How is Bell's palsy diagnosed?

How to Diagnose Bell's Palsy

Bell's palsy is diagnosed clinically when a patient presents with acute unilateral facial nerve paresis or paralysis (onset <72 hours) after systematically excluding all other identifiable causes of facial weakness. 1

Diagnostic Criteria

Bell's palsy is fundamentally a diagnosis of exclusion. The diagnosis requires three essential elements 1:

• Acute onset: Symptoms develop in less than 72 hours
• Unilateral facial weakness: Affecting both upper and lower face (including forehead)
• No identifiable cause: After thorough evaluation to rule out other etiologies

Clinical Presentation to Confirm

Look for these specific features 1:

• Facial muscle weakness/paralysis on one side
• Inability to close the eyelid on affected side
• Drooping of mouth corner with oral incompetence
• Loss of forehead wrinkling on affected side
• Flattening of nasolabial fold

Associated symptoms may include:

• Ipsilateral ear or facial pain (common presenting symptom)
• Taste disturbance or loss (anterior tongue)
• Hyperacusis
• Eye or mouth dryness
• Drooling

Critical Red Flags Requiring Alternative Diagnosis

Bilateral facial palsy is rare and should prompt investigation for other causes 1. Immediately consider alternative diagnoses if:

• Gradual onset (>72 hours)
• Bilateral involvement
• Forehead sparing (suggests central/stroke etiology)
• Other cranial nerve deficits
• Progressive neurologic symptoms
• Recurrent episodes

Systematic Exclusion of Other Causes

You must actively rule out 1:

• Stroke (forehead typically spared in central lesions)
• Brain tumors
• Parotid gland or infratemporal fossa tumors
• Cancer involving facial nerve
• Infectious diseases:
  • Herpes zoster (Ramsay-Hunt syndrome - look for vesicles)
  • Lyme disease (check endemic areas, tick exposure)
  • Sarcoidosis
• Trauma or fractures
• Post-surgical complications

Diagnostic Testing

Routine Testing: NOT Required

Laboratory testing and imaging are NOT required for diagnosis of typical Bell's palsy 2. The diagnosis is clinical.

When to Order Tests

Order targeted investigations only when clinical features suggest alternative diagnoses 2:

• Lyme serology: If endemic area or tick exposure
• Herpes zoster testing: If vesicles present
• MRI with gadolinium: If atypical features, bilateral involvement, or progressive symptoms 3
• CT scan: If trauma suspected

Electrodiagnostic Testing

Clinicians may offer electrodiagnostic testing (EMG/ENoG) to patients with complete facial paralysis to assess prognosis for recovery 1. This is optional, not diagnostic, and helps predict outcomes rather than confirm diagnosis.

Grading Severity

Use the House-Brackmann facial nerve grading scale to quantify severity 1:

• Grade I: Normal function
• Grade II: Mild dysfunction (slight weakness on close inspection)
• Grade III: Moderate dysfunction (obvious but not disfiguring)
• Grade IV: Moderately severe dysfunction (obvious weakness/disfiguring asymmetry)
• Grade V-VI: Severe to total paralysis

Common Diagnostic Pitfalls

1. Missing central causes: Always test forehead movement - if forehead is spared, consider stroke
2. Overlooking Ramsay-Hunt syndrome: Examine ear canal and concha for vesicles
3. Ignoring bilateral presentation: This is NOT typical Bell's palsy - investigate further
4. Over-testing typical cases: Resist ordering unnecessary imaging in straightforward presentations
5. Delayed recognition of incomplete recovery: Reassess or refer if no improvement by 3 months 1

When to Refer

Reassess or refer to a facial nerve specialist if 1:

• New or worsening neurologic findings at any point
• Ocular symptoms developing at any point
• Incomplete facial recovery 3 months after onset

High-Risk Populations

Be particularly vigilant in 1, 4:

• Ages 15-45 years (highest incidence)
• Diabetes
• Pregnancy (especially with preeclampsia)
• Upper respiratory infections
• Immunocompromised states
• Obesity
• Hypertension

The diagnosis remains clinical and straightforward in most cases: acute unilateral facial weakness involving the forehead, onset within 72 hours, with no other identifiable cause after appropriate clinical evaluation.