The Eyes Have It
- Apr 1, 2013
Photo: Anne M. Eberhardt/The Horse
Researchers are scrutinizing the equine eye so they can treat ocular issues better.
An old expression says the eyes are the windows to the soul. And when the windows to our horses' souls become scratched, cloudy, ulcerated, or otherwise unhealthy, we turn to our veterinarians for help. Although veterinarians have a grasp on diagnosing, treating, and managing many eye problems, other equine ophthalmic puzzles remain unsolved. Fortunately, researchers around the world are working continuously to understand these conditions better and find solutions.
In late 2012, for instance, the American Association of Equine Practitioners hosted a Focus on Ophthalmology meeting in Raleigh, N.C., where veterinarians gathered to further their understanding of the equine eye; how to examine it; and how to diagnose, treat, and manage specific ocular conditions. The following is a recap of some of the information presented. For recaps on additional lectures from other presenters, search "Focus on Ophthalmology 2012" on TheHorse.com.
Managing Equine Recurrent Uveitis
"Equine recurrent uveitis has been an interest of mine for as long as I've been an ophthalmologist," said Brian C. Gilger, DVM, MS, Dipl. ACVO, a professor of ophthalmology in North Carolina State University's Department of Clinical Sciences. "Unfortunately, it's still an interest because we can't seem to get this disease under control."
Gilger explained that there are three distinct types of ERU that can affect horses: classic, insidious, and posterior. Classic, or anterior (affecting the front of the eye), ERU is the most common type and causes concurrent bouts of pain and inflammation lasting approximately two weeks. "Active" periods are interspersed with periods of "quiet," a pattern that continues indefinitely until the eye goes blind.
Brian C. Gilger, DVM, MS, Dipl. ACVO, a professor of ophthalmology at North Carolina State University's Department of Clinical Sciences, recommended the following management practices to protect the eyes of horses with equine recurrent uveitis (ERU):
- Use a quality fly mask with ultraviolet protection;
- Decrease dust;
- Change bedding;
- Eliminate sharp objects from around the barn and pasture;
- Eliminate low branches;
- Decrease training and show schedule, and minimize trailering; and
- Avoid using haynets.
Horses with insidious, or subclinical, uveitis typically don't show signs of pain, but persistent low-grade inflammation following the active phase damages internal structures. These horses often slowly develop signs of chronic ERU, but owners might not recognize it until significant damage is done.
Posterior uveitis (in the back of the eye) is the least common type seen stateside. Most of its associated inflammation develops behind the lens.
Gilger noted that veterinarians see classic uveitis most frequently in Appaloosas, insidious ERU in Appaloosas and draft horses, and posterior ERU in Warmbloods.
There are several theories about the pathogenesis of both uveitis and ERU. "It's becoming more and more likely we have a genetic component to this," he noted. Environmental factors, ocular injury, certain infections (such as leptospirosis), and the presence of certain cells (such as T-lymphocytes, or T-cells) are also believed to contribute to disease development.
"The clinical diagnosis of ERU is based on the presence of characteristic clinical signs and a history of documented recurrent or persistent episodes of uveitis," Gilger said. In most horses a normal uveitic spell lasts two to three weeks before it calms down, even without therapy. Recurrence points to ERU.
Veterinarians use traditional medical treatments (e.g., topical and systemic non-steroidal anti-inflammatory drugs [NSAIDs], and corticosteroids) to reduce inflammation and minimize ocular damage, Gilger said; however, they do not effectively prevent recurrence.
Surgical intervention might be a viable option if medical approaches prove unsuccessful. Options include performing a vitrectomy (a removal of some or all of the aqueous humor, the fluid that fills the eye) or implanting a sustained-release cyclosporine device (CsA).
Although simply removing the cells and vitreous from inside the eye might "sound remarkably straightforward," Gilger said, a vitrectomy is a very expensive option and carries a high complication rate, such as cataract development. Therefore, some veterinarians opting for surgery recommend a CsA device instead.
In these cases a surgeon implants a small disc into the affected eye's sclera (white outer covering of the eyeball), Gilger said, adding that the entire procedure takes, on average, 15 minutes. The device releases medication into the eye at a predetermined dosage rate for at least four years. Veterinarians have completed more than 500 procedures worldwide, and he said owners have reported a significant decrease in the number of active episodes these horses develop.
Once acute episodes are under control, there are several relatively simple daily steps owners can take to help keep their charges comfortable and healthy. Decreasing sun exposure can offer pain relief, for instance, and changing stable or pasturemates if the horse seems prone to injure his eyes when kept with particular horses can help prevent further injury.
Equine recurrent uveitis and uveitis are two different diseases, Gilger stressed. Although there is no cure for ERU, an immune-mediated disease, treatment is often effective in controlling clinical signs and active episodes. But even with aggressive treatment, affected horses can go blind in a matter of years.
Diagnosing and Treating Glaucoma
The term glaucoma, Gilger explained, describes "increased ocular pressure to a level that is incompatible with the health of the eye." In the normal eye aqueous humor ebbs and flows, and the body regulates this so that pressure remains steady. An abnormally developed outflow pathway, or drain, in an eye with no other ocular deficits can decrease outflow, causing pressure to build (called primary glaucoma). Alternately, damage to the drain, such as scarring, debris accumulation, or vascularization due to ocular disease such as ERU, can cause secondary glaucoma.
Gilger said the most common clinical signs of early primary glaucoma include corneal edema (fluid swelling), minimal discomfort, and other minor ocular signs; as the disease progresses, corneal striae (stretch marks), mydriatic pupil (long-continued or excessive pupil dilation), globe (eyeball) enlargement, retinal or optic nerve degeneration, mild to moderate discomfort, and blindness might develop.
Early secondary glaucoma can be seen as unrelenting corneal edema, severe discomfort, chronic uveitis, and blindness, Gilger said. As the disease progresses, globe enlargement, corneal edema, uveitis signs, and vision loss become more common.
"We don't have a cure for this, so (with treatment) we're just trying to control it," Gilger said. Human and/or canine glaucoma medications aren't effective in horses, so he recommended using systemic anti-inflammatories to manage inflammation and using a topical eye drop that reduces aqueous humor production.
Although this process might sound relatively straightforward, Gilger said it's not so simple: "This is difficult to treat. The most effective medicine we have needs to be given three times per day, forever."
Veterinarians can pursue several surgical approaches to equine glaucoma, but none cure the disease. One option involves cycloablation (laser destruction of the aqueous humor-producing part of the ciliary body, a muscular ring located in the front part of the eye), which effectively lowers intraocular pressure and allowed up to 59% of horses in a recent study to retain vision. Topical medications are still necessary post-surgery in most horses, Gilger noted, and ocular inflammation following the surgery is common.
He also discussed implanting drains in the eye to eliminate excess aqueous humor. Veterinarians often use this method in conjunction with cycloablation, but Gilger said only about 50% of horses that recently underwent this procedure maintained proper ocular pressure post-surgery. Researchers are examining the use of implants to administer anti-glaucoma drugs, but he said "it's far away" at the moment.
In many cases affected globes become large and painful, despite treatment, and veterinarians must manage these cases quickly, Gilger said. Enucleation (surgical removal of the eye and associated structures) is an ideal option in these cases, and he noted that the veterinarian might recommend intraocular silicone or cosmetic prostheses for these animals.
Researching Ocular Drugs
Alison B. Clode, DVM, Dipl. ACVO, an assistant professor of ophthalmology in North Carolina State University's Department of Clinical Sciences, studies the drugs used to treat a variety of equine ocular ailments, including squamous cell carcinoma of the cornea and ocular fungal and bacterial infections, as well as other ocular anti-inflammatory drugs.
"With all the drug studies we're doing ... we're finding out a lot about what antifungal (and antibiotic) medications may or may not work," she says. "It's a lot of putting the drug on the eye and figuring out where the drugs go and how readily they get to the parts of the eye that they need to be."
She's also evaluating the many non--steroidal anti-inflammatories available for treating equine eyes. "They're really expensive, so we don't want to say, 'Put your horse on this medication,' without knowing that it's actually going to work or has potential to work," she says. "We don't want to be spending a lot of owners' money without justification for it.
"A lot of the studies I do, as soon as I have the results saying this drug does get into the cornea when you give it topically, we can go and start treating animals with it," she continues. "That's a slight oversimplification, but that's pretty much how it is. It's going to help their animals and be the most effective use of their finances."
Clode is also performing histopathologic evaluations (microscopic examination of cells) of nonhealing corneal ulcer biopsy samples to get to the root of what's going on in affected horses. And, in the future, she hopes to not only better understand how to control fungal disease but also figure out where it starts.
"Horses get fungal keratitis (corneal inflammation) more than any species we treat, by far," she says. "They actually get it more than humans in some areas of the country. We see fungal keratitis here (in Central North Carolina) in horses all the time, yet it's not as common in people in this area. I would like take some steps back and figure out what about the horse makes them more susceptible to it, because we don't really know."
Finally, Gilger touched on the importance of closely monitoring the horse's "good" eye. In primary glaucoma cases the healthy eye is predisposed to developing glaucoma, so Gilger recommends checking its pressure at least quarterly. Horses with secondary glaucoma are less prone to developing glaucoma in the opposite eye because there are underlying disease mechanisms at work other than the primary problem.
"It is very important for the equine clinician to recognize early clinical signs of glaucoma, be able to measure the intraocular pressure of horses, and use appropriate anti-glaucoma therapy," he -summarized.
Identifying Orbital Disorders
When a horse presents with clinical signs of an orbital injury (e.g., an injury to the bones of the skull that house and protect the eyeball, sometimes evidenced by a prominent third eyelid, acute swelling, eye bulging, etc.), the veterinarian should perform a complete diagnostic exam to pinpoint the exact problem. Gilger recommends first performing ultrasound on the eye to determine if the globe appears normal.
"This is one of the most satisfying, easy diagnostic procedures you can do," he said.
In some cases, the veterinarian ultrasounds the other eye to compare the structures. Common problems identified in this exam include vitreal hemorrhage and retinal detachment, he said, both of which can lead to blindness. If the globe appears normal, Gilger recommends pursuing more advanced imaging--skull radiographs or the gold-standard, computed tomography (CT)--to try to identify the problem.
Following imaging, he advised veterinarians to palpate the eye, a procedure he stressed the horse must be sedated for. Palpation is useful for locating fractures and replacing fragments to their appropriate positions ("Because these bones are non-weight-bearing, they will often heal once replaced to proper position," he added).
Once the veterinarian makes a diagnosis, he or she can begin treatment or refer the horse to a specialist:
Orbital Fractures How the veterinarian treats the fracture depends on whether the surrounding skin is open or closed, Gilger said. Closed skin fractures typically heal well if the fragment is replaced, he said. In his experience with closed skin fractures, the horses' eyes return to normal function post-treatment.
When dealing with an open fracture, Gilger said the veterinarian should clean the defect well and consider minimal debridement before wiring the bone into place.
"Depending on the extent of the contamination, some or all the wound is left open for adequate drainage, or drains are placed to facilitate healing," he added.
Orbital Neoplasia Tumors on the equine orbit are rare, Gilger said. When they do develop, the most common types are squamous cell carcinoma, lymphosarcoma, and anaplastic sarcoma.
Veterinarians treat orbital neoplasia surgically, with or without adjunct therapy. "Although surgical removal and salvage of the eye is possible early in the disease process, many cases require exenteration of the orbit (removal of all the tissues and structures within the orbit)," he said.
"Diseases of the equine orbit may not be as common as diseases in other parts of the equine eye, but the impact of orbital disease is very high," Gilger concluded. Be vigilant about watching horses' eyes for signs of trouble; this can mean prompt treatment, increasing the horse's chances for a successful recovery.
Understanding Cataract Surgery
While some cataracts cause few vision problems for horses, others impact vision substantially and require surgical removal. The procedure of choice, Gilger said, is called phacoemulsification and aspiration. He added that implanting intraocular lenses has become common in cataract surgery as well.
Veterinarians should perform a thorough ocular exam to evaluate the case prior to surgery. "We only take (surgical) cases that are really good candidates because this isn't an easy process," he said. Before proceeding, it's crucial to ensure that horse (and owner) are able to handle the long-term care required post-surgery.
Gilger said many veterinarians employ systemic and topical antibiotics prior to surgery to reduce the risk of bacterial contamination and endophthalmitis (inflammation of the eye's internal structures). In the 24 hours leading up to surgery, veterinarians administer intravenous antibiotics. Twelve to 18 hours prior to surgery veterinarians use topical atropine to dilate the pupil, and they administer systemic NSAIDs at least an hour before surgery.
Many surgeons use gastric protectants to reduce the risk of the horse developing ulcers, Gilger added.
Once a horse recovers from general anesthesia, veterinarians monitor him for short-term postoperative eye complications, including uveitis, corneal edema, blood in the eye, corneal dehiscence (failure of the healing incision), and retinal detachment. They also watch out for colic, cecal impactions, colitis (inflammation of the colon), and laminitis, as these complications can sometimes prove fatal, Gilger said.
Veterinarians observe the horse for long-term issues, including glaucoma, uveitis, corneal edema, capsular fibrosis (clouding over the lens capsule), and retinal detachment.
Many patients require medical therapy for up to three months following surgery, Gilger said, and some horses benefit from baseline doses of anti-inflammatories to help keep the affected eye(s) comfortable.
An abundance of horses recover well from cataract surgery and retain vision. Careful planning with a veterinarian and dedication to recovery efforts can help improve surgical outcomes.
Free Download: 15 Fascinating Facts about Equine Eyes
About the Author
Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.
POLL: Rabies Vaccination