Bell's palsy

Definition | Aetiology | Pathophysiology | Risk Factors | Signs and Symptoms | Investigations | Management

Definition

Bell’s palsy is an acute, unilateral facial nerve (cranial nerve VII) paralysis of unknown cause, resulting in sudden weakness or paralysis of facial muscles.

Aetiology

The exact cause of Bell’s palsy is unknown, but it is thought to be due to viral-induced inflammation of the facial nerve.

Possible Triggers:

• Viral infections: herpes simplex virus (HSV-1) is the most common suspected cause.
• Other viral associations: epstein Barr virus (EBV), varicella-zoster virus (VZV), cytomegalovirus (CMV).
• Autoimmune response: inflammation induced nerve damage.
• Cold exposure: sudden exposure to cold temperatures may be a risk factor.

Pathophysiology

• Viral reactivation leads to inflammation and oedema of the facial nerve.
• Compression of the nerve within the facial canal disrupts conduction.
• Results in paralysis of the muscles of facial expression on the affected side.

Risk factors

• Recent viral infection (e.g., HSV, VZV).
• Pregnancy (particularly in the third trimester).
• Diabetes mellitus.
• Hypertension.
• Family history of Bell’s palsy.

Signs and symptoms

Bell’s palsy presents as sudden onset unilateral facial weakness.

Key Features:

• Unilateral facial weakness: unable to close the eye or move the mouth on the affected side.
• Loss of forehead wrinkling: unlike strokes, Bell’s palsy affects the forehead due to lower motor neuron involvement.
• Drooping mouth: difficulty eating and drinking.
• Loss of nasolabial fold: flattening of the facial features.
• Hyperacusis: increased sensitivity to sound in the affected ear.
• Reduced taste sensation: affects the anterior two-thirds of the tongue.
• Dry eyes or excessive tearing: due to impaired lacrimal gland function.

Differentiate from Stroke:

• Bell’s palsy affects the entire half of the face, including the forehead.
• Stroke (upper motor neuron lesion) spares the forehead due to bilateral cortical innervation.

Investigations

• Clinical diagnosis: based on sudden onset unilateral facial paralysis.
• Exclude stroke: if upper face is spared, consider urgent stroke assessment.
• Blood tests: HbA1c for diabetes, inflammatory markers if an autoimmune cause is suspected.
• Electromyography (EMG): if symptoms persist beyond 3 months to assess nerve function.
• MRI brain: if an alternative diagnosis (e.g., tumour, multiple sclerosis) is suspected.

Management

1. Medical Management:

• Corticosteroids: prednisolone 50 mg daily for 10 days (best if started within 72 hours).
• Antivirals: Consider acyclovir or valacyclovir if herpes virus is suspected.

2. Eye Care:

• Artificial tears: to prevent corneal dryness.
• Eye patch at night: to protect the eye from injury.
• Tape eyelid shut: if incomplete eye closure causes exposure keratitis.

3. Physiotherapy and Rehabilitation:

• Facial exercises to improve muscle tone and prevent contractures.
• Massage therapy to stimulate facial muscles.

4. Follow-Up and Prognosis:

• Most cases resolve within 3–6 months.
• Severe cases may have residual weakness or synkinesis (involuntary movements).
• Urgent referral if no improvement after 3 months.