Equine corneal transplants' success rate growing
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.
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.
"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.
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.