What are the causes of Bell's palsy?

Causes of Bell's Palsy

Bell's palsy has no identifiable cause by definition—it is a diagnosis of exclusion made only after ruling out all other potential etiologies of acute unilateral facial nerve paralysis. 1, 2

Defining Bell's Palsy as Idiopathic

• The American Academy of Otolaryngology-Head and Neck Surgery explicitly states that currently, no cause for Bell's palsy has been identified, and the condition is diagnosed only when no other medical etiology can be found. 1, 2
• Bell's palsy is defined as acute unilateral facial nerve paresis or paralysis with onset in less than 72 hours without an identifiable cause—making it fundamentally a diagnosis of exclusion. 1, 3

Suspected Pathophysiologic Mechanisms (Not Proven Causes)

While the exact mechanism remains unknown, the American Academy of Otolaryngology-Head and Neck Surgery notes that a viral etiology is suspected but unproven. 1

Viral Reactivation Hypothesis

• Herpes simplex virus (HSV) reactivation within the geniculate ganglion has been proposed as a trigger, followed by inflammation and nerve compression in the narrow temporal bone canal. 1, 4, 5
• Research has detected HSV-1 DNA in only 13% of Bell's palsy patients within 48 hours of onset—a rate not significantly different from healthy controls—calling into question its causative role. 6
• Varicella-zoster virus (VZV) was detected in only 3% of early Bell's palsy cases, with no significant difference from controls. 6
• Human herpesvirus-6 (HHV-6) DNA was found in 61% of Bell's palsy patients with significantly higher viral loads than controls, suggesting a possible association, though causation remains unproven. 6

Autoimmune Hypothesis

• A cell-mediated autoimmune mechanism against myelin basic protein has been proposed, with some authors suggesting Bell's palsy may represent a mononeuritic variant of Guillain-Barré syndrome. 7
• This theory posits that viral infection or reactivation may provoke an autoimmune reaction against peripheral nerve myelin, leading to demyelination of the facial nerve. 7

Bacterial Infection Hypothesis

• Some clinical observations have noted increased neutrophils (rather than lymphocytes) in certain Bell's palsy patients, with reported improvement following antibiotic treatment—suggesting bacterial infection may play a role in select cases. 8

Conditions That Must Be Excluded Before Diagnosing Bell's Palsy

The American Academy of Otolaryngology-Head and Neck Surgery mandates exclusion of the following identifiable causes: 1, 2

Vascular Causes

• Stroke (distinguished by forehead sparing and additional neurologic deficits such as dizziness, dysphagia, diplopia, or limb weakness) 1, 2

Neoplastic Causes

• Brain tumors 1, 2
• Parotid gland or infratemporal fossa tumors 1, 2
• Cancer involving the facial nerve 1, 2, 9

Infectious Causes

• Herpes zoster (Ramsay Hunt syndrome) 1, 2, 9
• Lyme disease (especially in endemic areas; can cause bilateral facial palsy) 1, 2, 9
• Sarcoidosis (granulomatous inflammation of the facial nerve) 1, 2, 9

Traumatic Causes

• Temporal bone fractures 1, 2
• Surgical injury to the facial nerve 1

Systemic Risk Factors (Not Causes, But Associations)

• Diabetes mellitus increases susceptibility through ischemic and neuropathic mechanisms. 1, 9
• Pregnancy and preeclampsia are associated with higher incidence. 1, 6
• Hypertension and compromised immune systems are risk factors. 1, 6
• Obesity elevates risk. 1

Critical Clinical Distinctions

• Bilateral Bell's palsy is extremely rare and should immediately trigger investigation for Guillain-Barré syndrome, Lyme disease, or sarcoidosis rather than accepting it as idiopathic. 1, 2, 9
• Recurrent facial paralysis on the same side is not Bell's palsy and requires imaging (MRI) to exclude tumor or other structural lesions. 2, 9
• Involvement of any other cranial nerve excludes Bell's palsy and indicates central pathology or skull base disease. 1, 2

Common Pitfalls

• Accepting Bell's palsy as a diagnosis without thoroughly excluding stroke, tumor, Lyme disease, or other identifiable causes can lead to missed serious pathology. 1, 2
• Ordering extensive viral serologies or imaging in typical presentations delays treatment beyond the critical 72-hour window without improving outcomes. 1, 3
• Attributing recurrent episodes to "idiopathic Bell's palsy" when recurrence mandates investigation for underlying systemic disease or structural lesions. 9