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.
The first equine uveal disorders many horse owners think of are uveitis and equine recurrent uveitis. But there are actually several other important equine uveal problems veterinarians occasionally encounter.
Brian C. Gilger, professor of ophthalmology at North Carolina State University's College of Veterinary Medicine reviewed these relatively uncommon anomalies at the 2012 American Association of Equine Practitioners' Focus on Ophthalmology conference, held Sept. 6-8 in Raleigh, N.C.
Gilger noted straight away that many of these abnormalities cause little to no discomfort or vision loss for the horse; however their differential diagnoses can be serious and damaging diseases. Thus, he said, it's important to understand these innocuous conditions to avoid incorrect diagnoses.
Multiple Congenital Ocular Anomalies (MCOA)--Most commonly seen in Rocky Mountain Horses, Miniature Horses, and Kentucky Mountain Saddle Horses, MCOA is a recessive trait associated with the silver dapple coat color, Gilger said. This nonprogressive disease commonly includes ciliary body cysts (a congenital uveal abnormality) and megaloglobus (eyeball enlargement).
Persistent Pupilary Membranes--This anomaly, in which the remnants of an ocular fetal membrane remain and appear as strands of tissue in the pupil, is rarely clinically significant and typically does not necessitate treatment, Gilger said.
Aniridia--This rare condition--in which an iris never develops so the horse can't constrict his pupils--is typically diagnosed in foals, Gilger said. Aniridia is believed to be an autosomal dominant trait, he said, and is most often identified in Thoroughbreds, Quarter Horses, and Belgain Draft Horses.
Iris Coloboma--This disorder, in which some of the ocular structures are missing due to incomplete intraocular fissure during gestation, also is typically clinically insignificant, Gilger said. Iris coloboma often appears as a notch-like defect in the bottom center of the iris. The main associated complication is photophobia, he added, however if a horse doesn't show clinical signs associated with the disorder, he's not likely to develop any.
Corpora Nigra Cysts--"This is one of my favorite diseases because it's a lot of fun to treat," Gilger relayed with a grin. Although multiple cysts can develop on the corpora nigra (the irregularly shaped body at the edge of the iris), the condition typically has no effect on vision, unless the cysts grow large enough to obstruct the horse's vision. At this point, treatment entails deflating the cysts via a laser. "I don't let the others treat them because they're too much fun (to deflate)," he admitted with a laugh.
Iris Hypoplasia--These dark, bulging areas on blue or light colored irises' surface "are completely insignificant, but they can be really dramatic," Gilger said. Essentially, these bulges are the result of the aqueous pressure pushing on a weak portion of the iris. Photophobia is a possible side effect, Gilger said.
Medulloepithelioma--This is one uveal disorder that has major clinical significance. "These tumors arise from undifferentiated medullary epithelial cells that line the inner portion of the embryonic optic cup," Gilger explained Medulloepitheliomas are most commonly diagnosed in young horses and grown slowly, however they are often associated with other potentially serious ophthalmic disorders, he said. The treatment of choice, Gilger said, is enucleation.
Uveal Melanoma--"Although relatively rare overall, intraocular pigmented masses are reported in older gray horses, usually with concurrent cutaneous (skin) melanoma," Gilger said.
Early clinical signs of uveal melanoma include focal corneal edema with a dark mass visible in the eye. Clinical signs of more advanced disease include blindness, eyelid twitching, watery eyes, eyeball enlargement, diffuse corneal edema, aqueous flare, and anterior chamber masses, he said.
Uveal melanoma isn't life threatening on its own and won't spread to other areas of the body, Gilger said, however the fact that it often occurs concurrently to cutaneous melanoma is cause for concern. Treatment options include laser removal (although masses are known to grow back) and iridectomy (when less than 20% of the iris is involved), however enucleation or exenteration are the most common and effective choices, he added.
Ocular Lymphoma--This disorder most commonly occurs in horses with systemic lymphoma, Gilger said. The ocular manifestation, he added, is easier to manage than the systemic manifestations. Uveal lymphoma's clinical signs are nonspecific but include eyelid twitching, episcleral injection, corneal edema and vascularization, aqueous flare, hyopoyon, hyphema, and iris congestion and swelling.
"On the basis of ocular signs alond it is usually not possible to differentiate intraocular lymphoma from other potential causes of anterior uveitis," he noted. Treatment options include steroid therapy or enucleation.
Take-Home Message
The majority of the noninflammatory uveal disorders in horses are clinically insignificant, Gilger noted. "Some of these can be pretty bizarre, but most of them are going to be cosmetic defects," he added. It's important, however, to have any ocular abnormality examined by a veterinarian to rule out differential, and potentially more serious, diagnoses.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.