How is idiopathic Bell's palsy diagnosed?

Diagnosis of Idiopathic Bell's Palsy

Bell's palsy is diagnosed clinically when acute unilateral facial weakness involving the forehead develops within 72 hours and no other identifiable cause can be found through history and physical examination alone. 1

Clinical Diagnostic Criteria

Required features for diagnosis:

• Rapid onset of unilateral facial weakness or paralysis occurring in less than 72 hours 1, 2
• Complete hemifacial involvement including the forehead (inability to wrinkle forehead or raise eyebrow on affected side) 1, 2
• No other cranial nerve deficits on examination 1
• No identifiable alternative cause after clinical evaluation 1, 3

Essential History Elements

Document the following to support diagnosis:

• Sudden onset within 72 hours (gradual progression beyond 3 weeks suggests tumor or infection, not Bell's palsy) 1
• Associated symptoms: ipsilateral ear or facial pain, taste disturbance on anterior two-thirds of tongue, hyperacusis, dry eye or mouth 1
• Recent viral prodrome or upper respiratory infection 3
• Risk factors: diabetes, hypertension, pregnancy (especially third trimester), obesity 3
• Absence of stroke symptoms: no dizziness, dysphagia, diplopia, limb weakness, or altered mental status 1

Physical Examination Requirements

Perform systematic facial nerve testing:

• Ask patient to raise eyebrows, close eyes tightly, smile, puff out cheeks 1
• Confirm forehead involvement (this distinguishes peripheral from central lesions; forehead sparing indicates stroke) 1, 2
• Test taste on anterior two-thirds of tongue if feasible 1
• Assess for hyperacusis (stapedius muscle involvement) 1
• Evaluate eye closure completeness to assess corneal exposure risk 2

Complete cranial nerve examination is mandatory:

• Any involvement of cranial nerves other than CN VII excludes Bell's palsy and mandates imaging for central pathology or skull-base lesion 1, 2

Diagnostic Testing: What NOT to Order

Routine laboratory testing is NOT recommended for typical Bell's palsy presentations, as it delays treatment beyond the critical 72-hour window without improving outcomes 1, 2. Laboratory tests cannot confirm or exclude Bell's palsy and should only be ordered when specific alternative diagnoses are suspected (e.g., Lyme serology in endemic areas, glucose if undiagnosed diabetes suspected) 1.

Routine imaging is NOT recommended for new-onset Bell's palsy with classic features 1, 2. Observational studies show no outcome advantage and highlight the risk of delayed treatment 2.

Electrodiagnostic testing is NOT indicated for patients with incomplete facial paralysis 1, 2. It may be offered only to patients with complete facial paralysis, ideally performed 3–14 days after symptom onset 2.

Red Flags Requiring MRI with Contrast

Order MRI (with and without contrast) when any of these atypical features are present:

• Recurrent paralysis on the same side (suggests tumor) 1, 2
• Isolated branch paralysis (only upper or lower face affected) 1, 2
• Any other cranial nerve involvement (indicates central or skull-base pathology) 1, 2
• Bilateral facial weakness (extremely rare in Bell's palsy; suggests Guillain-Barré syndrome, Lyme disease, or sarcoidosis) 1, 3
• Forehead sparing (classic sign of central stroke, not Bell's palsy) 1, 2
• Progressive weakness beyond 3 weeks 1, 2
• No recovery after 3 months 1, 2
• New or worsening neurologic findings at any time 1, 2

Mandatory Exclusions Before Diagnosis

The following conditions must be ruled out clinically:

• Stroke (distinguished by forehead sparing, additional neurologic deficits, vascular risk factors) 1, 3
• Herpes zoster (Ramsay Hunt syndrome: look for vesicles in ear canal or on tongue) 1, 3
• Lyme disease (especially in endemic areas and with bilateral palsy) 1, 3
• Sarcoidosis (consider with bilateral involvement or systemic symptoms) 1, 3
• Tumors (brain, parotid gland, infratemporal fossa, or facial nerve malignancy) 1, 3
• Trauma (temporal bone fracture or iatrogenic surgical injury) 1, 3
• Guillain-Barré syndrome (especially if bilateral facial weakness) 1, 3

Common Diagnostic Pitfalls

Failing to test forehead function leads to missed stroke diagnoses, as central lesions spare the forehead while Bell's palsy always involves it 1, 2.

Ordering unnecessary labs or imaging increases cost, delays critical corticosteroid therapy beyond 72 hours, and offers no clinical benefit in typical presentations 1, 2.

Missing other cranial nerve involvement on examination postpones appropriate diagnosis of serious central pathology or skull-base lesions 1, 2.

Accepting bilateral facial weakness as Bell's palsy is a critical error; bilateral involvement is exceedingly rare and mandates immediate evaluation for Guillain-Barré syndrome, Lyme disease, or sarcoidosis 1, 3.

Delaying treatment to await test results compromises the 72-hour therapeutic window, which is essential for optimal recovery with corticosteroids 2.