Surgical removal of an eye is a traumatic event, but cosmetic options might help owners cope

by Kenneth L. Marcella, D.V.M.

THE HORSE has the largest eye of all land mammals. Because the equine eye is so big and it is located at the widest point of the skull to allow for a horse's unique, almost 360¡ field of vision, eye injuries are common. Cuts, blunt-impact trauma, punctures, and abrasions account for most ocular problems in the horse. Injuries often are so severe that eye removal, or enucleation, is one of the most common equine ophthalmologic surgeries.

In many instances, eye injuries or diseases can progress to blindness associated with significant pain. These situations warrant eye removal, and surgeons should advise owners about the benefits of the many options for prosthetic replacement.

Removal of a horse's eye is always a traumatic event, but many equine ophthalmologists feel it is often more stressful for the owner than for the horse. If a surgeon provides the horse owner with an understanding of how he or she reached the decision to recommend eye removal and then makes information available about the cosmetic options for the horse after surgery, it might help reduce that stress.

Eye injuries

Direct kicks, trauma from falls, and contact with starting gates, machinery, and any number of stable obstacles can result in eye injuries. Elasticized or stretchy rubber ties are a particular hazard. Many horses pull against cross ties that break under pressure and snap back, causing eye lacerations or punctures, so horsemen need to be especially cautious when using them.

If an injury causes loss of integrity of the globe (eyeball), the eye may lose fluids and begin to deteriorate or shrink. If trauma causes severe enough bleeding within the eye, the increase in pressure might damage internal structures and the optic nerve, which results in blindness and a swollen, painful eye. A puncture that causes a bacterial infection also can cause blindness and a swollen, painful eye if it goes untreated or if the infection becomes unresponsive to antibacterial therapy.

Extensive abrasions to the surface of the eye (cornea) result in thick whitish or gray plaques across the eye. These thickenings can be severe enough to render the horse functionally blind.

Equine eye trauma can be a very serious situation and always should be addressed aggressively with an immediate, complete examination and appropriate medical or surgical care. Yet, because of the often-severe nature of equine eye trauma, many cases do not respond or simply are unable to be treated, and eye removal should be considered.

Diseases affecting the eye

The most common non-traumatic ocular disease that might lead to eye removal is equine recurrent uveitis, also known as moon blindness or periodic ophthalmia. During bouts of equine recurrent uveitis, the immune system operating within the eye launches a hypersensitivity reaction in the eye directed against an infectious agent.

Many organisms have been implicated in this disease. These range from bacterial:

•Leptospirosis;

•Brucellosis;

•Streptococcus;

•Rhodococcus;

•Escherichia coli; and

•Borrelia to viral:

•herpesvirus types 1 and 4;

•influenza;

•equine viral arteritis; and

•equine infectious anemia or swamp fever to parasitic:

•onchocerca; and

•toxoplasmosis.

In some individuals, the immune response directed against these potential invaders becomes excessive. Rather than ridding the eye of the infection, the changes that occur in the eye as part of the immune response actually begin to damage the eye, causing deposits of thick fibrin in the ocular fluid. The constant inflammation within the eye damages all its structures and causes tearing (lacrimation), squinting (blepharospasm), and light sensitivity. In chronic stages the horse becomes blind but still experiences painful flare-ups associated with inflammation.

Many horses respond to a number of treatment protocols, and progress is being made with this age-old disease. Still other horses do not respond or are not treated aggressively enough and eventually will require enucleation.

Tumors of the eye are frequently seen in the horse, with squamous cell carcinoma (SCC) being the most commonly reported type of cancer. This tumor is most often seen in draft breeds, Appaloosas, and gray-and-white Paints. Recent evidence implicates a specific gene that can make individuals in these high-risk breeds up to eight times more likely to develop SCC than horses lacking the gene. SCC commonly affects the third eyelid, but it can invade the eye itself and the orbit or socket, making eye removal necessary.

Remove the pain

The decision to remove an eye is not one to be made hastily, but Neal Ashton, M.R.C.V.S., senior partner in Oakham Veterinary Hospital near Leicester, England, provided a straightforward approach.

"The decision to remove an eye is inevitably based on the need to remove pain, and, as such, it is rarely a difficult one to make," Ashton said. "If the horse loses sight but is not in pain, there is no point in removal."

The type of problem necessitating eye removal often determines where (clinic or field) and how (which approach and technique) the procedure will be done. The actual surgery is relatively uncomplicated. Usually, general anesthesia is recommended to control pain during the procedure and allow the surgeon time for possible placement of a prosthetic eye. In some cases, eye removal can be done successfully as a field surgery using injectable anesthetics. Eye removal surgery also can be performed on a tranquilized, standing horse, utilizing local anesthetic blocks of the face and eye, but this procedure usually is reserved for horses that are too old to withstand the stress of general anesthesia.

Complications of enucleation are generally excessive bleeding or postoperative infection. Because the optic nerve must be severed during the procedure, infection has the potential to travel via this nerve into the brain. So, while eye removal in the horse is generally regarded as a straightforward procedure, the surgeon must use caution to reduce the possibility for complications.

A natural appearance

When an equine eye is removed, several choices exist as to the exact method of surgery and the final appearance of the horse. Owners often want their horses to look as natural as possible after the eye is removed, so post-surgical cosmetics usually are of great concern, especially with certain breeds of show horses, as well some performance horses.  (Some horse-show rules state that a horse can continue to compete with "the appearance of two eyes," not necessarily two functional eyes.) Given these concerns, owners can choose from among several different ocular prosthetics.

Horses that have experienced trauma that led to thickened, non-visual corneas or horses with excessive scarring of the cornea can be fitted with tinted or dark-colored contact lenses. These lenses are fitted over removed portions of the eye. If the entire eye must be removed, options still exist to provide the horse with a more normal appearance after surgery.

If trauma to the eye is severe, often the entire eye, including lids, conjunctiva (the pink tissue surrounding the globe of the eye), and associated muscle must be removed. An implant or prosthetic eye can be placed in the depression created by the surgery, and the skin closed over the top. Horses that have this procedure lack eyelids but otherwise look as if they are holding the eye closed.

Prosthetic eye

If the associated eye structures are not damaged, the surgeon can make an incision along the white (sclera) of the eye and remove the fluid, lens, and retina. A dark silastic, or silicon, ball then can be placed in the remaining sac of the eye and the incision closed. Horses repaired in this way are usually well accepted by their owners. According to Ralph Hamor, D.V.M., a board-certified veterinary ophthalmologist at the University of Illinois Veterinary Teaching Hospital, "This procedure leaves the eyelids, conjunctiva, glands, and eye muscles intact, so the horse can still move its eye normally, and there is a fairly normal appearance."

For some sensitive owners, the desire to replace an exact duplicate of the horse's eye leads to the use of ceramic or glass prosthetics. This type of repair requires intact eyelids and eye muscles to hold the hand-painted glass shell in place. A glass or ceramic prothesis must be removed and cleaned daily, but it can produce a horse with a truly matching, normal-appearing eye. The cost of these specially fit and individually designed eyes can be expensive with the implant, fittings, and surgery costing $3,000 to $4,000.

Brian Gilger, D.V.M., M.S., a professor of veterinary ophthalmology at North Carolina State University and author of the textbook Equine Ophthamology, has begun using ocular prosthetics made of hydroxyapatite. A natural material derived from marine coral, hydroxyapatite is currently the most common material used for human prosthetic eyes. Hydroxyapatite is biocompatible with living tissue, lightweight, and porous, which allows small blood vessels and fibrous tissue to grow into it, making the implant very secure.

Scleral samples are being harvested from horses euthanized for other reasons, and these tissues are fitted with hydroxyapatite implants and then put in horses that need them. This relatively new equine prosthetic provides a secure base for the ceramic shell that is then made, fitted to the implant, and painted to match the other eye.

Not every horse with eye trauma needs a high-tech eye for special occasions, but many injured horses that require eye removal can be successfully treated to eliminate pain and to produce an acceptable cosmetic result.

Kenneth L. Marcella, D.V.M., is a practicing veterinarian in Canton, Georgia.