Is forehead movement a reliable way to differentiate between Bell's palsy and stroke?

Forehead Movement in Differentiating Bell's Palsy from Stroke

Forehead movement is a reliable clinical sign to differentiate Bell's palsy from stroke, as Bell's palsy affects the entire facial nerve including forehead muscles, while central causes like stroke typically spare forehead movement due to bilateral cortical innervation of the upper face. 1, 2

Anatomical Basis for Differential Diagnosis

The distinction between Bell's palsy and stroke relies on understanding facial nerve innervation patterns:

• Bell's Palsy (Peripheral Facial Weakness):

  • Affects the entire facial nerve (cranial nerve VII)
  • Causes weakness of both upper and lower face on the affected side
  • Key finding: Inability to raise eyebrow or wrinkle forehead on affected side
  • Complete loss of movement in all facial muscles on the affected side 1

• Stroke (Central Facial Weakness):

  • Upper motor neuron lesion in the brain
  • Spares the forehead due to bilateral cortical innervation of upper facial muscles
  • Key finding: Preserved forehead movement despite lower facial weakness
  • Patient can still raise eyebrows and wrinkle forehead despite weakness of lower face 3, 4

Clinical Assessment Using House-Brackmann Scale

The American Academy of Otolaryngology-Head and Neck Surgery recommends using the House-Brackmann scale to quantify facial nerve function 2:

• In Bell's palsy, forehead movement is affected according to the severity:
  • Grade 2: Slight weakness noticeable on close inspection
  • Grade 3: Slight to no movement of forehead
  • Grade 4: No movement of forehead
  • Grade 5-6: Progressive worsening to total paralysis 1

Diagnostic Pitfalls and Caveats

• Beware of rare exceptions: Some pontine strokes can mimic Bell's palsy with complete facial weakness including forehead 4
• Additional distinguishing features:
  • Sudden onset (within hours) may suggest stroke rather than Bell's palsy, which typically develops over 1-3 days 5
  • Associated symptoms like nausea, vomiting, slurred speech, or other neurological deficits suggest stroke 4
  • Hypertension may be present in both conditions but is more concerning for stroke when severe 4

Diagnostic Algorithm

1. Assess forehead movement:

   • Ask patient to raise eyebrows/wrinkle forehead
   • If forehead movement is preserved with lower face weakness → suspect stroke
   • If forehead movement is absent along with lower face weakness → suspect Bell's palsy

2. Look for red flags suggesting stroke:

   • Sudden onset (minutes to hours)
   • Other neurological symptoms
   • Severe hypertension
   • Risk factors for stroke (diabetes, hypertension, smoking, atrial fibrillation)

3. When in doubt:

   • Obtain urgent neuroimaging (MRI preferred over CT)
   • Remember that small pontine infarcts can be missed on initial imaging and may require careful review 4

Important Clinical Consideration

The forehead movement test is highly valuable but not infallible. In elderly patients with vascular risk factors, even with clinical features suggesting Bell's palsy, maintain a high index of suspicion for stroke, particularly if presentation is hyperacute or atypical 6. Some pontine infarcts can present with isolated facial palsy that mimics Bell's palsy, including loss of forehead movement 4.