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Bell palsy

contributed by Dr Frank Gaillard on May 2, 2008

System: Central Nervous System

Bell's palsy is characterised by rapid onset facial nerve paralysis, often with resolution in 6 - 8 weeks. Long thought to be idiopathic, strong evidence is now present to implicate reactivation of herpes simplex virus with latent infection in the geniculate ganglion. As there are numerous causes of facial nerve palsy, many of which can be acute in onset, it is currently a diagnosis of exclusion supported by typically presentation. Other than rapid onset paralysis, up to 70% of patients will describe altered taste sensation in the week preceding paralysis, due to nervus intermedieus compression. It should be remembered that sensory nerves have thinner myelin and are more susceptible to dysfunction than motor nerves.

Prognosis

  • 80 - 90% full recovery
  • rest chronic facial paralysis
  • corneal drying is the most problematic complication

Risk factors for incomplete recovery:

  • older age of onset
  • complete paralysis

Radiographic features

MRI is not performed in all patients with a Bell's palsy, as this is not deemed cost effective. It is most useful in two situations:

  • decompressive surgery is being contemplated
  • atypical signs and symptoms (diagnosis in question)
    • slow progressive palsy
    • spasm preceding palsy
    • multiple cranial nerves involved
    • recurrent palsy
    • no recovery after 6 - 8 weeks

Enhancement of the nerve is not seen in all patients with Bell's palsy, reported variably between 57 - 100%. Typically long segments of the facial nerve enhance in a uniformly linear fashion. Nodularity should raise suspicion of a neoplastic cause. The facial nerve on either side of the geniculate ganglion is most frequently involved, from the distal IAM to the distal tympanic segment. The mastoid and extratemporal segments are less frequently involved.

Care should be taken to to mistake normal facial nerve enhancement on MRI for that seen in Bell's palsy. Focal enhancement in the most lateral aspect of the IAM is probably the most useful feature, and has been proposed as a marker of severity and prognosis.

Enhancement may persist well beyond clinical improvement (up to 1 year).

Etymology

Sir Charles Bell (Scottish anatomist, surgeon, and physiologist, 1774 -1842) [3]