Equine Temporohyoid Osteoarthropathy Studied

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Originally published on TheHorse.com

Although scientists still don't know what causes a relatively rare equine neurologic disorder, a research team in England recently took steps to further characterize the most common presenting signs, diagnostic methods, and treatment options associated with equine temporohyoid osteoarthropathy.

Temporohyoid osteoarthropathy (THO) is a disorder of the hyoid apparatus (voice box) and associated structures. The hyoid apparatus and larynx are suspended from the skull via the temporohyoid joint (THJ), which is located in the temporal region of the horse's head (between the larynx and the base of the ear). Essentially, THO is a bony growth of the tympanic bulla, proximal stylohyoid, and petrous temporal bones. The result is fusion of the temporohyoid joint and, in some cases, fracture of the small bones that make up the hyoid apparatus. The cause of the disorder is unknown.

To better characterize affected horses presenting signs, management, and outcomes, Viktor Palus, DVM, MRCVS, ECVN, of Dick White Referrals Veterinary Specialist Centre in Suffolk, England, recently completed a retrospective review of THO cases in England.

Palus and colleagues examined the medical records of horses presented to two English referral clinics from 2005 to 2011. Horses were included in the study if a thickened stylophoid bone was identified on endoscopic examination and they had received a THO diagnosis; seven horses met the criteria. Four mares, two geldings, and one colt, aged one to 18 years, were included in the study.

Of the seven study horses:

• Four horses exhibited peripheral vestibular syndrome with facial paralysis (of those, one had nystagmus [involuntary movement of the eyeballs] and two had concurrent dysphagia [difficulty swallowing]);
• One horse presented with dysphagia and facial hyperesthesia (hypersensitivity to touch and sound);
• One horse presented with facial hyperesthesia and facial paralysis;
• One horse presented with head shaking; and
• Two horses had corneal ulcers at the time of presentation; Palus noted this is a common secondary problem with THO due to facial paralysis and decreased tear production.

All THO cases were confirmed via guttural pouch endoscopy, Palus said. Additional diagnostic techniques, including nuclear scintigraphy (bone scans) and computed tomography (CT), were used in several cases:

• Mildly increased radioisotope update was noted in one of two horses that underwent a bone scan;
• CT revealed hypertrophy of the hyoid bone in two cases;
• In one of the two CT-scanned horses, one fracture of the stylohyoid bone was identified and two fractures of the bone were identified in the other case.

Palus explained that five of the seven study horses underwent surgery to correct the problem; in all instances, veterinarians chose a ceratohyoidectomy (complete removal of the ceratohyoid bone). He said the team did not identify any major post-operative complications, and two horses enjoyed complete resolution of neurologic signs. Two owners reported improvement after surgery, but noted that some neurologic signs remained two months after surgery. Follow up information was not available for the last horse treated surgically.

The final two horses were treated medically, Palus said, noting that both horses only displayed mild signs of THO at presentation. He said that one of the two was treated with doxycycline for six weeks and the other received phenylbutazone for six weeks. The phenylbutazone-treated made a full recovery and returned to work slowly, while the doxycycline-treated horse showed improvement but retained neurologic deficits at five months following treatment.

"This small series demonstrates the spectrum of neurologic signs that can occur with the condition," Palus said in closing. "The prognosis is generally good and the ceratohyoidectomy should be considered in horses that do not respond to conservative medical treatment."

Study co-author Celia M. Marr BVMS, MVM, PhD, Dipl. EIM, ECEIM, MRCVS, an associate at Rossdales Veterinary Surgeons in Newmarket, England, noted it's crucial for horse owners to identify possible THO early and have the horse evaluated by a veterinarian: "It is really important that THO is detected as early as possible. If it can be diagnosed and treated before the bone fractures, the debilitating neurological damage that results when the bone breaks can be avoided.

"Unfortunately in many (cases), the horse shows no clinical signs until fracture occurs," she continued." Others will have subtle signs such as mild pain associated with eating and drinking, abnormal head carriage when being ridden or being ridden."

In the mean time, researchers will continue trying to determine THO's cause, be it "wear and tear leading to arthritis," infection, or something entirely different, Marr said. "Unraveling this would be very useful as it might lead to more effective treatments," she concluded.

The study, "Retrospective study of neurological signs and management of seven English horses with temperohyoid osteoarthropathy," appeared in Equine Veterinary Education in August 2012. The abstract is available online.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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