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Bell's Palsy

Summary

  • Acute, unilateral facial nerve paralysis of unknown etiology
  • Characterised by sudden onset of facial weakness, typically affecting one side of the face
  • Diagnosis primarily clinical, with imaging used to rule out other causes1

Pathophysiology

  • Exact cause unknown, but believed to involve inflammation of the facial nerve (CN VII)
  • Possible triggers include:
    • Viral infections (e.g., herpes simplex virus, varicella-zoster virus)
    • Immune-mediated processes
    • Vascular ischaemia
  • Inflammation leads to compression and oedema of the facial nerve within the facial canal

Demographics

  • Annual incidence: 15-30 cases per 100,000 population
  • Affects all age groups, but peak incidence in 15-45 years
  • Slightly higher prevalence in:
    • Pregnant women
    • Patients with diabetes mellitus
    • Individuals with upper respiratory tract infections

Diagnosis

  • Primarily clinical, based on:
    • Sudden onset of unilateral facial weakness
    • Inability to close eye or wrinkle forehead on affected side
    • Drooping of corner of mouth
  • Physical examination:
    • House-Brackmann scale to grade facial nerve function
    • Assessment of taste and lacrimation
  • Exclusion of other causes (e.g., stroke, tumour) through history and examination

Imaging

  • Not routinely required for diagnosis
  • May be used to rule out other causes or in cases of atypical presentation
  • Modalities:
    • MRI:
    • T1-weighted with gadolinium: enhancement of the facial nerve
    • FLAIR: hyperintensity of the facial nerve
    • CT:
    • Limited role in acute setting
    • May show bony erosion in chronic cases
  • Findings:
    • Enhancement and swelling of the facial nerve, particularly in the labyrinthine and tympanic segments
    • Normal brain parenchyma

panels-1

  • 50-year-old patient presented with right sided facial weakness affecting the forehead.
  • MRI showed a FLAIR hyperintensity, mild thickening, and pathological enhancement (labyrinthine, tympanic and mastoid segments; red arrow) of the right facial nerve and a normal left facial nerve (blue arrow).

Treatment

  • Early corticosteroids (± antiviral) and eye protection
  • Most recover completely within 3–6 months

Differential diagnosis

Imaging differential Differentiating feature
Ramsay Hunt syndrome Enhancement of CN VII and VIII in the internal auditory canal, often with labyrinthine enhancement
Facial schwannoma Focal enhancing, expansile mass along the nerve rather than smooth linear enhancement
Perineural tumour spread (parotid malignancy) Thickened enhancing CN VII extending from a parotid/stylomastoid mass, widening the stylomastoid foramen
Leptomeningeal disease Additional cranial nerve/pial enhancement

Mild geniculate and tympanic-segment enhancement can be normal; asymmetric labyrinthine, canalicular or fundal enhancement suggests pathology.

  1. Zhang et al. The etiology of Bell's palsy: a review. 2020. Journal of neurology - Open in new tab. ↩