Vestibular disease may result from a number of infectious, traumatic and non-infectious conditions. Temporohyoid osteoarthropathy
and head trauma are the most common causes of vestibular nerve disease in horses. Other causes include equine protozoal myelitis
(EPM), neoplasia, brain abscess, West Nile, eastern and parasitic migration. Diagnostic tools include endoscopy of the guttural
pouches, radiographs, computed tomography, and assessment of spinal fluid. Individual cases will be presented and discussed
during the lecture.
Clinical signs of vestibular disease:
Head tilt: ventral deviation of the poll to the affected side (muzzle points away)
Nystagmus: With peripheral vestibular disease, the nystagmus is horizontal, and fast phase is away. With central vestibular
disease, the nystagmus can be horizontal, vertical or rotary, and the type of direction of nystagmus may change with head
Recumbency/leaning: affected horses may prefer to lie on the side of the lesion
Circling: toward the lesion. The body may be flexed, with a concavity toward the lesion (due to extensor hypotonia on the
affected side and mild extensor hypertonia on the contralateral side).
Ventrolateral strabismus: can be observed on the ipsilateral side. Best evaluated when head is elevated and extended. The
strabismus due to vestibular disease results from abnormal upper motor neuron input to the oculomotor nerve
Ataxia but no weakness: are common in horses with severe vestibular disease.
Facial nerve paralysis: frequently occurs with vestibular nerve disease due to the proximity of the facial nerve to the vestibular
nerve. Clinical signs of facial nerve paralysis include muzzle deviation away from the affected side, absent menace response,
absent palpebral response, ear droop, and decreased nostril flare. Corneal ulceration can occur to decreased ability to blink
and decreased tear production.
The most common cause of acute vestibular nerve disease in horses
Pathophysiology: Temporohyoid osteoarthropathy is a chronic boney proliferation of the temporohyoid joint (stylohyoid and
petrous temporal bones). The onset of neurologic dysfunction is acute. However the disease is actually due to a chronic boney
proliferative process of the stylohyoid and petrous temporal bones. It is not clear if the boney proliferation is due to trauma,
infection (otitis media) or nonseptic degenerative arthrosis. The proliferation results in ankylosis of the temporohyoid joint.
Onset of neurologic signs occurs when a subsequent fracture results.
Clinical signs: Acute onset of either unilateral facial and/or vestibular nerve disease. In a few cases, there is discharge
from the affected ear (blood, purulent exudate or spinal fluid). Dysphagia can result from pain from the fracture or damage
to the vagus and glossopharyngeal nerve, but is not a common occurrence. Some owners report head throwing, ear rubbing, and
bridling problems for several weeks or months prior to development of neurological signs.
Diagnosis: Endoscopic examination of the guttural pouches is usually the easiest way to confirm a diagnosis. Osseous proliferation
of the proximal end or the entire stylohyoid bone confirms the diagnosis. It is helpful to compare the both stylohyoid bones
to each other. Bilateral disease is reported. Skull radiographs (lateral and ventrodorsal) can reveal osseous proliferation
of the proximal stylohyoid bone and tympanic bulla. It may be difficult to adequately position the head in standing horses
with vestibular disease (due to a head tilt and ataxia). Computed tomography provides the best imaging of this area, but requires
Treatment: Most horses will respond to conservative medical management (broad spectrum antibiotics, non-steroidal drugs, and
if necessary treating corneal ulcers). The majority of horses will significantly improve, and can be used athletically. However,
most will still have mild facial or vestibular nerve disease. Acute death, meningitis, and seizures have been rarely reported.
Surgery: Removal of a portion of the stylohyoid bone has been done to prevent further fractures of the temporohyoid joint.
However, regrowth can occur. Alternatively a ceratohyoidectomy (removal of the entire ceratohyoid) is an easier surgery. The
best surgical candidates are horses with temporohyoid osteoathropathy that have not yet developed neurological signs. This
surgery has also been performed on horses with neurological disease.