THO (Temporohyoid Osteoarthropathy), or inner ear disease, is the fusion of the stylohyoid and temporal bones located between the larynx and base of the ear. The bone remodeling and fusion is thought to be the result of degenerative joint disease (arthritis) and inflammation.
Tongue movements like chewing, swallowing, and whinnying can cause fractures or inflammation in the joint. These irritations can compromise the nerves at the base of the skull and a horse can quickly show signs of nurologic disease. A horse may suffer facial paralyses, head shaking, and present with a head tilt. In severe cases they may be unable to stand.
THO is relatively uncommon and symptoms can mimic EPM. If your horse is negative for EPM a guttural pouch endoscopy can confirm a THO diagnosis.
Cases of THO that do not respond to conservative treatments are candidates for a ceratohyoidectomy, removal of the ceratohyoid bone. Prognosis with a ceratohyoidectomy is good, however, some horses have permanent defects. In some cases clinical signs can take a year to resolve.