Editor's Note: This article is part of TheHorse.com's ongoing coverage of topics presented at the 2012 American Association of Equine Practitioners' Focus on Ophthalmology conference, held Sept. 6-8 in Raleigh, N.C.
An underlying theme was apparent when a worldwide authority on equine ophthalmology presented a lecture the 2012 American Association of Equine Practitioners' Focus on Ophthalmology conference: "Horses can't do anything easily." Specifically, in this case, the natural mechanisms in the horse's eye do not always cooperate when veterinarians are working to repair corneal defects.
At the conference, held Sept. 6-8 in Raleigh, N.C., Dennis Brooks, DVM, PhD, Dipl. ACVO, professor of ophthalmology at the University of Florida's College of Veterinary Medicine, reviewed the challenges the equine eye presents when faced with corneal ulcers and/or ulcerative keratitis, and methods that can be used to improve the outcomes for patients.
The Cornea Reviewed
Brooks began with a brief review of the equine cornea and the types of ulcers that can develop on the equine eye.
"The normal equine cornea is about 2,000 square millimeters, and about 1 millimeter thick," he relayed. "Despite that thinness, it's very, very strong tissue. And the equine cornea heals tremendously well, but the diseases that attack the cornea are some of the post powerful in comparative ophthalmology."
When injured, the corneal epithelium grows at rate of approximately one millimeter per day, starting from the outside edges of the eye (the limbus) and growing inward to the center, Brooks explained. For example, he said, a 6-millimeter ulcer should take about six days to heal, barring complications. However it takes about six weeks for the new epithelial cells to solidly attach to the stroma, leaving the horse at risk for re-injury, he said.
There are several different types of ulcers that can affect the cornea, Brooks said, including:
- Superficial ulcers, which, although involve the fewest structures in the eye, "can be just as difficult to treat and remove the risk of infection," he said;
- Deep ulcers, which often require surgery to heal properly;
- Descemetoceles, a type of deep ulcer that can leave as little as "six red blood cells (worth of material) holding the eye together" when they're at their worst;
- Melting ulcers, which typically require extensive treatment to repair; and
- Ulcers with iris prolapsed, which were "once thought of as a hopeless condition," but can now be treated and sight saved in some instances.
Brooks explained that the equine cornea is constantly exposed to microbes, however, the epithelium and tears serve as the barriers against infection. When those barriers are damaged or ulcerated, bacteria, fungus, and other pathogens can enter the eye and prompt infection development, he said.
"When a horse cornea gets injured, something in horse tears attracts neutrophils," Brooks relayed. Although neutrophils are typically associated with the healing process, tear film neutrophils--along with some bacteria and fungi--are associated with a highly destructive enzyme pronteinase and collagenase enzymes (collectively termed proteases), which often results in stromal thinning (the thickest, middle layer of the cornea), descemetocele formation, melting ulcers, and corneal perforation, he explained.
"But horses can't do anything easily," he said, noting that when protease levels are high in one eye, they are also elevated in the opposite, healthy eye. Thus, it's important to be on the lookout for problems in the "healthy" eye when a corneal ulcer is found.
When protease levels return to normal, the eye is often able to heal. The challenge is returning those levels to normal.
"Neutrophils are the real problem," he stressed.
Diagnostic Procedures
Brooks discussed equine eye exams in detail in a prior lecture at the conference. However, he touched on the basics of diagnosing corneal ulcers, recommending all corneal injuries be stained with flourescein (to detect corneal ulcers) and rose bengal (to detect a tear film instability). He also suggested microbiologic culture for horse with "rapidly progressing and deep corneal ulcers" to determine what, if any, fungus or bacteria is present.
Current Medical Treatment Options
"Medical therapy almost always comprises the initial major thrust in ulcer control," Brooks said. Medical therapy is effective in treating many corneal ulcers, and often begins with topical atropine application to dilate the pupil and prevent synichia (when the iris adheres to the cornea or the lens capsule) from occurring.
Brooks stressed, "Atropine can reduce digestive motility to the point of dangerous colic, so we need to watch these horses really closely."
Once the pupil is dilated, other medical measures can be implemented. In many cases, Brooks noted, veterinarians will employ a lavage system to aid in the treatment of nervous or fractious horses, or those with painful eyes that require frequent treatment.
Veterinarians often select broad-spectrum topical antibiotics as a treatment option, and culture test results can aid in drug selection, he said. He noted that solutions, rather than ointments, appear to interfere less with corneal healing during the application process.
Autogenous serum, which is derived from the horse, is another popular treatment option known to reduce tear film and corneal protease activity, Brooks said. "The serum can be administered topically as often as possible and should be replaced by new serum every five days," he added.
In severe cases, acetylcysteine and/or ethylenediaminetraacetic acid (commonly referred to as EDTA) can be used in conjunction to the serum to promote healing.
Finally, Brooks indicated that most veterinarians will employ systemic and topical nonsteroidal anti-inflammatory drugs (NSAIDs) when treating corneal ulcers. Phenylbutazone and flunixin meglumine are common choices for systemic NSAIDS, he said, used to reduce uveal exudation and relieve ocular discomfort. Common topical choices include profenol, flurpbiprofen, and diclofenamic acid, Brooks said, which are used to reduce uveal swelling.
"Horses with corneal ulcers and secondary uveitis should be stall rested until the condition is healed," he added. "Intraocular hemorrhage and increased severity of uveitis are sequelae to overexertion."
Current Surgical Treatment Options
If medical therapy proves ineffective, surgical intervention might be required. Brooks explained that there are several different surgical options appropriate for different problems.
The application of a bandage soft contact lens might be appropriate for some cases to protect the new epithelium and reduce ocular pain, Brooks said. "Disadvantages include an occasional poor fit in horses, thereby resulting in limited retention times," he noted.
Surgical debridement or a keratectomy procedure (which entails removing a small part of the cornea) can speed healing, minimize scarring, and decrease the potential for secondary uveitis development, Brooks said. Plus, he added with a smile, "Cutting cornea is exciting. It's like cutting Jell-O!"
Similarly, a grid keratotomy (which involves etching grid lines on the surface of the cornea over and around the ulcer) can promote healing in indolent ulcers, however Brooks stressed it must be used properly and in appropriate situations to avoid complications.
When faced with deep or melting corneal ulcers, descemetoceles, and perforated corneal ulcers with or without iris prolapsed, Brooks said veterinarians often choose a conjunctival graft procedure. This procedure involves pulling conjunctiva across the ulcer and suturing it into place. Some scarring typically occurs at the ulcer site with conjunctival grafts, Brooks noted, and inflammation might recur after the graft is removed.
Amniotic membrane flaps are also used to treat indolent or deep, melting ulcers, Brook said. This procedure, which has been shown to decrease fibrosis, reduce vascularization, and promote re-epithelialization involves suturing a piece of amniotic membrane over the defect, with or without the aid of a conjunctival graft.
Finally, if all other surgical options fail, Brooks recommended enucleation as a humane alternative.
The Future
Additionally, Brooks relayed that he and colleagues recently started using limbal stem cells they've proliferated to treat some chronic ulcers. Although data is limited, he reported success in the early stages of clinical use.
He also noted that his team has started growing corneal endothelial cells, which are located on the inner surface of the cornea.
"We're getting close to where we can help some more of these horses," Brooks said of treatment options in the works.
Take-Home Message
Although the natural mechanisms in equine eyes sometimes complicate corneal ulcer recovery, veterinary technology has advanced to the point practitioners and surgeons can successfully treat most eye problems. Prompt care is still required, as early treatment typically means better success rates.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.
