What is the cause of Bell's palsy contraction?

How Bell's Palsy is Contracted

The Exact Cause Remains Unknown

The American Academy of Otolaryngology-Head and Neck Surgery explicitly states that currently, no cause for Bell's palsy has been identified, making it a diagnosis of exclusion when no other medical etiology can be found. 1

Proposed Mechanisms and Theories

Inflammatory-Compression Pathway (Most Accepted Mechanism)

• Bell's palsy occurs due to inflammation and edema of the facial nerve, causing compression of the nerve within the narrow temporal bone canal of the temporal bone. 2
• This neural edema develops within 72 hours, leading to mechanical compression that disrupts nerve impulse conduction to facial muscles and associated structures. 2, 3
• The compression may result in temporary or permanent nerve damage depending on severity and duration. 2

Viral Etiology Hypothesis (Suspected but Unproven)

• Herpes simplex virus type 1 (HSV-1) is the most commonly proposed infectious agent, with the theory suggesting viral reactivation within the geniculate ganglion triggers the inflammatory cascade. 3, 4
• Varicella zoster virus (VZV) can cause a similar presentation (Ramsay Hunt syndrome), though this is technically a distinct entity from Bell's palsy. 5
• The viral theory is supported by the association with upper respiratory tract infections as a risk factor. 2, 3
• However, the viral etiology remains unproven and controversial, with laboratory and clinical evidence being inconclusive. 4

Alternative Bacterial Theory (Minority View)

• Some evidence suggests bacterial infection may play a role in certain cases, based on neutrophil elevation rather than lymphocytosis in blood counts and response to antibiotic therapy. 6
• This remains a minority hypothesis and is not widely accepted. 6

Ischemic Component

• Ischemic compression of the seventh cranial nerve may contribute, particularly in patients with diabetes mellitus who have both ischemic and neuropathic vulnerability. 2, 3, 7

Risk Factors That Increase Susceptibility

While the exact cause is unknown, certain conditions increase the likelihood of developing Bell's palsy:

• Diabetes mellitus increases risk through possible ischemic and neuropathic mechanisms. 2, 3
• Pregnancy, especially in the third trimester, increases susceptibility. 2, 3
• Upper respiratory tract infections increase risk, supporting the viral hypothesis. 2, 3
• Compromised immune systems increase vulnerability, consistent with an inflammatory or infectious mechanism. 2, 3
• Age 15-45 years represents peak incidence, though no age is immune. 2, 3, 8
• Hypertension and obesity are additional risk factors. 8

Clinical Implications of Unknown Etiology

• Because Bell's palsy is idiopathic, it is diagnosed by excluding all other identifiable causes of facial nerve paralysis including stroke, tumors, Lyme disease, sarcoidosis, trauma, and other infectious or systemic diseases. 1, 3
• The unclear etiology affects treatment approaches, which remain largely symptomatic rather than targeting a specific causative agent. 8, 6
• Despite uncertain etiology, corticosteroids are recommended as they improve outcomes, suggesting an inflammatory component regardless of the trigger. 1
• The addition of antiviral agents to steroids remains controversial, with some evidence suggesting modest benefit but no definitive proof. 1, 4, 5

Important Caveats

• Bell's palsy is not "contracted" in the traditional infectious disease sense—it is not contagious and cannot be transmitted from person to person. 1
• The condition represents an acute inflammatory neuropathy of uncertain trigger rather than a directly acquired infection. 2, 3
• Bilateral presentation is extremely rare and should immediately raise suspicion for alternative diagnoses such as Guillain-Barré syndrome or sarcoidosis rather than true Bell's palsy. 1, 3