How do you differentiate and manage a patient presenting with sudden onset of facial weakness, to determine if it's Bell's palsy or a stroke?

Differentiating Bell's Palsy from Stroke

The critical distinguishing feature is forehead involvement: Bell's palsy causes complete hemifacial weakness including the forehead (peripheral pattern), while stroke typically spares forehead muscles (central pattern) because of bilateral cortical innervation of upper facial muscles. 1

Primary Clinical Distinction

Forehead Function is the Key Differentiator:

• In Bell's palsy, the patient cannot wrinkle their forehead or raise their eyebrow on the affected side because the facial nerve lesion is peripheral (after it exits the brainstem), affecting all ipsilateral facial muscles 1
• In stroke, forehead movement is preserved because upper motor neuron lesions spare the forehead due to bilateral cortical innervation 1, 2
• Test this by asking the patient to raise both eyebrows and look for asymmetry 1

Additional Neurologic Findings Point to Stroke

Stroke typically presents with other neurologic deficits that are absent in Bell's palsy:

• Dizziness, dysphagia (difficulty swallowing), or diplopia (double vision) 1
• Weakness in extremities, speech difficulties, or altered mental status 1
• Involvement of other cranial nerves (CN V, VI, VIII, IX, X, XI, or XII) 1
• Document function of all cranial nerves to exclude stroke and other central causes 1

Temporal Pattern Differences

Onset timing helps distinguish the conditions:

• Bell's palsy has rapid onset over less than 72 hours, which is a cardinal diagnostic feature 3, 1, 2
• Stroke typically has sudden onset (seconds to minutes) and often occurs with vascular risk factors 1
• Gradual progression over days to weeks suggests neoplastic or infectious causes, not Bell's palsy 3, 2

Associated Features of Bell's Palsy

Bell's palsy has characteristic associated symptoms that stroke lacks:

• Ipsilateral ear or facial pain is common 1
• Taste disturbance or loss from the anterior two-thirds of the tongue 1
• Hyperacusis (increased sensitivity to sound) 1
• Dry eye or excessive tearing 1
• Sagging of the eyelid or corner of mouth 1

Critical Red Flags Requiring Imaging

Do not assume Bell's palsy if any of these are present:

• Any other cranial nerve involvement beyond CN VII—this excludes Bell's palsy and mandates urgent imaging 1
• Bilateral facial weakness (extremely rare in Bell's palsy; consider Lyme disease, sarcoidosis, or Guillain-Barré syndrome) 1, 2
• Gradual onset or recurrent episodes (requires MRI to exclude structural lesions) 3, 4
• Atypical presentation with facial swelling and redness (consider Lyme disease or Ramsay Hunt syndrome) 2

Diagnostic Approach

History and physical examination are sufficient for typical Bell's palsy:

• Confirm rapid onset within 72 hours 3, 1
• Test all facial muscles: raise eyebrows, close eyes tightly, smile, puff out cheeks 1
• Examine all other cranial nerves systematically 1
• Routine laboratory testing and imaging are not recommended for typical presentations 1

When to order imaging:

• MRI with and without contrast is indicated if symptoms persist beyond 2 months, presentation is atypical, or other neurologic findings suggest stroke 1
• CT of temporal bone is complementary for evaluating fractures or osseous anatomy 1

Management Differences

For confirmed Bell's palsy (after excluding stroke):

• Prescribe oral corticosteroids immediately: prednisolone 50 mg daily for 10 days OR prednisone 60 mg daily for 5 days followed by a 5-day taper 1, 5
• Consider adding antivirals (valacyclovir 1 g three times daily for 7 days) to reduce synkinesis rates 5
• Implement eye protection immediately: lubricating drops, ophthalmic ointment, and eye taping or patching to prevent permanent corneal damage 1

For suspected stroke:

• Activate stroke protocol immediately with urgent neuroimaging 1
• Time-sensitive thrombolytic therapy may be indicated 1

Common Pitfalls to Avoid

• Do not assume all acute facial weakness is Bell's palsy—30% of facial palsies have identifiable causes requiring different management 3, 2
• Always test forehead function; this single examination finding is the most reliable way to distinguish peripheral from central causes 1, 2
• Never overlook other cranial nerve involvement, which excludes Bell's palsy and suggests serious central pathology 1
• A rare pontine infarct can present with isolated facial palsy mimicking Bell's palsy, but careful examination will reveal subtle additional neurologic signs 6