What causes Bell's palsy?

Causes of Bell's Palsy

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

Primary Suspected Cause

The most widely accepted theory regarding the cause of Bell's palsy is:

• Viral infection - particularly herpes simplex virus type 1 (HSV-1) reactivation within the geniculate ganglion 1
• The facial nerve becomes inflamed and swollen within the narrow facial canal in the temporal bone
• This swelling leads to compression of the nerve, resulting in temporary or permanent nerve damage 2

Pathophysiology

Bell's palsy involves:

1. Inflammation and edema of the facial nerve
2. Compression of the nerve within the narrow bony canal
3. Resulting paresis (weakness) or paralysis (complete loss of movement) of facial muscles 2

The facial nerve carries impulses to:

• Facial muscles
• Lacrimal glands
• Salivary glands
• Stapedius muscle
• Taste fibers from the anterior tongue
• General sensory fibers from the tympanic membrane and ear canal

This explains why patients may experience:

• Facial weakness/paralysis
• Eye or mouth dryness
• Taste disturbance or loss
• Hyperacusis (increased sensitivity to sound)
• Ipsilateral pain around the ear or face 2, 1

Risk Factors

Bell's palsy occurs more frequently in:

• People aged 15-45 years
• Patients with diabetes mellitus
• Individuals with upper respiratory infections
• Those with compromised immune systems
• Pregnant women 2, 1
• Patients with severe preeclampsia 1

Alternative Theories

While viral etiology is the most accepted theory, other potential causes have been proposed:

• Autoimmune mechanism - A cell-mediated autoimmune reaction against myelin basic protein, potentially making Bell's palsy a mononeuritic variant of Guillain-Barré syndrome 3
• Bacterial infection - Some cases show increased neutrophils rather than lymphocytes, and may respond to antibiotic treatment 4
• Ischemic compression - Vascular factors causing compression of the facial nerve 5

Important Clinical Considerations

• Bell's palsy is a diagnosis of exclusion
• Other conditions that can cause facial paralysis must be ruled out, including stroke, brain tumors, parotid tumors, and systemic/infectious diseases (zoster, sarcoidosis, Lyme disease) 2, 1
• Bilateral Bell's palsy is rare and should prompt investigation for other causes 2
• Bell's palsy has an acute onset (less than 72 hours) 2
• Approximately 70-94% of patients recover completely without treatment, though up to 30% may have incomplete recovery 1, 6

Treatment Implications

Understanding the viral etiology has led to treatment recommendations:

• Oral corticosteroids are strongly recommended within 72 hours of symptom onset 1
• Antiviral therapy (acyclovir or valacyclovir) may be offered as an adjunct to steroids, though evidence for additional benefit is mixed 1, 7
• Eye protection is crucial for preventing corneal damage in patients with impaired eye closure 1

The suspected viral cause and resulting inflammation pathway provides the rationale for the anti-inflammatory and antiviral treatment approach that has become standard care for Bell's palsy.