Summary

Bell's palsy is an acute, idiopathic, unilateral lower motor neuron (LMN) facial nerve (CN VII) palsy. It is the most common cause of acute facial paralysis, affecting approximately 20-30 per 100,000 people annually. The presumed aetiology involves viral reactivation (particularly HSV-1) causing inflammation and swelling of the facial nerve within the temporal bone. The hallmark is sudden-onset unilateral facial weakness affecting the entire side of the face, including the forehead (distinguishing it from upper motor neuron lesions). Treatment with corticosteroids within 72 hours improves outcomes. Eye care is essential to prevent exposure keratopathy. Prognosis is generally excellent, with 70-85% of patients achieving complete recovery.

Key Facts

• Incidence: 20-30 per 100,000 per year
• Aetiology: Presumed viral (HSV-1) reactivation
• Key feature: LMN facial weakness — forehead affected
• Treatment: Prednisolone 50mg OD for 10 days (start within 72h)
• Eye care: Critical to prevent exposure keratitis
• Prognosis: 70-85% full recovery without treatment; improves with steroids

Clinical Pearls

Forehead Is Key: LMN (Bell's) = forehead and lower face affected (cannot wrinkle forehead). UMN (Stroke) = forehead spared (bilateral cortical innervation).

Bell's Phenomenon: When the patient attempts to close the eye, the eyeball rolls upward, exposing the sclera. This is a normal protective reflex, visible because eyelid closure is weak.

Start Steroids Early: Maximum benefit if prednisolone started within 72 hours of symptom onset. After 72 hours, benefit is less clear.

Why This Matters Clinically

Bell's palsy is the most common cause of acute facial weakness. Rapid recognition, early steroid treatment, and appropriate eye care prevent complications and optimise recovery. It is also critical to distinguish from stroke and other causes of facial weakness.

Structured Approach

Observe at Rest:

• Facial asymmetry
• Nasolabial fold flattening
• Mouth droop

Active Movement:

• Raise eyebrows (frontalis — preserved in UMN)
• Close eyes tight (orbicularis oculi)
• Smile (zygomatic, buccal branches)
• Blow out cheeks
• Whistle or purse lips

Bell's Phenomenon:

• Ask patient to close eyes
• Observe incomplete closure; eye rolls up

Ear Examination:

• Look for vesicles (Ramsay Hunt if present)

Neurological Examination:

• Other cranial nerves (especially V, VIII, IX)
• Limb examination if stroke suspected

Special Tests

Recovery Rates

Timeline

Prognostic Factors

Key Guidelines

1. NICE Clinical Knowledge Summaries: Bell's Palsy — UK guidance.
2. AAO-HNSF Clinical Practice Guideline: Bell's Palsy (2013) — American Academy of Otolaryngology.

Landmark Trials

Sullivan et al. (2007) — Early treatment with prednisolone

• 496 patients
• Key finding: Prednisolone within 72h significantly improved recovery (NNT ~6)
• PMID: 17914058

Cochrane Review (2015) — Antivirals for Bell's palsy

• Key finding: Antivirals alone or with steroids provide no additional benefit over steroids alone
• PMID: 26564643

Evidence Strength

What is Bell's Palsy?

Bell's palsy is a sudden weakness on one side of your face. It happens when the nerve that controls the muscles of your face becomes inflamed and swollen.

What causes it?

The exact cause is not known, but it is thought to be triggered by a viral infection (like the cold sore virus). It is not a stroke.

How is it different from a stroke?

In Bell's palsy, the whole side of your face is weak — including your forehead. In a stroke, your forehead is usually spared. If you are unsure, always seek urgent medical advice.

How is it treated?

1. Steroid tablets (prednisolone): Work best if started within 3 days. Reduce inflammation and improve recovery.
2. Eye care: Very important. Use artificial tears during the day and ointment at night. Tape your eye closed at night to protect it.
3. Antiviral tablets: Not usually needed for Bell's palsy but given if there are blisters in the ear (Ramsay Hunt syndrome).

What to expect

• Most people (70-85%) make a full recovery
• Recovery usually starts within 2-4 weeks
• It may take 3-6 months to fully recover
• Some people have mild weakness or twitching afterwards

When to seek help

See a doctor urgently if:

• You have sudden facial weakness (to confirm the diagnosis)
• Your eye becomes red, painful, or vision changes
• You notice vesicles (blisters) in your ear