An 8-year-old Thoroughbred gelding was presented for evaluation of compulsive head-shaking and spooking when asked to move to his right. The referring veterinarian had attempted treatment with cyproheptadine and melatonin with no improvement in behaviors. The owner reported that the head-shaking behavior ceased in low light conditions.
Physical examination findings were unremarkable with no evident lameness. A CBC and chemistry panel were also normal.
Ocular examination was largely unremarkable as well, with the exception of a smooth pigmented mass at the ventrolateral aspect of the right pupil that moved slightly with head motion. The corpora nigra appeared normal as did the remainder of the iris (figure 1, above).
A pigmented anterior segment mass in a horse may be neoplastic or cystic in origin. Most pigmented anterior segment tumors are melanomas or melanocytomas (the benign variant, which can still be destructive to the eye). Uveal cysts in horses are often associated with the corpora nigra, but can also be free-floating or affixed to the iris in other areas. A simple way to differentiate between tumors and cysts is to try to determine whether the structure transilluminates (whether light seems to pass through it). Try to use a bright light source and illuminate the eye from a variety of angles (figure 2: In this photo of a corpora nigra cyst, small gray areas can be seen in the cyst as indicated by the red arrow. These are areas where light is passing through more lightly pigmented areas).
In cases where transillumination is not conclusive, an ultrasound of the eye may be performed using a high frequency (usually 20 MHz or higher) probe. Cystic structures will have a hyperechoic rim with a dark center, while tumors and other solid structures will be echogenic throughout.
In the case of this horse, the mass was suspected to be a cyst given its shape and lack of confluence with normal iris tissue, but transillumination was equivocal. An ultrasound was performed that confirmed the structure was a cyst (figure 3).
Cysts are benign nonpainful lesions that generally do not correlate with significant ocular disease in horses. They can, however, lead to behavioral changes when they are positioned within the visual axis, particularly if they are somewhat mobile as this one was. Veterinary ophthalmologists can deflate uveal cysts noninvasively using a diode laser-focused with a special lens and headset. The wavelength of light produced by the diode laser is selectively absorbed by melanin, leading to energy release at the surface of the cyst and creation of a hole in the wall of the cyst. The cyst noted in this gelding was treated with the diode laser and was noted to deflate immediately (figure 4).
The horse was treated with topical atropine and dexamethasone/neomycin/polymyxin and oral Banamine for a short period of time as cyst deflation can generate low-grade inflammation in the eye. Two weeks after cyst deflation the owner reported a total cessation in head-shaking and shying behaviors.
While this horse dramatically altered its behavior following treatment, the following points should be made during discussion with the owner:
- Cysts can be incidental findings and may not be related to the behaviors seen. The only way to determine whether the cyst is the cause of the behavior is to try to rule out other causes, then treat the cyst and observe for changes.
- Orthopedic, neurologic, and temperamental or psychological issues can all produce head-shaking, shying, spooking, and other undesirable behaviors. Sometimes behaviors in horses can become ingrained and will not change despite addressing the cyst.
- New cysts can form in the future and can require treatment.
- Complications with diode laser treatment are far less common than with invasive surgical treatments for cysts but are still possible, and can include uveitis, hyphema, cataract formation, and retinal damage.
About Dr. Stephanie Pumphrey
A veterinary ophthalmologist, Dr. Stephanie Pumphrey provides medical and surgical treatment to a variety of veterinary patients with eye issues. Along with patients at the Hospital for Large Animals, she treats small animals, exotics, and wildlife species. She completed her ophthalmology residency at Cummings School of Veterinary Medicine in 2012 and became board-certified the same year. Commonly treated conditions include cataracts, glaucoma, corneal ulcers, eyelid and eyelash disorders like entropion or distichiasis, and uveitis and other inflammatory diseases.