Temporohyoid Osteoarthropathy (THO)

What is temporohyoid osteoarthropathy?

A progressive syndrome, temporohyoid osteoarthopathy (THO) results in bone thickening and the eventual fusion of the temporohyoid joint (stylohyoid and petrous temporal bones) to the skull. The hyoid apparatus connects to the tongue and larynx, so this fusion can result in reduced mobility of these structures, which can be painful for affected horses. Once fused, actions such as swallowing and chewing can cause fractures, potentially resulting in nerve deficits.

Although THO most commonly affects adult horses, it can occur at any age and there are no known sex or breed predispositions. The exact cause of THO is not well understood; suggested causes include infection, inflammation, trauma, and osteoarthritis.

What are the clinical signs of temporohyoid osteoarthropathy?

Early clinical signs of THO can include head shaking/tossing, difficulty chewing food, resistance to the bit, behavioral problems, and ear rubbing. Neurological signs may develop as the disease progresses. These can include loss of balance, head tilt, and involuntary movement of the eyes (nystagmus). Signs of facial nerve paralysis may include deviation of the muzzle to one side, a droopy ear, and inability to close the eyelids. Damage to nerves that affect the eye can lead to decreased tear production and abnormal blinking, which can cause significant ulceration of the cornea. The first and most common nerve deficit is hearing loss, which might go unnoticed if the horse is partially deaf. A hearing test called brainstem auditory evoked response (BAER) will detect if hearing loss is partial, complete, affecting one ear or both.

How is temporohyoid osteoarthropathy diagnosed?

Diagnosis of THO can be made through imaging, including x-rays, CT, and MRI. Endoscopy may also be utilized to examine the temporohyoid joint. Some horses may have both sides affected, even if only one side is clinically apparent.

How is temporohyoid osteoarthropathy treated?

Treatment for THO can include medical and surgical management. Medically, inflammation and pain can be reduced through the use of non-steroidal anti-inflammatory drugs (NSAIDs), corticosteroids, gabapentin, and antimicrobials if infection is present or suspected.

Surgically, there are two treatments intended to reduce stress on the hyoid apparatus, thereby reducing the risk of fracture - a partial stylohyoidectomy and a ceratohyoidectomy. A partial stylohoidectomy removes part of the stylohyoid bone. A certohyoidectomy removes the ceratohyoid bone. Complications of partial stylohoidectomies, including damage to the artery in the tongue, as well as damage to facial nerves, have been reported. The complication rate is lower and reported outcomes as better with ceratohyoidecomies, and this is currently the recommended treatment.

What is the prognosis for temporohyoid osteoarthropathy?

The prognosis for THO depends on the severity of clinical signs at the time the horse is diagnosed. The prognosis is good to excellent for horses treated surgically, and many return to performance. The prognosis is fair for horses that are only managed medically. Although improvements may be seen within one to two months, rehabilitation can take up to two years. Neurological signs may persist in some cases and hearing loss is permanent. Early diagnosis and treatment are key to a positive prognosis.

How can temporohyoid osteoarthropathy be prevented?

Since the cause of THO is currently unknown, preventative measures have not been identified.

For more information

Tanner, J., Spriet, M., Espinosa-Mur, P., Estell, K.E., Aleman, M. 2019. The prevalence of temporal bone fractures is high in horses with severe temporohyoid osteoarthropathy. Vet Radiol Ultrasound 60(2): 159-166.

Espinosa, P., Nieto, J.E., Estell, K.E., Kass, P.H., Aleman, M. 2017. Outcomes after medical and surgical interventions in horses with temporohyoid osteoarthropathy. Equine Vet J 49(6): 770-775.

Aleman, M., Spriet, M., Williams, D.C., Nieto, J.E. 2016. Neurologic Deficits Including Auditory Loss and Recovery of Function in Horses with Temporohyoid Osteoarthropathy. J Vet Intern Med 30(1): 282-8.

Hilton, H., Puchalski, S.M., Aleman, M. 2009. The computed tomographic appearance of equine temporohyoid osteoarthropathy. Vet Radiol Ultrasound 50(2): 151-6.

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