The temporohyoid joint is the joint between the skull and the hyoid apparatus, which is a bony structure that supports the base of the tongue.

For a variety of reasons, arthritis of this joint can develop, and the joint can ultimately become rigid. Stress on the long lever arm of the hyoid bones from tongue movement puts stress on the joint, which does not move properly. Ultimately that can result in a fracture of the skull that breaks into the brain case.

Depending on the specific nature of the fracture, there is damage or irritation to several vital cranial nerves in this area - the 7th (Facial Nerve) and 8th (Vestibulo-cochlear) nerves. Damage to these two nerves causes the signs of facial nerve paralysis and vestibular dysfunction respectively (head tilt and abnormal eye movements).

THO is a poorly understood condition that may result from middle ear infection, trauma or simply may be a degenerative or arthritic condition of the joint. It is generally a disease of middle aged horses and is probably more common in Quarter Horses and crosses. As horses age, we know that there is degeneration of this joint that looks very much like the condition seen in horses affected by this syndrome.

Early signs of the condition include head shaking, head tilt, resistance to movement of head and neck. Once there is fracture of the area or advanced arthritic change in the joint, the most common signs include facial and ear droop on the affected side, head tilt to the affected side, wobbliness and loss of balance. In addition there may be abnormal eye movement (nystagmus). In rare cases, affected horses lose the ability to swallow.

One other important consequence of damage to the 7th nerve is "Dry Eye" - loss of the lacrimal gland's ability to produce tears. This combined with an inability to close the affected eye results in exposure damage to the cornea (the clear front part of the eye). This can be serious, and can result in blindness if not aggressively treated.

Diagnosis is with endoscopy of the guttural pouches, and radiography. More detailed information can be obtained via a CT scan of the head.

Treatment usually involves medical management. Steroids, NSAIDs and supportive nursing care are a critical part of managing the affected horse. Surgery can be helpful in some cases.