What causes facial weakness?

Causes of Facial Weakness

Facial weakness results from damage to the facial nerve (CN VII) anywhere along its pathway from the brainstem through the temporal bone to the facial muscles, with Bell's palsy accounting for approximately 70% of cases while the remaining 30% have identifiable causes including stroke, tumors, infections, trauma, and systemic diseases. 1

Primary Categories of Facial Weakness

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

• Acute unilateral facial nerve paresis or paralysis with onset in less than 72 hours without identifiable cause 1, 2
• Suspected viral etiology (possibly herpes simplex virus type 1), though exact mechanism remains unknown 1, 3
• Results from facial nerve inflammation and edema within the narrow temporal bone canal, leading to nerve compression 1
• Involves forehead muscles (distinguishing it from central causes) 2, 4
• Bilateral presentation is rare and should prompt investigation for alternative diagnoses 1, 4

Central Nervous System Causes (30% of non-Bell's palsy cases)

• Stroke: Typically spares forehead muscles (upper motor neuron pattern), may present with isolated facial palsy rarely 1, 4
• Brain tumors: Gradual onset, often with additional neurologic symptoms 1
• Multiple sclerosis: Intra-axial brainstem lesions affecting facial nuclei 1
• Vascular malformations within the pons 1

Cerebellopontine Angle and Temporal Bone Pathology

• Acoustic neuromas (vestibular schwannomas): Most common cerebellopontine angle tumor 1
• Meningiomas: Extra-axial masses compressing facial nerve 1
• Cholesteatomas: Erosive lesions within temporal bone 1
• Paragangliomas: Vascular tumors along nerve pathway 1
• Epidermoid cysts: Slow-growing lesions in cerebellopontine angle 1

Infectious Causes

• Lyme disease: Accounts for up to 25% of facial paralysis cases in endemic areas, often bilateral 1, 4
• Herpes zoster (Ramsay Hunt syndrome): Vesicular rash in ear canal, more severe prognosis than Bell's palsy 1, 4
• Systemic viral infections: Various viruses can cause secondary facial nerve palsy 5, 6
• Local infections: Otitis media, mastoiditis affecting facial nerve 5, 6

Traumatic Causes

• Temporal bone fractures: Direct injury to facial nerve within bony canal 1
• Facial trauma: Penetrating injuries or blunt force 1
• Surgical injury: Iatrogenic damage during parotid, mastoid, or skull base surgery 1, 4

Neoplastic Causes

• Parotid gland tumors: Direct invasion or compression of extracranial facial nerve 1
• Perineural tumor spread: Carcinomas tracking along facial nerve 1
• Skin cancers of head/face: Squamous cell or basal cell carcinoma with perineural invasion 1
• Intrinsic facial nerve tumors: Rare schwannomas or neurofibromas 1

Systemic and Inflammatory Diseases

• Sarcoidosis: Granulomatous inflammation affecting facial nerve 1, 4
• Guillain-Barré syndrome: Bilateral facial weakness with ascending paralysis 4
• Diabetes mellitus: Increased risk for Bell's palsy and microvascular complications 1, 3
• Autoimmune disorders: Various immunological conditions 5, 6

Other Causes

• Hemifacial spasm: Vascular compression of facial nerve (hyperactivity rather than weakness) 1
• Medications: Drug-induced facial nerve dysfunction 5, 6
• Degenerative CNS diseases: Progressive neurological conditions 5

Critical Distinguishing Features

Timing of Onset

• Bell's palsy: Rapid onset over less than 72 hours 1, 2
• Neoplastic/infectious causes: Gradual progression over days to weeks 1
• Stroke: Sudden onset, often with other neurologic deficits 1

Forehead Involvement

• Peripheral causes (including Bell's palsy): Forehead muscles affected, cannot raise eyebrow 1, 2
• Central causes (stroke): Forehead spared due to bilateral cortical innervation 4

Associated Symptoms Suggesting Alternative Diagnoses

• Dizziness, dysphagia, diplopia: Suggest brainstem pathology, not Bell's palsy 1
• Multiple cranial nerve involvement: Indicates cerebellopontine angle mass or systemic disease 1
• Vesicular rash in ear: Herpes zoster (Ramsay Hunt syndrome) 1
• Arthritis or erythema migrans rash: Lyme disease 1
• Recurrent episodes on same side: Suggests tumor or structural abnormality 1

Common Pitfalls to Avoid

• Assuming all acute facial weakness is Bell's palsy: 30% have identifiable causes requiring different management 1
• Missing bilateral presentation: Bilateral facial weakness is rare in Bell's palsy and mandates investigation for Lyme disease, sarcoidosis, or Guillain-Barré syndrome 1, 4
• Failing to assess forehead function: Critical for distinguishing peripheral from central causes 2, 4
• Overlooking gradual onset: Progressive symptoms over weeks suggest tumor rather than Bell's palsy 1
• Not checking for other cranial nerve deficits: Additional findings exclude Bell's palsy diagnosis 1