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.
Although it shares the same name, equine glaucoma is a different animal than human glaucoma, and the variations between the two could be one reason the disease is not commonly recognized and treated in horses, one veterinarian suggested.
At the 2012 American Association of Equine Practitioners' Focus on Ophthalmology conference, held Sept. 6-8 in Raleigh, N.C., Brian Gilger, DVM, MS, Dipl. ACVO, professor of ophthalmology at the North Carolina State University College of Veterinary Medicine, reviewed the proper diagnostics and treatments for equine glaucoma.
Horses, Gilger noted, tend to tolerate glaucoma better than some other species. The term glaucoma, he 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 equine recurrent uveitis, can cause secondary glaucoma.
Gilger said the most common clinical signs associated with early primary glaucoma include corneal edema (fluid swelling), minimal discomfort, and other minor ocular signs; as the disease progresses, signs might include corneal striae (stretch marks), mydriatic pupil (long-continued or excessive pupil dilation), an enlarged globe, retinal or optic nerve degeneration, mild to moderate discomfort, and blindness.
Early secondary glaucoma is evidenced by unrelenting corneal edema, severe discomfort, chronic uveitis, and blindness, Gilger said. As the disease progresses, eye enlargement, diffuse corneal edema, uveitis signs, and vision loss become more common.
Gilger relayed that veterinarians should diagnose glaucoma using an instrument called a tonometer, which measures intraocular pressure. He recommended using nerve blocks on the horse's eyelids to relieve any tension that might artificially increase the ocular pressure and avoiding sedation if possible, as tranquilizers tend to artificially lower the intraocular pressure.
When using the tonometer, Gilger stressed it's important to test at the most normal-looking part of the cornea and to be consistent in where testing takes place to ensure accurate measurements.
Gilger also said that a thorough ocular exam and an ultrasound of the affected eye can yield valuable information about its internal status and differential diagnoses that could be identified.
"We don't have a cure for this, so (with treatment) we're just trying to control it," Gilger said. Medications available to treat human and/or canine glaucoma aren't effective in horses, he noted. Therefore, he recommended using systemic anti-inflammatories to manage underlying inflammation in addition to a topical eye drop that reduces aqueous humor production.
Although this sounds like a relatively straightforward treatment regimen, Gilger stressed that 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."
There are several surgical options available to treat equine glaucoma, he said; however, like medical treatment, 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 one recent study to maintain vision. Topical medications are still necessary post-surgery in most horses, Gilger noted, and ocular inflammation following the surgery is common.
Gilger also discussed the implanting drains in the eye to eliminate excess aqueous humor. Veterinarians often use this method in conjunction with cycloablation; however, Gilger noted that only about 50% of horses that recently underwent this surgery maintained proper ocular pressure.
He also noted that researchers are currently looking at employing implants to administer anti-glaucoma drugs, but "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.
Finally, Gilger touched on the importance of closely monitoring the horse's "good" eye. In primary glaucoma cases the other eye will be predisposed to developing glaucoma, so Gilger recommends checking the opposite eye's pressure at least quarterly to ensure it remains healthy. In cases of secondary glaucoma, the horse is not necessarily more prone to developing glaucoma in the opposite eye since there are underlying disease mechanisms at work other than the primary problem.
"Equine glaucoma is a frustrating disease that is different in clinical appearance, pathogenesis, and response to treatment compared to canine and human glaucoma," Gilger concluded. "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."
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.