How do you differentiate Bell's palsy from stroke?

Differentiating Bell's Palsy from Stroke

The key distinguishing feature is forehead involvement: Bell's palsy causes complete hemifacial weakness including the forehead, while stroke causes central facial weakness that spares the forehead. 1

Primary Distinguishing Features

Forehead Involvement (Most Critical)

• Bell's palsy affects the entire ipsilateral side of the face, including the forehead, because the facial nerve lesion occurs after it exits the brainstem, causing loss of innervation to all ipsilateral facial muscles 1
• Stroke spares the forehead due to bilateral cortical innervation of the upper facial muscles, resulting in central facial weakness that preserves forehead wrinkling 1, 2
• This anatomical difference is the most reliable clinical sign for differentiation 1

Additional Neurologic Findings

• Bell's palsy presents with isolated facial nerve dysfunction with no other cranial nerve abnormalities or neurologic deficits 3, 4
• Stroke typically presents with additional neurologic symptoms such as dizziness, dysphagia, diplopia, or other cranial nerve involvement 5
• The clinician should document function of all other cranial nerves to exclude stroke and other central causes 5

Diagnostic Criteria for Bell's Palsy

Temporal Pattern

• Onset must occur within 72 hours of symptom development 1, 2
• Symptoms typically peak in the first week and gradually resolve over three weeks to three months 3

Associated Features Suggesting Bell's Palsy

• Ipsilateral ear or facial pain (common presenting symptom) 5
• Taste disturbance or loss from anterior tongue 5, 1
• Hyperacusis (sensitivity to sound) 5, 1
• Dry eye or mouth 5, 1
• Sagging of eyelid or corner of mouth 5, 1

When to Suspect Stroke Instead

Red Flags Requiring Alternative Diagnosis

• Presence of other neurologic deficits including weakness in extremities, speech difficulties, or altered mental status 5
• Forehead sparing on physical examination 1, 2
• Gradual onset beyond 72 hours 1
• History of vascular risk factors with acute onset of symptoms 5

Diagnostic Approach

History and Physical Examination

• A thorough history and physical examination are sufficient to diagnose Bell's palsy when typical features are present 5, 1
• Specifically assess for symptoms suggesting central pathology: dizziness, dysphagia, diplopia 5
• Document all cranial nerve function to identify any additional deficits 5
• Confirm rapid onset within 72 hours 1, 2

When Imaging is NOT Needed

• Routine diagnostic imaging should not be performed for patients with new-onset Bell's palsy who have typical presentation 5
• Laboratory testing is not routinely indicated unless history suggests specific alternative diagnoses 5

When to Consider Imaging

• Atypical presentation with other neurologic findings suggesting stroke 5
• Gradual onset or progression beyond typical timeframe 5
• Bilateral facial weakness (extremely rare in Bell's palsy and should raise suspicion for Guillain-Barré syndrome or sarcoidosis) 1, 2

Common Pitfalls to Avoid

• Do not assume forehead involvement rules out stroke—you must confirm that forehead weakness is present to support Bell's palsy diagnosis 1
• Do not miss bilateral presentation, which is extremely rare in Bell's palsy and should immediately prompt investigation for alternative diagnoses 1, 2
• Do not overlook other cranial nerve involvement, which excludes Bell's palsy and suggests central pathology 5, 3
• Do not forget that Bell's palsy is a diagnosis of exclusion—other identifiable causes must be ruled out through history and examination 1, 2