The cornea is the clear window of the eye. It is to your eye what the watch crystal is to your wristwatch. It covers the colored part of the eye (the iris) and the dark central opening of the iris (the pupil). Light passes through the cornea, in through the pupil into the eye. The cornea is round and dome-shaped, much like a tennis ball cut in half. Because of its curved shape, the cornea functions as a lens, focusing light onto the retina, allowing us to see.
In a normal eye, the cornea is clear, the surface is smooth and regular, and because of this, the eye sees well. If trauma or a disease process affects the clarity, shape or surface smoothness, the vision can be significantly decreased.
The internal surface of the cornea is lined with a layer of cells called endothelial cells, which function like a pump to remove excess fluid from the cornea. If trauma or disease damages the endothelial cells, this function is impaired and the cornea can develop edema (swelling) and become cloudy, interfering with vision.
There are a number of disease processes that can affect the corneal endothelium and cause corneal swelling. A large group of these are called corneal dystrophies, inherited disorders that usually affect the eye later in life. Other disease processes can affect the shape of the cornea, thus altering the way the cornea focuses light into the eye. One such disease is called Keratoconus, a disease process in which the cornea becomes thinned and cone-shaped. This affects vision by making the corneal shape very irregular, thus affecting the way light is focused on the retina and thereby decreasing vision.
Two other ways that the cornea can be damaged are by infection and trauma. Infection by bacteria, viruses or fungi can, despite treatment with antibiotics, antiviral and antifungal medications, cause corneal scarring and decreased vision. Trauma by sharp objects can cause laceration of the cornea, which may require surgical repair. The scarring and subsequent changes in the shape of the cornea can cause significant loss of vision. Injuries from chemicals, especially alkaline materials such as lye, and blunt trauma may also cause corneal scarring and subsequent loss of vision.
If the cornea becomes damaged and visual loss results, there are several ways of improving vision. Sometimes glasses or contact lenses can improve vision. Glasses are usually helpful only in changes in the shape of the cornea. Rigid contact lenses can sometimes help to improve vision in cases of corneal scarring or alteration in the shape of the cornea by diseases like Keratoconus. In superficial scarring, the excimer laser can be used to remove or reduce the density of the scar. This treatment is known as Phototherapeutic Keratectomy (PTK). This technique can be highly effective, and, in some cases, can prevent the need for corneal transplant surgery.
In more severely damaged corneas, corneal transplantation (also known as Penetrating Keratoplasty or PK) is the only way to restore vision. Corneal transplantation is a procedure in which the central part of the cornea is removed with a round blade called a trephine (sort of like a cookie cutter) and is replaced with a donor cornea, similarly cut with a trephine. The corneal donor tissue is sutured (stitched) into place with very fine suture material which is approximately 1/3 the diameter of a human hair.
The corneas used in transplantation are taken from donor eyes, removed within a few hours of the death of the donor. Corneal donor tissue is very carefully inspected to assure that it is usable and a wide range of tests are performed on the donor to assure that no transmittable disease is present.
Corneal healing is a slow process because the cornea has no blood vessels. The stitches remain in place for several months and may remain in place for several years. Visual improvement after surgery takes several months and the best, final vision may not be attained for more than a year. Usually, with selective suture removal and glasses or contact lenses, good vision can be obtained early in the postoperative course.
Long-term follow-up is very important in corneal transplant surgery, as there is a risk of corneal graft rejection. This risk is usually highest in the first two years, but corneal transplant patients must be monitored regularly to assure adequate healing and to check for signs of corneal graft rejection. The most important signs of rejection that a patient might notice are pain, redness and decreased vision. If any of these symptoms occur, the patient should report them immediately and come in for an eye examination and treatment, as the vast majority of graft rejection episodes are treated and resolved with anti-inflammatory drops.
Overall, the success rate for corneal transplantation is very high, with clear corneas and excellent vision. The success rate varies with the preoperative eye problem. Certain disease entities such as many Corneal Dystrophies and Keratoconus do very well, whereas other problems such as severely scarred corneas may not achieve the success of less complicated problems. For most patients, corneal transplantation is an excellent surgical procedure and offers a very viable option to patients who see poorly secondary to corneal disease.