Etiology

• Facial nerve injury occurs secondary to:
  EitherTrauma.
  OrCompression.
  OrInflammation of the nerve as it courses through the middle ear, the dorsal aspect of the guttural pouch, the ventral ramus of the mandible, and ventral to the facial crest.
• Proximal facial nerve or facial nuclei injuries may be caused by equine protozoal myelitis, aberrant parasite migration, and cranial trauma.
• Fractures of the petrous temporal bone or hemorrhage into the middle ear, otitis media/interna, arthritis of the temporohyoid joint, guttural pouch mycosis, parotid lymph node abscessation, or fractures of the vertical ramus of the mandibular can also lead to proximal facial nerve injury.
• Distal facial nerve injury may be a result of blunt trauma to the side of the face, tight halters, and recumbency from anesthetic procedures.

Pathophysiology

• The facial nerve provides motor function to the muscles of facial expression, ie NOT the masster muscles.
• Signs are due to alteration or loss of motor function to muscles of facial expression.
• The facial nerve can be injured to various degrees of severity: neurapraxia is loss of function only; axonotmesis is loss of axons; neurotmesis is severance of axons and their nerve sheaths.

• Damage:
  EitherAt the base of brain.
  OrGuttural pouch.
  OrHyoid bone.
  OrMandible.
• Injury results in a loss of muscle tone on the affected side.
• The loss of function may affect the auricular muscles, the orbicularis occuli muscle, the buccal muscles, or the muscles of the upper lip, nostrils, lower lip or chin, depending on the location of the injury.
• Closed trauma to the side of the face usually results in degrees of neurapraxia and axonotmesis.
• Neurotmesis occur with open wounds to the side of the face and with fractured temporal bones.

Timecourse

• Function normally returns within 14 days, and often does so abruptly.

Axonotmesis

• Requires that the damaged axons regrow down the Schwann cell sheaths.
• It can take 6 months for recovery from this type of injury.
• The quality of recovery is never 100%.
• When no skin is lacerated, functional recovery usually is obtained.

Neurotmesis

• Depending on alignment of the damaged nerve fibers and depending on the degree of local tissue reaction, a proportion of the damaged fibers may regrow to their respective muscles of facial pression in 6-12 months; however this does not result in full return of function.
• Atrophy of the masseter can seriously interfere with food ingestion   →   recovery is signaled by a gradual improvement in the animal's ability to feed.