Causes of Bell's Palsy
The Fundamental Answer
Bell's palsy is, by definition, idiopathic—meaning it has no identifiable cause. 1 The condition is diagnosed only after all other potential causes of facial nerve paralysis have been excluded through clinical evaluation. 1, 2
Suspected Pathophysiologic Mechanisms
While the exact cause remains unknown, the American Academy of Otolaryngology-Head and Neck Surgery acknowledges that a viral etiology is suspected, though the precise mechanism is currently unproven. 1 The proposed pathophysiology involves:
- Facial nerve inflammation and edema within the narrow bony canal of the temporal bone, leading to nerve compression and potential temporary or permanent damage 1
- Possible viral reactivation, particularly herpes simplex virus (HSV-1) and varicella-zoster virus at the geniculate ganglion, though this remains unconfirmed 3, 4
- Potential autoimmune mechanisms, with some researchers suggesting Bell's palsy may represent a cell-mediated autoimmune demyelinating cranial neuritis, possibly a mononeuritic variant of Guillain-Barré syndrome 3
What Bell's Palsy Is NOT
The American Academy of Otolaryngology-Head and Neck Surgery explicitly states that Bell's palsy must be distinguished from other identifiable causes of facial paralysis, including: 1
- Stroke (central causes spare forehead function)
- Brain tumors
- Parotid gland or infratemporal fossa tumors
- Cancer involving the facial nerve
- Infectious diseases: herpes zoster (Ramsay Hunt syndrome), Lyme disease, sarcoidosis 1
- Trauma or fractures
- Post-surgical complications
Risk Factors for Development
While not "causes" per se, Bell's palsy occurs more commonly in: 1, 2
- Age 15 to 45 years (peak incidence)
- Diabetes mellitus
- Upper respiratory infections (recent or concurrent)
- Compromised immune systems
- Pregnancy
Clinical Implications
The idiopathic nature of Bell's palsy has important treatment implications. Since viral reactivation is suspected but unproven, corticosteroids remain the only definitively recommended treatment (addressing inflammation), while antiviral therapy may be offered as an adjunct but should never be used alone. 5, 6 The fact that corticosteroids work effectively suggests an inflammatory or possibly autoimmune component, though this does not establish causation. 3
The diagnosis requires rapid onset (<72 hours) of unilateral facial weakness without any other identifiable medical etiology. 1 If another cause is found, it is not Bell's palsy by definition—it is facial paralysis from that specific identified cause.