Consider Temporal Nerve Problem in Neurologic Horse Diagnosis

When presented with a horse demonstrating facial nerve paralysis and/or a head tilt, neurologists say veterinarians should consider temporohyoid osteoarthropathy, a disorder of the hyoid apparatus (voice box) and associated structures. Although it is not the most commonly diagnosed neurologic condition in horses, it's an important and treatable condition that horse owners and their veterinarians should be aware of, according to Steve Reed, DVM, Dipl. ACVIM, and J. Brett Woodie, DVM, Dipl. ACVS, of Rood & Riddle Equine Hospital in Lexington, Ky.

The hyoid apparatus and larynx are suspended from the skull via the temporohyoid joint, which is located in the temporal region of the horse's head (the region between the larynx and the base of the ear). Temporohyoid osteoarthropathy is a bony proliferation (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.

"While the actual growth of the bones and loss of movement in the joint can be uncomfortable for affected horses, the bigger problem is that fracture of the bones of the hyoid apparatus can occur, which often damages the nearby facial, vagus, glossopharyngeal, vestibular cochlear, and hypoglossal nerves," Reed said.

Clinical signs in the early stages of the disease, prior to fracture of the bones of the hyoid apparatus, can include ear rubbing, head shaking and tossing, pain on manipulation of the ears and temporal region, bit avoidance, and a reluctance to eat due to pain while chewing.

In cases where the bones of the hyoid apparatus actually fracture and damage one or more cranial nerves, affected horses can present with unilateral or bilateral neurologic signs, including:

  • Facial paralysis;
  • Muzzle deviation;
  • Ear drooping;
  • Head tilt;
  • Corneal ulceration (secondary to eyelid dysfunction and decreased tear production); and
  • An unsteady gait or collapse (not loss of consciousness) due to acute onset nerve damage.

Clinical signs temporohyoid osteoarthropathy

A horse with left sided temperohyoid osteoarthropathy, showing left ear droop and muzzle deviation to the right.

According to Reed, two of the major concerns with this condition are that horses can either lose an eye due to damage to the nerve that controls eyelid movement and tear production, or the neurological defects can be severe enough that he horses are unsafe for athletic purposes.

According to Woodie, one of the best ways for veterinarians to diagnose this condition is by examining the guttural pouches with an endosocope.

"What we see is a thickening of the proximal (upper) part of the stylohyoid bone, which can be quite dramatic," said Woodie.

Both Woodie and Reed agreed that while medical treatments are available, aggressive surgical intervention should be the first choice, in order to avoid problems associated with nerve injury.

Woodie explained that of the two surgical options currently available, complete removal of the ceratohyoid bone (ceratohyoidectomy) is the "newer procedure on the block."

He noted, "Unlike the traditional approach of selectively removing part of the stylohyoid bone, the ceratohyoidectomy is less likely to accidentally injure blood vessels and nerves during the surgical procedure."Regardless, both procedures are effective. The most important factor for achieving a favorable outcome is to not "wait and see," they said.

"Early surgical intervention is imperative to avoid permanent and lifelong problems," emphasized Reed.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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