Temporohyoid Osteoarthropathy

This graphic shows the anatomy of the hyoid apparatus and the temporohyoid joint. Note the ceratohyoid bone at the bottom of the skull. This is the bone that is removed to aid in the petrous temporal bone healing. The temporohyoid joint is circled in red. (Photo courtesy of Science Direct)

By Lauren Lamb, DVM

Temporohyoid osteoarthropathy, also known as THO, is a neurologic disease seen in horses. It is a rare disease, but when diagnosed and treated early in the disease process, a horse can make a full recovery. THO is caused by arthritis, inflammation or infection of the temporohyoid joint.

The temporohyoid joint is made up of the stylohyoid bone and the petrous temporal bone. The petrous temporal bone is on the base of the skull within the guttural pouch. The stylohyoid bone is connected to the hyoid apparatus, which is a series of bone in the horse’s throatlatch that suspend the larynx (voice box) and the tongue from the base of the skull.

The temporohyoid joint has a significant amount of movement when the horse chews or moves its tongue. Two cranial nerves run through the petrous temporal bone, cranial nerve seven and eight. The facial nerve (cranial nerve seven) is responsible for movement of the lips, ears, blinking of the eyelids and production of tears.

The vestibulocochlear nerve (cranial nerve eight) is responsible for helping the horse keep its balance, walk in a straight line and hold its head in a normal position. The exact cause for THO is unknown. One possible theory is inflammation or infection of the THO joint, which will cause degeneration of the joint.

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