SAVING VISION THROUGH CORNEAL TRANSPLANTATION
From Washington University Physicians, posted November 3, 2008, written by Mary Jo Blackwood, R.N., M.P.H
Our precious vision depends on the cornea as its first line of defense against injury. Besides vision-correcting surgery to the cornea such as LASIK, recent advances in cornea transplant procedures to restore the vision have increased in effectiveness. Whether the problem is scarring from contact lens overuse, a chemical splash, or any of a myriad of diseases that affect the eye, the ophthalmologists at the Washington University Eye Center and Cornea Service are on the leading edge of therapies. Corneal specialists include Andrew J.W. Huang, MD, Anthony Lubniewski, MD, and Anjali K. Pathak,MD..
Whatever the cause, when the cornea is irreparably damaged, a corneal transplant is a good option.
Dr Huang performs corneal transplantation using the newest techniques at partial-thickness corneal transplant to make use of the body’s own functional tissue, and reduce the chance of rejection from donor corneas.
“We no longer have to do a full-thickness corneal transplant when it’s not necessary. Previously, we had no way to access the inner corneal surface, so we had to remove the whole cornea and replace it with a healthy one. Now we can transplant either the inner corneal surface (covered by endothelial cells) by itself, or the outer cornea (made of epithelial cells and stoma), thus reducing the chances for both rejection of the cornea and the visual distortion that can come from transplanting an entire cornea.” There are three distinct areas of the cornea that can be transplanted or modified:
Inner cornea: “In a procedure called DSAEK, we first remove the diseased or damaged endothelial cells and put the precut thin donor tissue only on the inner surface by making a small incision, rolling up the tissue and slipping it through the incision, using a tiny air bubble to unroll it and make it stick to the back surface of the cornea. There is no visual distortion because the curved front part of the cornea is untouched.” This procedure can be used for patients with Fuchs’ corneal dystrophy, a hereditary condition with endothelial abnormalities; and patients with damage of endothelia cells after complicated cataract or glaucoma surgery.
Middle cornea: The bulk of the corneal tissue is a spongy matrix material (stroma) without blood vessels. In a procedure called DALK, or the big bubble technique, a larger air bubble is injected into the cornea to push the undersurface away from the bulk of the cornea, creating a cushion. Then the front part of the diseased corneal tissue is removed, leaving the undersurface intact to protect the eye. After the unneeded underlayer from the donor cornea is removed, the remainder, the cap, is placed onto the eye. (The inner surface of the host cornea can be used for another procedure.) “We put in a few stitches to suture the donor cornea to the surrounding tissue until it becomes adhered to the undersurface. There will be much less distortion than a full-thickness transplant because we are maintaining the donor curvature. This procedure can be used for patients with corneal scars from previous infection or stromal injuries, as well as patients with keratoconus, a corneal thinning condition with high myopia and irregular astigmatism; and various hereditary corneal dystrophies characterized by abnormal protein deposits in the stroma.
Front corneal layer: The outer layer is made up of an epithelial cell barrier to prevent invasion of microorganisms or conditions such as dryness. To function properly, the epithelial cells must have a good anchor into the deeper layer of the matrix. With lid blinking, if the cells are not anchored, they can be blinked away or scratched off. “We use a procedure called phototherapeutic keratectomy (PTK) to clean off the epithelial cells and polish the matrix so when new cells grow back, they can adhere to a smoother surface and can become better anchored. This procedure can be performed for patients with recurrent corneal erosions and Map-Dot-Fingerprint dystrophy, both conditions associated with painful corneal surface abrasions.
The Role of Corneal Stem Cells
Huang is doing primary research on corneal epithelial stem cells, which are located in an area to either side of the cornea called the limbus. When contact lenses don’t fit properly; or chemical injuries occur; or in some eye diseases such as Stevens-Johnson syndrome, limbal stem cells can become damaged and die. When that barrier becomes ineffective, conjunctival tissue (the covering white part of the eye) can move into the cornea, bringing with it blood vessels and inflammation that potentially can compromise the vision or damage the cornea.
Some older corneal transplants failed because stem cells weren’t able to regenerate or to maintain their barrier function. Now surgeons can transplant and restore stem cells, if needed, before they do corneal transplants.