What is Temporohyoid osteoarthropathy (THO) in horses? Signs, diagnosis, treatment, and recovery
When a horse suddenly begins shaking their head or abnormally chewing, it can be easy to dismiss. Learn more about the signs, treatment and recovery process for a serious equine disease called temporohyoid osteoarthropathy (THO).
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What is temporohyoid osteoarthropathy (THO)?
Temporohyoid osteoarthropathy (THO) is characterized by bony proliferation and eventual fusion of the affected temporohyoid joint. This is the small connection where the hyoid apparatus (the bones supporting the tongue and larynx) attaches to the base of the skull.
What causes THO in horses?
The cause remains unclear but is speculated to include primary degenerative joint disease, inflammation or infection of the middle or inner ear, and infection of the guttural pouch, which is an air-filled sac located beneath each ear.
What are the signs of THO?
Clinical signs include ear sensitivity to touch, head tossing, ear rubbing, behavioral changes, and changes in chewing. Most horses with THO also develop neurologic signs involving the head. Horses can develop ear droop, muzzle deviation, decreased upper eyelid function, and corneal ulceration, as well as a head tilt, circling, ataxia, and abnormal rapid eye movements. It is possible that neurologic deficits can occur acutely and severely, and in some cases, this leads to the horse falling over. While most horses with THO do not appear painful, altered sensation can lead to signs of irritation (head shaking, ear rubbing, abnormal chewing).
How is THO diagnosed?
The easiest and a very reliable diagnostic method is endoscopy of the upper airway, specifically into the horse’s guttural pouch. A head CT, however, is the gold standard for diagnosis since it provides much more detailed anatomy of all structures involved and facilitates surgical planning.
What are the treatment options for THO?
Surgical removal of a portion of the tongue bone system alleviates pressure on the degenerated, fused, or fractured joint. When the pressure is removed, any soft tissue swelling can be reduced and bony remodeling can occur leading to removal of inflammation around affected nerves.
What is the prognosis for horses with THO?
With surgery, the prognosis for recovery from neurologic disease and other clinical signs of THO is good. In most (~70%) affected horses, nerve dysfunction resolves within six to 12 months of surgery, and sometimes faster.
Can horses return to riding after THO treatment?
The short answer is yes. However, before riding, a veterinarian comfortable evaluating neurologic function and return of function should see the horse, because balance can take longer to recover and may require special testing.
CSU Veterinary Health System equine specialists have treated several horses that have returned to athletic function, including careers in eventing, following surgery to treat THO.
If you have concerns about your horse, please contact CSU Veterinary Health System equine services to schedule an appointment at (970) 297-5000, option 3.