What can cause an unreasonable neurosis of the facial nerve, also known as Bell's palsy or facial nerve paralysis?

Causes of Facial Nerve Paralysis (Bell's Palsy)

Bell's palsy is idiopathic by definition—meaning no identifiable cause can be found—but the most widely accepted mechanism involves inflammation of the facial nerve within the temporal bone canal, possibly triggered by herpes simplex virus reactivation, though this remains unproven. 1

Primary Etiologic Categories

Idiopathic (Bell's Palsy - 70% of Cases)

• Bell's palsy is diagnosed only after excluding all other causes of acute unilateral facial nerve paralysis 2, 1
• The American Academy of Otolaryngology-Head and Neck Surgery explicitly states that no definitive cause has been identified for Bell's palsy 2
• The suspected mechanism involves facial nerve inflammation and edema within the narrow temporal bone canal, causing nerve compression 3
• Herpes simplex virus reactivation within the geniculate ganglion has been proposed but remains unproven 4, 5
• Autoimmune demyelination similar to Guillain-Barré syndrome has been suggested as an alternative mechanism 5

Infectious Causes (Must Be Excluded)

• Lyme disease: Accounts for up to 25% of facial paralysis cases in endemic areas, often presents bilaterally 2, 3
• Herpes zoster (Ramsay Hunt syndrome): Characterized by vesicular rash in the ear canal, worse prognosis than Bell's palsy 6, 3
• HIV/AIDS: Can cause facial nerve involvement through direct viral effects or opportunistic infections 2
• Bacterial infections: Bacterial meningitis, chronic otitis media with temporal bone involvement 2
• COVID-19: Emerging evidence suggests facial palsy as a late neurologic manifestation 7

Central Nervous System Pathology

• Stroke: Typically spares forehead muscles (upper motor neuron pattern), may have additional neurologic deficits like dizziness, dysphagia, diplopia 2, 1
• Brainstem lesions: Infarction, vascular malformations, multiple sclerosis affecting the facial nerve nucleus in the pons 2
• Brain tumors: Gradual onset with other neurologic symptoms 3

Structural/Neoplastic Causes

• Facial or vestibular schwannomas: Affect the nerve in the cerebellopontine angle or internal auditory canal 2
• Meningiomas: Compress the facial nerve along its intracranial course 2
• Parotid gland tumors: Direct invasion or compression of the extracranial facial nerve 2, 3
• Cholesteatomas: Erode the temporal bone and involve the facial nerve 2
• Paragangliomas and epidermoid cysts: Affect the nerve in the cerebellopontine angle 2
• Perineural tumor spread: Carcinomas and sarcomas tracking along the facial nerve 2

Traumatic Causes

• Temporal bone fractures: Direct injury to the facial nerve within the bony canal 2, 3
• Surgical trauma: Iatrogenic injury during skull base or parotid surgery 2

Systemic/Inflammatory Diseases

• Sarcoidosis: Granulomatous inflammation affecting the facial nerve 2, 6, 3
• Guillain-Barré syndrome: Bilateral facial weakness with ascending paralysis 2, 6, 3
• Melkersson-Rosenthal syndrome: Recurrent facial paralysis with facial edema 2
• Diabetes mellitus: Increases susceptibility through ischemic and neuropathic mechanisms 6

Other Causes

• Leukemia: Infiltration of the facial nerve 2
• Multiple idiopathic cranial nerve neuropathies: Involvement of other cranial nerves excludes Bell's palsy 2

Critical Distinguishing Features

Timing of Onset

• Bell's palsy: Rapid onset over less than 72 hours 1, 3
• Neoplastic/infectious causes: Gradual progression over days to weeks 3
• Stroke: Sudden onset, often with other neurologic deficits 3

Anatomic Distribution

• Peripheral (Bell's palsy): Entire ipsilateral face affected including forehead 2, 1
• Central (stroke): Forehead spared due to bilateral cortical innervation 1, 3

Associated Features Suggesting Alternative Diagnoses

• Bilateral facial palsy: Extremely rare in Bell's palsy; investigate for Lyme disease, sarcoidosis, or Guillain-Barré syndrome 2, 1, 6
• Other cranial nerve involvement: Excludes Bell's palsy, suggests brainstem pathology 2, 1
• Isolated branch paralysis: Not consistent with Bell's palsy, suggests focal structural lesion 2
• Recurrent episodes on same side: Requires workup for structural lesions, sarcoidosis, or diabetes 6

Common Pitfalls to Avoid

• Do not assume all acute facial weakness is Bell's palsy—30% have identifiable causes requiring different management 3
• Always assess forehead function to distinguish peripheral from central causes 1, 3
• Do not overlook other cranial nerve involvement, which excludes Bell's palsy and suggests central pathology 1
• Bilateral presentation should never be accepted as idiopathic without extensive workup 2, 1, 6
• Recurrent facial palsy is not Bell's palsy—it requires identification of an underlying etiology 6