Equine corneal transplants' success rate growing

Equine corneal transplants' success rate growing

Better technology, pharmacology needed to reduce tissue rejection, expert says
Jul 01, 2008

Treatment of equine eye diseases, especially through surgery, has made dramatic strides in the last 25 years. As late as the mid-1980s, many veterinarians expected to fail when treating horses' eyes in the mistaken belief that they heal poorly, one expert recalls.

Photo 1: A deep stromal abscess is present to cause severe uveitis.
Actually, a horse's eye heals well from surgery, says Dennis Brooks, DVM, PhD, Dipl. ACVO, who with his colleagues has performed 236 equine corneal transplants since 1995 at the University of Florida's College of Veterinary Medicine, with a high success rate. "It just took us time to learn how to help it heal," he says.

Corneal transplantation for horses may be performed for optical, therapeutic, tectonic and cosmetic reasons, Brooks says. "Optical transplants restore or improve vision in cases of corneal edema. Therapeutic grafts attempt to control medically refractory corneal disease by removing necrotic and infected tissue. Tectonic grafts are done to preserve or restore the structural integrity of the eye when corneal tissue is missing and cosmetic grafts improve the appearance of the eye without necessarily improving vision."

The successful transplants by Brooks and colleagues were mostly for therapeutic and tectonic reasons. "Initially we did it, not just because we wanted to do that surgery, but because we had no choice," Brooks says.

The equine cornea is the anterior transparent portion of the fibrous tissue of the eye that supports the intraocular contents and transmits and refracts light. With disease, such as fungal infection, neoplasm, ulcers and/or infection due to laceration, its integrity and function may be compromised.

Photo 2: The superficial cornea is elevated to expose the abscess, the abscess removed and the missing tissue replaced with split-thickness cornea in the posterior lamellar keratoplasty (PLK) procedure. This image shows corneal scarring one month postoperatively.
"Indications for surgical intervention include progression of corneal melting, increasing depth of an ulcer, persistence or progression of iridocyclitis despite appropriate medical therapy, posterior corneal location of a stromal abscess, stromal abscess rupture through the Descemet's membrane and suspicion of a corneal foreign body," explains Heidi Denis, DVM, Dipl. ACVO with Animal Eye Associates in Maitland, Fla.

"Two basic surgical procedures have been described for the horse: penetrating keratoplasty (PK) for full-thickness stromal abscesses or ulcer/iris prolapses, and a split-thickness form of PK, the posterior lamellar keratoplasty (PLK), for deep stromal corneal abscesses (DSA) with a clear overlying anterior stroma," Brooks explains.

Photo 3: Little scarring is present one year postoperatively.
"A third type of split-thickness PK procedure, the deep lamellar endothelial keratoplasty (DLEK), has been used recently for DSA in the horse where the superficial cornea may be vascularized but is otherwise normal," Brooks says, pointing out that all three procedures have good visual outcomes, although partial graft rejection and scar formation are unavoidable.

Both fresh and frozen grafts are used in horses, though fresh ones "maximize endothelial cell preservation, minimizing post-operative corneal opacity," Brooks says. Frozen cornea may be damaged by changes in pH, osmolality, solute concentration and by ice-crystal formation. Though this damage may occur to tissue, it may be stored for up to six months and used satisfactorily. Because fresh grafts are not often available, success in horses is still quite good with frozen cornea.

Photo 4: A deep stromal abscess is causing severe pain and inflammation inside the eye.
For all transplant methods there is a high success rate, good visual outcome in 75 percent to 90 percent of cases and shorter treatment times than without surgery. Unfortunately, no matter how successful, all corneal transplants in horses vascularize, have some degree of opacity and are to some degree considered partially rejected.