. In many cases, rejection can be halted if the patient is treated at the first signs of rejection.

Posterior Lamellar Keratoplasty (PLK) Some eye surgeons are using a different technique for certain patients in need of a cornea transplant. It's called a posterior lamellar keratoplasty, or PLK. Traditional cornea transplantation removes all the layers of the cornea. In PLK, a tiny cut is made into the cornea (similar to that used in cataract surgery). Then surgeons peel off the diseased cells in the Descemet's membrane and the endothelium (the two innermost layers). Next, an endothelial layer from a donor cornea is folded and slipped through the cut in the cornea. Air is used to unfold and position the donor graft. Once in place, the pumping cells in the donor endothelium begin to function. This pumping action permanently holds the donor cells to the rest of the patient's own cornea.

Since only a tiny cut is made into the cornea, there is no need for sutures and recovery time is much quicker (about 2 months versus 18 months for traditional cornea transplant, or PK). PLK also leaves the outer layer of the cornea intact, reducing the risk of corneal injury or changes in vision.

PLK is only used for patients who have disease or damage to the innermost, or endothelial, layer of the cornea. If the damage extends into other layers of the cornea, a traditional transplant is needed.

For information about the cornea or corneal diseases: Cornea Research Foundation of America, http://www.cornea.org The Corneal Dystrophy Foundation, http://www.fuchs-dystrophy.org The Eye Bank for Sight Restoration, http://www.eyedonation.org FuchsSupport, http://fuchsdystrophy.org National Eye Institute, http://www.nei.nih.gov