Temporohyoid osteoarthropathy (THO) is a progressive syndrome that describes a primary degenerative joint disease of the temporohyoid joint, ultimately leading to fusion between the skull and hyoid apparatus, stricture of the external ear canal, and obliteration of the lumen of the tympanic bulla.
Horses from 6 months to older than 20 years of age have been affected with a mean age of around 10 years. About half of the reported cases are in Quarter Horse and Quarter Horse–related breeds. In most affected horses both temporohyoid joints are involved, although typically one side is worse than the other such that signs tend to be on one side only. It is possible for horses to present with disease on both sides. Deafness can also occur.
Fusion of the THJ causes disruption to the normally well-coordinated movements of the larynx, tongue and hyoid apparatus. The fused joint is placed under stress during movements of the head and neck, chewing feed, swallowing and vocalization. Placement of a mouth gag for dentistry or passing a nasogastric (stomach) tube can also put stress on the fused joint. Unfortunately these can singly or collectively lead to fractures of the stylohyoid bone, or more commonly the petrous temporal bone. The facial nerve (cranial nerve VII) and vestibulocochlear nerve (CN VIII) run within the petrous temporal bone and can be impinged on by bony proliferation or damaged by acute fracture and haemorrhage. There is also the possibility of infection around the brainstem after fracture from contamination via the middle or inner ear.
Signs
The earliest signs of THO can include pain with reluctance to chew and dropping of feed, head shaking, ear rubbing, heightened sensitivity when the base of the ear is touched, and reluctance to take a bit. There is a sudden onset of neurological signs after fracture. In some horses there are no abnormalities until this occurs. Almost all horses will show damage to the facial nerve, including drooping of the lower lip, upper eyelid, and ear on the affected side. The muzzle is pulled towards the unaffected side. Weakness of the buccinator muscle, which runs from the level of the canine teeth past the lower premolars and molars, cases retention of feed in the buccal pouch and dropping of feed (‘quidding’). Damage to the lacrimal branch of the facial nerve leads to reduction in production of tears. The inability to fully close the eyelids, coupled with reduced tear production, quickly leads to corneal ulcers. These are often missed early because the common signs of corneal ulcer disease (excessive tearing and squinting) are missing because of the nerve damage.
Damage to the vestibulocochlear nerve (CN VIII) leads to head tilt, neck turn, body lean, and tight circling, all towards the affected side. There is an ataxic gait without limb weakness. These signs may be revealed or exacerbated by blindfolding. In the acute stages, there may be horizontal nystagmus, with the fast phase away from the side of the lesion.
Diagnosis
The diagnosis is suspected when there is a sudden onset of signs referable to damage to cranial nerves V and VIII. The easiest diagnostic test is direct endoscopic examination of contour of the temporohyoid joint from within the guttural pouches. Plain radiographs or CT imaging provide more detail, including changes not seen on endoscopy.
Treatment and Outcome
Treatment can include medical and surgical options. Medical therapy can include non-steroidal anti-inflammatory drugs and antibiotics. Temporary suturing of the eyelids (tarsorrhaphy) is used to protect the eye along with topical antibiotics and artificial tears. Return of eyelid and lacrimal function can take many months. The surgical option is a procedure involves the removal of the ceratohyoid bone on the affected side. Most horses will return to athletic activity after weeks to months of convalescence. The surgery is recommended to prevent re-fracture of the fused temporohyoid joint.
Tags: Neurology