Causes of Bell's Palsy
Bell's palsy has no identifiable cause—it is by definition an idiopathic condition diagnosed only after excluding all other potential etiologies of facial nerve paralysis. 1
Primary Mechanism (Not a "Cause")
While the exact etiology remains unknown, the pathophysiologic mechanism involves:
- Inflammation and edema of the facial nerve within the narrow temporal bone canal, leading to nerve compression and subsequent temporary or permanent damage 1, 2
- Neural edema develops within 72 hours, causing mechanical compression that disrupts nerve impulse conduction 2
- A viral etiology is suspected but unproven, with herpes simplex virus type 1 being one proposed infectious agent 1, 3
Diagnostic Exclusion Criteria
Bell's palsy is fundamentally a diagnosis of exclusion—you must rule out identifiable causes before making this diagnosis 1, 4:
Conditions That Must Be Excluded:
- Stroke (central facial weakness sparing the forehead) 1, 2
- Brain tumors 1, 2
- Parotid gland or infratemporal fossa tumors 1, 2
- Cancer involving the facial nerve 1, 2
- Infectious diseases: Herpes zoster (Ramsay Hunt syndrome), Lyme disease 1, 2
- Systemic diseases: Sarcoidosis, Guillain-Barré syndrome (especially if bilateral) 1, 2
- Trauma or fractures 1
Risk Factors (Not Causes)
These factors increase susceptibility but do not cause Bell's palsy:
- Age 15-45 years (peak incidence) 1, 2, 4
- Diabetes mellitus (increases risk through possible ischemic and neuropathic mechanisms) 1, 2, 4, 3
- Upper respiratory tract infections (supports viral hypothesis) 1, 2, 4
- Pregnancy (especially third trimester) 1, 2, 4
- Compromised immune systems 1, 2, 4
- Hypertension and obesity 5
Proposed Etiologic Theories (Unproven)
Research suggests multiple potential contributing factors, though none are definitively established:
- Viral infection (herpes simplex virus type 1 most commonly implicated) 3, 6
- Ischemic compression of the seventh cranial nerve 5, 7, 6
- Immune-mediated inflammation 7, 6
- Cold stimulation responsivity 6
- Anatomical structure (narrow bony canal predisposing to compression) 6
Critical Clinical Caveat
Bilateral Bell's palsy 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, 2, 4. If you encounter bilateral facial weakness, you are likely dealing with a different condition entirely.