What are the causes of Bell's palsy?

Causes of Bell's Palsy

Bell's palsy is an idiopathic condition with no definitively identified cause, though viral etiology (particularly herpes simplex virus reactivation) is strongly suspected as the primary mechanism leading to facial nerve inflammation and edema. 1, 2

Primary Suspected Mechanisms

• Viral infection: The most widely accepted theory

  • Herpes simplex virus type 1 (HSV-1) reactivation within the geniculate ganglion 1, 3
  • Varicella zoster virus (VZV) reactivation (when without skin lesions, may appear indistinguishable from Bell's palsy) 4
  • Viral inflammation leads to facial nerve swelling within the narrow facial canal 1

• Immune-mediated mechanisms:

  • Cell-mediated autoimmune reaction against myelin basic protein 5
  • Possibly a mononeuritic variant of Guillain-Barré syndrome with cell-mediated immunity against peripheral nerve myelin antigens 5
  • A viral infection may trigger this autoimmune response 5

• Ischemic compression:

  • Inflammation causes swelling of the facial nerve within its bony canal 1, 6
  • This leads to nerve compression and temporary or permanent nerve damage 1

Risk Factors and Associated Conditions

Bell's palsy occurs more frequently in certain populations:

• Age: Most common in people 15-45 years old 1, 6
• Pregnancy 1, 6
• Diabetes mellitus 1, 6, 3
• Upper respiratory infections 1, 6
• Compromised immune systems 1
• Severe preeclampsia 1

Less Common Theories

• Bacterial infection: Some cases show increased neutrophils rather than lymphocytes, and some patients respond to antibiotics, suggesting possible bacterial etiology in certain cases 7

Important Clinical Considerations

• Bell's palsy is a diagnosis of exclusion 1, 2

• Other conditions that can cause facial paralysis must be ruled out:

  • Stroke
  • Brain tumors
  • Tumors of the parotid gland or infratemporal fossa
  • Cancer involving the facial nerve
  • Systemic and infectious diseases (including herpes zoster, sarcoidosis, Lyme disease)
  • Trauma or surgical injury 1, 2

• Bilateral Bell's palsy is rare and should prompt investigation for other causes 1

• Typical presentation includes:

  • Acute onset (<72 hours)
  • Unilateral facial weakness or paralysis
  • May include ipsilateral pain around ear or face
  • Possible associated symptoms: eye dryness, mouth dryness, taste disturbance, hyperacusis 1

Prognosis

• 70-94% of patients recover completely without treatment, depending on severity 2
• Up to 30% of patients with complete paralysis may not recover completely without treatment 2
• Early treatment with corticosteroids within 72 hours significantly improves recovery rates 2

Understanding the suspected viral and immune-mediated mechanisms has important treatment implications, supporting the use of both corticosteroids (for inflammation) and potentially antiviral medications (for viral replication) in the early management of Bell's palsy.