The cornea is the only ocular structure that can be removed and replaced with tissue donated by another human being. The procedure for doing so is referred to as a corneal transplant and is indicated in circumstances in which vision is significantly reduced by corneal scarring, edema or corneal irregularity. Only a central segment of the cornea is actually replaced, and this is typically a button-shaped segment measuring seven to nine millimeters in diameter. A similarly sized segment from a donor cornea is used to replace the extracted tissue and is secured in place with sutures. The sutures are made of nylon and can be left in place for a year or longer. They are finer than a human hair and knots are buried beneath the tissue surface to ensure comfort.
A corneal transplant is an intricate eye surgery that is sometimes combined with the removal of cataracts. It can be performed under local or general anesthesia. The immediate risks include intraocular hemorrhage, infection and injury to other ocular structures. The long-term risks include transplant rejection, glaucoma and failure to recover vision. Most patients recover vision, although eyeglass or contact lens correction of refractive error is usually necessary. Rejection can damage the corneal tissue and is a constant danger. Signs of rejection include increased redness, light sensitivity and blurred vision. Episodes of rejection can be successfully treated with steroid eye drops, if given promptly. Delayed treatment of rejection may result in permanent clouding of the cornea, referred to as failure of the graft.
Descemet’s Stripping Endothelial Keratoplasty
The aim of Descemet’s stripping endothelial keratoplasty (DSEK) is to replace only the dysfunctional endothelial layer of the cornea in diseases including Fuchs’ dystrophy and bullous keratopathy. In patients with these conditions, the cornea is swollen and cloudy because of a shortage of corneal endothelial cells that pump aqueous fluid out of the cornea. During a DSEK procedure, a thin layer of eye bank prepared corneal tissue lined with endothelial cells is placed in the anterior chamber and is positioned with an air bubble against the rear surface of the recipient cornea. Within minutes, a strong adhesion develops due to hydrostatic forces, which allows for removal of the air bubble. The donor endothelial cells repopulate the back surface of the recipient cornea and clear the cornea by reestablishing its dehydrated state.
DSEK has proven to be a major advance for patients with corneal endothelial failure because of rapid visual rehabilitation after surgery and benefits associated with the much smaller surgical incision compared to a traditional corneal transplant. These benefits include less induced astigmatism, reduced need to remove and replace sutures and maintenance of the natural integrity of the eye and its resistance to trauma. Like a full thickness corneal transplant, a DSEK graft can be rejected by the recipient’s immune system. Another problem encountered in some cases is dislocation of the graft immediately after surgery. If this occurs, it may require a return visit to the operating room for repositioning of the graft and a secondary attempt at using an air bubble to support it while adhesion develops. As with all forms of intraocular surgery, infection and intraocular bleeding can complicate this procedure.
Pterygium Excision & Conjunctival Graft
Removal of pterygia is advised in those cases in which vision is, or is likely to be, compromised by further growth of the lesion. It is also recommended for those patients who have chronic inflammation of the pterygium that is not responsive to medical therapy. Historically, a “bare sclera” excision was performed, but in the 1980s it was recognized that simple removal of the lesion had a rate of recurrence of more than 50 percent, which was unacceptably high. The additional surgical maneuver of placing a free or rotated conjunctival graft over the conjunctival defect has been found to reduce the rate of recurrence to less than 20 percent. This graft can be sutured or held in place with tissue adhesive. The site from which the graft is harvested heals uneventfully. Other recognized methods of preventing pterygium recurrence include application of Mitomycin-C and beta irradiation.