How do you differentiate between facial nerve palsy and stroke based on facial examination?

Differentiating Facial Nerve Palsy from Stroke on Facial Examination

The critical distinguishing feature is forehead involvement: Bell's palsy (peripheral facial nerve palsy) causes complete ipsilateral facial weakness including inability to wrinkle the forehead, while stroke (central facial palsy) typically spares forehead movement due to bilateral cortical innervation of the upper facial muscles. 1

Key Examination Findings

Forehead Function - The Primary Discriminator

• Bell's palsy (peripheral/LMN lesion): Complete inability to wrinkle forehead, raise eyebrow, or close eye on affected side 1
• Stroke (central/UMN lesion): Forehead movement preserved or only mildly affected; patient can wrinkle forehead and raise eyebrow 1
• Important caveat: Recent evidence challenges the traditional teaching—up to 76% of stroke patients may demonstrate some degree of upper facial weakness, though typically milder than lower facial involvement 2

Lower Face Involvement

• Bell's palsy: Affects entire ipsilateral face uniformly, including loss of nasolabial fold, inability to smile symmetrically, and drooping of mouth corner 1
• Stroke: Predominantly affects lower face with asymmetric smile and mouth droop, but the pattern may be similar to Bell's palsy in severity 1, 2

Critical Associated Neurological Signs

Features Suggesting Stroke (Not Bell's Palsy)

• Other cranial nerve involvement: Diplopia, dysphagia, or dizziness indicate brainstem pathology 1
• Limb weakness: Any arm or leg weakness ipsilateral or contralateral to facial weakness 1
• Sensory deficits: Unilateral sensory loss 1
• Speech abnormalities: Aphasia (not just dysarthria from facial weakness) 1
• Gaze abnormalities: Forced eye deviation or gaze palsy 1
• Ataxia or incoordination: Out of proportion to any weakness 1
• Altered consciousness: Lethargy or encephalopathy 3

Features Consistent with Bell's Palsy

• Isolated facial weakness: No other neurological deficits 1
• Acute onset: Symptoms develop within 72 hours 1
• Associated symptoms: Ipsilateral ear pain, taste disturbance, hyperacusis, or eye dryness 1
• Preserved corneal reflex: Though facial weakness present 3

Examination Algorithm

Step 1: Test Forehead Function

• Ask patient to raise eyebrows and wrinkle forehead
• If forehead completely paralyzed: Strongly suggests peripheral (Bell's palsy) 1
• If forehead movement preserved: Suggests central lesion (stroke), but does not exclude it if mildly weak 1, 2

Step 2: Assess for Additional Neurological Deficits

• Test all other cranial nerves systematically 1
• Assess limb strength bilaterally 1
• Check sensation, coordination, and gait 1
• If ANY other deficits present: Stroke until proven otherwise—requires immediate imaging 1, 4

Step 3: Evaluate Onset and Progression

• Sudden onset with encephalopathy or hypertension: Consider stroke even with isolated facial palsy 3
• Gradual progression beyond 72 hours: Consider alternative diagnoses including tumor 1

Critical Pitfalls to Avoid

Rare Stroke Presentations Mimicking Bell's Palsy

• Dorsal pontine infarcts: Can present as isolated facial palsy affecting entire face, mimicking Bell's palsy perfectly 3
• Frontal lobe strokes: May present with peripheral-pattern facial weakness 5
• Upper facial weakness in stroke: Occurs in 76% of stroke patients when carefully assessed, particularly with severe strokes and greater lower facial involvement 2

When to Image Despite "Typical" Bell's Palsy Appearance

• Multiple cranial nerve involvement: Requires immediate MRI regardless of forehead sparing 4
• Acute presentation with severe hypertension: Consider hypertensive encephalopathy with pontine involvement 3
• Symptoms persisting beyond 2 months: Warrants MRI to exclude structural lesions 1
• Bilateral facial weakness: Extremely rare in Bell's palsy, suggests alternative diagnosis 1

Time-Sensitive Considerations

• Stroke patients: May benefit from thrombolysis if within treatment window—facial palsy recovery improved with IV tPA 6
• Bell's palsy patients: Do not require emergency imaging if isolated facial weakness with typical features 1
• Diagnostic uncertainty: When in doubt, treat as stroke and obtain urgent neuroimaging—the consequences of missing stroke far outweigh unnecessary imaging 3

Examination Severity Grading

• Tight eye closure assessment: Most sensitive indicator for detecting upper facial weakness in stroke (42% sensitivity) 2
• Complete paralysis vs. paresis: Complete inability to move any part of face suggests higher stroke severity (NIHSS correlation) 2, 6