What is the difference between stroke and Bell's palsy?

Distinguishing Stroke from Bell's Palsy

The critical difference is that Bell's palsy causes complete unilateral facial weakness involving the forehead, while stroke causes facial weakness that spares the forehead due to bilateral cortical innervation of the upper face. 1

Key Clinical Distinctions

Forehead Involvement (Most Important)

• Bell's palsy affects the entire ipsilateral face including the forehead because the lesion is in the peripheral facial nerve (CN VII) after it exits the brainstem, causing loss of all ipsilateral facial muscle innervation 1
• Stroke spares forehead movement because upper facial muscles receive bilateral cortical innervation from both cerebral hemispheres, so a unilateral central lesion does not completely paralyze forehead function 2, 1
• Failure to test forehead function is a critical pitfall that leads to misdiagnosis 3

Associated Neurologic Symptoms

• Stroke typically presents with additional neurologic deficits including dizziness, dysphagia (swallowing difficulty), diplopia (double vision), weakness in extremities, speech difficulties, altered mental status, or involvement of other cranial nerves 1
• Bell's palsy is an isolated facial nerve problem without other neurologic findings; the presence of any other cranial nerve dysfunction excludes Bell's palsy and suggests central pathology like stroke 4, 1
• Document function of all cranial nerves to exclude stroke and other central causes 1

Onset and Timing

• Bell's palsy develops rapidly over less than 72 hours with acute unilateral facial weakness 4, 3
• Stroke onset is typically sudden (seconds to minutes) rather than progressive over hours to days 1

Associated Symptoms Specific to Bell's Palsy

• Ipsilateral ear or facial pain is common in Bell's palsy 1
• Taste disturbance on the anterior two-thirds of the tongue 1
• Hyperacusis (increased sensitivity to sound) 1
• Dry eye or mouth due to autonomic fiber involvement 1
• Sagging of eyelid or corner of mouth 1

Diagnostic Approach Algorithm

Step 1: Test Forehead Function

• Ask patient to raise eyebrows and wrinkle forehead
• If forehead is weak = peripheral lesion (Bell's palsy possible)
• If forehead is spared = central lesion (stroke likely) 2, 1

Step 2: Assess for Other Neurologic Deficits

• Test all other cranial nerves systematically 1
• Assess for limb weakness, speech problems, mental status changes
• Any additional neurologic findings = NOT Bell's palsy, consider stroke 1

Step 3: Evaluate Timing and Associated Features

• Confirm onset within 72 hours for Bell's palsy 4, 3
• Look for ear pain, taste changes, hyperacusis (supports Bell's palsy) 1
• Assess vascular risk factors (diabetes, hypertension, hypercholesterolemia) which increase stroke risk 5

Step 4: Imaging Considerations

• Routine imaging is NOT recommended for typical Bell's palsy presentation 3, 1
• Order MRI with and without contrast if: atypical presentation, other neurologic findings suggesting stroke, no recovery after 3 months, or bilateral facial weakness 3, 2
• A rare dorsal pontine infarct can mimic Bell's palsy with isolated facial palsy, making anatomic knowledge crucial 5

Critical Pitfalls to Avoid

• Never assume isolated facial palsy is always Bell's palsy—rare pontine strokes can present with complete hemifacial paralysis mimicking Bell's palsy 5
• Bilateral facial weakness is extremely rare in Bell's palsy and should immediately raise suspicion for Guillain-Barré syndrome, sarcoidosis, or Lyme disease rather than Bell's palsy 2, 1
• Missing subtle forehead weakness or inadequate eye assessment leads to misdiagnosis 3
• Overlooking other cranial nerve involvement excludes Bell's palsy and suggests central pathology 1

When to Suspect Stroke Instead of Bell's Palsy

• Forehead sparing on examination 2, 1
• Presence of limb weakness, speech difficulties, or altered mental status 1
• Other cranial nerve deficits 4, 1
• History of vascular risk factors with acute symptom onset 1
• Dizziness, dysphagia, or diplopia 1