What labs or imaging studies distinguish Bell's palsy from stroke?

Distinguishing Bell's Palsy from Stroke: Imaging and Laboratory Evaluation

Neuroimaging is the key diagnostic tool to distinguish Bell's palsy from stroke, with MRI being the preferred modality when facial weakness is suspected to be caused by stroke rather than Bell's palsy. 1

Clinical Features to Distinguish Bell's Palsy from Stroke

• Bell's palsy presents with acute unilateral facial nerve paresis or paralysis with onset in less than 72 hours without an identifiable cause, involving the entire hemiface including the forehead 2
• Stroke-related facial weakness typically spares the forehead due to bilateral upper motor neuron innervation of the frontalis muscle, while Bell's palsy affects the entire facial nerve distribution including the forehead 3, 4
• Bell's palsy is a diagnosis of exclusion that requires careful elimination of other causes through history and physical examination 2
• Presence of other neurological deficits (such as hemiparesis, dysarthria, or sensory changes) suggests stroke rather than Bell's palsy 5

Imaging Studies for Differentiation

When to Image

• Routine imaging is NOT recommended for typical Bell's palsy presentation 1, 2
• Imaging is indicated when:
  • Presentation is atypical (isolated branch involvement, other cranial nerve involvement) 1
  • Symptoms persist beyond 2-3 months 1
  • Symptoms worsen rather than improve 1
  • There is a history of trauma or tumor 1
  • Bilateral facial weakness is present 1

Recommended Imaging Modalities

• MRI with and without contrast is the imaging test of choice when stroke is suspected 1

  • Should include the entire course of the facial nerve from brainstem to periphery 1
  • High-resolution thin-cut sequences are recommended 1
  • Diffusion-weighted imaging (DWI) can detect pontine infarcts that may present as isolated facial palsy 5

• CT scan is recommended as the initial rapid neuroimaging for patients presenting with stroke-like symptoms to rule out hemorrhage 1

  • If MRI is contraindicated, contrast-enhanced CT can be used 1
  • Dedicated temporal bone CT with thin sections should be obtained instead of a head CT when evaluating the facial nerve 1

• Vascular imaging (CTA, MRA) should be performed in stroke patients to evaluate stroke mechanism and assess future stroke risk 1

Laboratory Testing

• Routine laboratory testing is NOT recommended for typical Bell's palsy 1, 2
• Laboratory testing may be appropriate in specific circumstances:
  • Lyme disease testing in endemic areas or with history of tick exposure 1
  • Testing for other infectious causes (HIV, syphilis) when clinically indicated 1, 6
  • No specific laboratory test can differentiate Bell's palsy from stroke 4

Key Diagnostic Algorithm

1. Initial Assessment: Determine if forehead is involved (suggests Bell's palsy) or spared (suggests stroke) 3, 4

2. Evaluate for Red Flags:

   • Sudden onset with other neurological deficits (suggests stroke) 5
   • Hypertension with encephalopathy (suggests stroke) 5
   • Isolated branch involvement (atypical for Bell's palsy) 1
   • Bilateral involvement (atypical for Bell's palsy) 1

3. Imaging Decision:

   • If stroke is suspected: Immediate CT to rule out hemorrhage, followed by MRI with DWI 1
   • If typical Bell's palsy presentation: No imaging needed 1, 2
   • If atypical features or no improvement after 2-3 months: MRI with and without contrast 1

Common Pitfalls and Caveats

• Small pontine infarcts can present as isolated facial palsy mimicking Bell's palsy and may be missed on initial imaging 5
• Approximately 0.8% of patients diagnosed with Bell's palsy in the ED receive an alternative diagnosis within 90 days 6
• Risk factors for misdiagnosis include increasing age, black race, and diabetes 6
• Stroke, herpes zoster, Guillain-Barré syndrome, and otitis media account for 85.4% of alternative diagnoses initially misdiagnosed as Bell's palsy 6
• COVID-19 infection has been reported as a potential cause of Bell's palsy, further complicating the diagnostic picture 7

Remember that while Bell's palsy is the most common cause of acute unilateral facial weakness, stroke must always be considered in the differential diagnosis, particularly in patients with vascular risk factors or atypical presentations 5, 6.