What are the causes of Bell's palsy?

Causes of Bell's Palsy

Primary Etiology

Bell's palsy is idiopathic by definition—meaning it has no identifiable cause—but is strongly suspected to result from viral reactivation (particularly herpes simplex virus type 1) triggering inflammation and edema of the facial nerve within the narrow temporal bone canal. 1, 2

The diagnosis of Bell's palsy can only be made after systematically excluding all identifiable causes of facial nerve paralysis through careful history and physical examination. 1, 2

Suspected Pathophysiologic Mechanisms

While Bell's palsy is defined as having no identifiable cause, several mechanisms are strongly suspected based on clinical and laboratory evidence:

Viral Reactivation Theory

• Herpes simplex virus type 1 (HSV-1) is the most commonly implicated pathogen, with reactivation from the geniculate ganglion leading to inflammation and nerve compression within the temporal bone canal. 2, 3, 4
• Herpes zoster virus reactivation has also been proposed as a trigger for the inflammatory cascade. 3, 4
• The viral hypothesis is supported by the frequent association with preceding upper respiratory tract infections. 2

Autoimmune Mechanism

• Bell's palsy may represent an autoimmune demyelinating cranial neuritis, potentially a mononeuritic variant of Guillain-Barré syndrome with cell-mediated immunity against peripheral nerve myelin antigens. 3
• A viral infection or reactivation may provoke an autoimmune reaction against peripheral nerve myelin components, leading to demyelination of the facial nerve. 3

Inflammatory Compression

• Neural edema develops within 72 hours of symptom onset, causing mechanical compression within the narrow temporal bone canal that disrupts nerve impulse conduction. 2
• This compression can lead to temporary or permanent nerve damage if not treated promptly. 2

Critical Differential Diagnoses That Must Be Excluded

Before diagnosing Bell's palsy, you must systematically rule out the following identifiable causes: 1, 2

Infectious Causes

• Herpes zoster (Ramsay Hunt syndrome): Look for vesicles in the ear canal or on the palate, severe otalgia, and hearing loss. 5, 2
• Lyme disease: Obtain history of tick exposure, erythema migrans rash, and geographic risk factors (endemic areas). Can cause bilateral facial palsy. 5, 2
• Chronic infections: Consider in immunocompromised patients or those with systemic symptoms. 5

Structural Lesions

• Stroke (central facial weakness): Forehead function is preserved in central lesions; other neurologic deficits typically present. 2
• Brain tumors: Consider with gradual onset, other cranial nerve involvement, or headache. 5, 2
• Parotid gland or infratemporal fossa tumors: Palpate for masses; consider with slowly progressive symptoms. 5, 2
• Cancer involving the facial nerve: Suspect with recurrent paralysis or atypical features. 5, 2

Systemic Diseases

• Sarcoidosis: Can cause recurrent facial palsy; check for bilateral hilar lymphadenopathy, uveitis, or systemic symptoms. 5, 2
• Guillain-Barré syndrome: Bilateral facial palsy is extremely rare in true Bell's palsy and should immediately trigger investigation for GBS, along with ascending weakness and areflexia. 5, 2, 3
• Diabetes mellitus: Increases susceptibility through ischemic and neuropathic mechanisms; check glucose/HbA1c. 5, 2

Trauma

• Temporal bone fractures or direct facial trauma: Obtain history of recent head injury or surgery. 2

Risk Factors for Bell's Palsy

Once identifiable causes are excluded, the following increase risk for idiopathic Bell's palsy:

• Age 15-45 years: Peak incidence occurs in this age range. 2
• Diabetes mellitus: Increases risk through possible ischemic and neuropathic mechanisms. 2
• Upper respiratory tract infections: Recent viral illness supports the viral reactivation hypothesis. 2
• Pregnancy (especially third trimester): Significantly increased risk during late pregnancy. 2
• Compromised immune systems: Immunosuppression may facilitate viral reactivation. 2

Special Considerations for Recurrent Cases

When facial paralysis recurs, it is no longer considered idiopathic Bell's palsy—you must identify an underlying etiology: 5

• Order MRI with and without contrast to exclude structural lesions not present during the first episode. 5
• Test for Lyme disease (serology) based on geographic risk and exposure history. 5
• Evaluate for sarcoidosis with ACE levels and chest imaging if clinically suspected. 5
• Check glucose/HbA1c to assess for diabetes or worsening glycemic control. 5
• Exclude herpes zoster (Ramsay Hunt syndrome), which can present with recurrent episodes. 5

Important Clinical Caveats

• Bilateral facial palsy is not Bell's palsy: This presentation is extremely rare in idiopathic Bell's palsy and mandates immediate investigation for Guillain-Barré syndrome, Lyme disease, or sarcoidosis. 5, 2
• Recurrence changes the diagnosis: A second episode on the same side requires full workup for identifiable causes rather than accepting it as idiopathic. 5
• Isolated branch paralysis is atypical: Complete or near-complete unilateral facial weakness is expected; isolated branch involvement suggests an alternative diagnosis. 2