What are the causes of Bell's palsy?

Causes of Bell's Palsy

Bell's palsy has no identified cause and is diagnosed only after excluding all other potential etiologies of acute unilateral facial nerve paralysis. 1

Primary Pathophysiologic Mechanism

While the exact cause remains unknown, the American Academy of Otolaryngology-Head and Neck Surgery describes Bell's palsy as occurring through inflammation and edema of the facial nerve within the narrow temporal bone canal, leading to compression and subsequent nerve damage. 2, 3 This neural edema develops within 72 hours and causes mechanical compression that disrupts nerve impulse conduction. 2

Suspected but Unproven Viral Etiology

• A viral etiology is suspected but remains unproven, with herpes simplex virus type 1 being one proposed infectious agent. 3
• Upper respiratory tract infections increase the risk of Bell's palsy, which supports the viral hypothesis. 2, 3
• Some evidence suggests infection with herpes simplex virus type 1 as a possible etiology, though this remains speculative. 4

Conditions That Must Be Excluded Before Diagnosis

Bell's palsy is strictly a diagnosis of exclusion—the American Academy of Otolaryngology-Head and Neck Surgery explicitly states that currently no cause has been identified, and diagnosis requires ruling out all identifiable causes of facial nerve paralysis. 1

Neurologic Causes to Exclude

• Stroke (distinguished by central facial weakness that spares the forehead and presence of other neurologic deficits such as extremity weakness, speech difficulties, or altered mental status). 3, 1
• Brain tumors affecting the facial nerve pathway. 3, 1
• Guillain-Barré syndrome (especially if bilateral facial palsy is present, which is extremely rare in true Bell's palsy). 3, 1

Infectious Causes to Exclude

• Herpes zoster (Ramsay Hunt syndrome). 3, 1
• Lyme disease. 3, 1

Structural Causes to Exclude

• Parotid gland or infratemporal fossa tumors. 3, 1
• Cancer involving the facial nerve. 3, 1
• Trauma or fractures affecting the temporal bone. 3, 1

Systemic Causes to Exclude

• Sarcoidosis. 3, 1

Risk Factors That Increase Susceptibility

While not causes per se, 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. 2, 3
• Age 15-45 years represents peak incidence. 2, 3
• Compromised immune systems increase susceptibility, consistent with an inflammatory mechanism. 2, 3

Critical Clinical Caveats

• 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. 3, 1
• The presence of other cranial nerve involvement excludes Bell's palsy and suggests central pathology. 1
• Routine laboratory testing and imaging are not recommended for typical presentations, but atypical features warrant further investigation. 1