Facial nerve neuropathy in Cats (Felis) | Vetlexicon
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Facial nerve neuropathy

ISSN 2398-2950

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Introduction

  • Uni or bilateral facial paralysis or paresis.
  • Cause: immune-mediated , inflammation, eg otitis media/interna, middle ear polyps, infection , myasthenia gravis, toxins , eg botulism , trauma, eg brain trauma, primary or metastatic brain tumor, middle ear tumor, rarely, idiopathic.
  • Signs: drooping and inability to move the ear and lip, drooling, widened palpebral fissure, absent abduction of nostril during inspiration, deviation of nose towards normal side, absent spontaneous or provoked blinking.
  • Treament: symptomatic for the facial nerve paralysis, specific treatment for the primary disease may reverse the facial neuropathy.
  • Prognosis: variable depending on inciting cause.

Presenting signs

  • Changed facial expression.
  • Widened palpebral fissure.
  • Absent spontaneous or provoked blinking.
  • Absent abduction of nostril during inspiration.
  • Deviation of the nose toward the normal side.
  • Food collecting in one side of mouth.
  • Dribbling saliva (sialosis).
  • Ear droop.
  • Lips retracted further than normal and nostril deviated to the affected side as a result of fibrosis with chronic denervation.
  • Keratoconjunctivitis sicca Eye: keratoconjunctivitis sicca and dry nose if concurrent involvement of parasympathetic supply of lacrimal and nasal glands respectively.

Pathogenesis

Etiology

Pathophysiology

  • Degeneration of facial nerve.
  • The facial canal that is adjacent to the tympanic cavity lacks a bony wall for a very short distance as it runs in the petrosal portion of the temporal bone. This leaves the facial nerve exposed to the cavity, and therefore to disease processes affecting the middle ear.
  • Inciting cause   →   axonal damage especially of large diameter myelinated fibers   →   loss of function   →   paresis/paralysis of facial muscles.

Diagnosis

Presenting problems

Client history

  • Head trauma.
  • Other concurrent neurological signs - peripheral or central.

Clinical signs

  • Paralyzed lip commissure.
  • Build-up of food in the cheek on the paralyzed side.
  • Dribbling.
  • Deviation of the nose away from the paralyzed side.
  • Deviation of the nose towards affected side with chronic disease (due to facial muscle contraction or fibrosis), or with hemifacial spasm.
  • Lips retracted further than normal and nostril deviated to the affected side as a result of fibrosis with chronic denervation.
  • Menace deficit with intact visual placing.
  • Absence of palpebral/corneal reflex (   →   corneal pathology).
  • Bilateral facial paralysis.
  • Drooping ear.
  • Facial spasm.
    Muscle twitching is either muscle or CNS in origin, so facial spasm is not usually a sign of facial neuropathy.
  • Dysfunction of the parasympathetic supply of lacrimal gland produces neurogenic keratoconjunctivitis sicca. This is mainly seen with lesions of the portion of the facial nerve located between the medulla and the middle ear. Lesions distal to the facial canal in the temporal bone will not affect the parasympathetic division of the facial nerve.

Diagnostic investigation

Other

  • Facial nerve biopsy Biopsy: nerve:
    • Active degeneration of large and small diameter myelinated fibers. Numerous macrophages and ovoids present. Schwann cell proliferation. Collateral sprouting. Various stages of remyelination.
  • Electromyography Electromyography:
    • Spontaneous denervation potentials in superficial facial muscles.
    • Stimulation of the facial nerve external to the stylomastoid foramen fails to cause muscle action potentials.
  • Schirmer tear test Schirmer tear test:
    • Dysfunction of the parasympathetic supply of lacrimal gland produces neurogenic keratoconjunctivitis sicca.
    • In normal cats, tear production ranges from 10-15 mm in 1 min.
  • CT  Computed tomography: head scan or MRI scan of skull  Magnetic resonance imaging: brain (middle ear) may reveal neoplasia within the middle ear, middle ear polyps, fluid within the affected tympanic bulla, or bulla osteomyelitis or thickening.
  • CT scan or MRI scan of brainstem may reveal focal neoplasia or inflammation ipsilaterally to facial neuropathy.
  • Otoscopic and pharyngeal examination under general anesthesia is necessary.
  • Swabs for cytology and culture (aerobic, fungal and yeast) from the middle ear cavity if tympanic membrane is ruptured.
  • Myringotomy Myringotomy with a 20-gauge spinal needle to obtain samples from the middle ear cavity for cytology and culture if tympanic membrane is intact, but bulging, or of an abnormal color.
  • Anti-acetylcholine receptor antibody titer to rule-out focal myasthenia gravis.
  • CSF analysis Cerebrospinal fluid: culture and sensitivity if CNS involvement is suspected.

Radiography

  • See radiography of the skull Radiography: skull (basic).
  • Examination of the tympanic bullae  Skull: normal tympanic bullae - rostro caudal open mouth view is important to identify any pathology in this area which may be involving the facial nerve eg otitis media  Skull: tympanic bulla disease - radiograph DV or neoplasia.

Confirmation of diagnosis

Discriminatory diagnostic features

  • History.
  • Clinical signs.

Definitive diagnostic features

Histopathology findings

  • Active degeneration of large and small diameter myelinated fibers.
  • Numerous macrophages and ovoids present.
  • Schwann cell proliferation.
  • Collateral sprouting.
  • Various stages of remyelination.

Treatment

Initial symptomatic treatment

Monitoring

  • Neurological signs.

Subsequent management

Treatment

Monitoring

  • Subsequent history and clinical signs.

Prevention

Prophylaxis

  • Care during bulla osteotomy and vertical canal ablation Ear: ablation - vertical canal.
  • Avoid long-term pressure on the side of the face during prolonged anesthesia.
  • Treat otitis externa Otitis externa  /media Otitis media/interna Otitis interna carefully and effectively.
  • Take action to reduce the risks of accidents, eg road traffic accidents.

Outcomes

Prognosis

  • Guarded.
  • Chronic lip paralysis may lead to permanent contracture.
  • Exposure keratitis can lead to loss of vision in the affected eye.
  • 20-47% of middle ear surgeries in cats result in permanent facial nerve paralysis.

Expected response to treatment

  • Improvement may take place in a few weeks or months, or not at all.
  • In general cats who present with facial nerve paralysis before surgery do not show improvement after the procedure.
  • Chronicity may result in muscle contracture and deform the facial expression.

Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Garosi L S, Lowrie M L, Swinbourne N F (2012) Neurologic manifestations of ear disease in dogs and cats. Vet Clin North Am Small Anim Pract 42 (6), 1143-1160 PubMed.
  • Bacon N J, Gilbert R L, Bostock D E et al (2003) Total ear canal ablation and bulla osetotomy in the cat: indications, morbidity and long-term survival. JSAP 44 (10), 430-434 PubMed.
  • Garosi L S, Dennis R, Schwarz T (2003) Review of diagnostic imaging of ear diseases in the dog and cat. Vet Radiol Ultrasound 44 (2), 137-146 PubMed.
  • Kern T J & Herb H N (1987) Facial neuropathy in dogs and cats: 95 cases (1975-1985). JAVMA 191 (12), 1604-9 PubMed.