Key Facts

Bell palsy is the commonest cause of acute facial nerve palsy. It is a diagnosis of exclusion — other causes must be considered. Early corticosteroids significantly improve outcome.

Epidemiology

Incidence ~20-30 per 100,000/year. Equal sex distribution. Peak age 15-45 years. More common in pregnancy (3rd trimester/early postpartum) and diabetes.

Aetiology

• Idiopathic by definition, but HSV-1 reactivation in the geniculate ganglion is the leading hypothesis
• Swelling of CN VII within the bony fallopian canal → nerve compression → demyelination ± axonal degeneration
• Risk factors: pregnancy, diabetes, family history, upper respiratory infection

Pathophysiology

Viral reactivation (HSV-1) in the geniculate ganglion → inflammation → oedema of the facial nerve within the rigid fallopian canal → nerve compression → ischaemia → demyelination (neuropraxia in mild cases) or axonal degeneration (in severe cases). Neuropraxia: good prognosis (recovery in weeks). Axonal degeneration: slower, incomplete recovery (months); risk of synkinesis (aberrant reinnervation).

Typical Presentation

• Acute onset (hours to 1-2 days): unilateral facial weakness affecting ALL muscles of ipsilateral face
• Forehead involved: inability to raise eyebrow, wrinkle forehead (KEY distinguishing feature from UMN)
• Eye closure weakness: inability to close eye (lagophthalmos) → corneal exposure → risk of corneal ulceration
• Loss of nasolabial fold: drooping of ipsilateral mouth corner
• Mouth drooping: drooling, difficulty eating/drinking
• Postauricular/mastoid pain: may precede weakness by 1-2 days (~50%)

Associated Features

• Hyperacusis (stapedius muscle paralysis)
• Altered taste (anterior 2/3 of tongue — chorda tympani involvement)
• Reduced lacrimation (greater petrosal nerve involvement)
• Dry mouth (submandibular/sublingual gland — chorda tympani)

Red Flags (NOT Bell Palsy)

• Bilateral facial palsy → sarcoidosis, GBS, Lyme disease
• Gradual onset (>2 weeks) → tumour (parotid, CPA schwannoma)
• Associated vesicles in ear (Ramsay Hunt syndrome — VZV)
• Other cranial nerve involvement → brainstem lesion, skull base pathology
• Failure to improve by 3-4 months → reconsider diagnosis

Clinical Diagnosis

• Bell palsy is a clinical diagnosis of exclusion
• No routine investigations required for typical presentation in a young, otherwise healthy patient

When to Investigate

• Atypical features: bilateral, gradual onset, recurrent, other CN involvement, vesicles
• Lyme serology: if risk factors (tick exposure, endemic area)
• MRI brain/temporal bone: if progressive, no improvement by 3 months, suspected tumour
• Blood glucose/HbA1c: screen for diabetes (associated)
• Pregnancy test: if appropriate
• VZV serology: if vesicles present (Ramsay Hunt)

Grading

• House-Brackmann scale: grades I (normal) to VI (total paralysis); guides prognosis and treatment assessment

Pharmacological (NICE CKS)

• Prednisolone 50mg OD × 10 days (or 25mg BD × 10 days; or 60mg × 5 days then taper over 5 days): started within 72 hours of onset; NNT ~10 for complete recovery
• Antivirals: aciclovir 400mg 5×/day or valaciclovir 1g TDS × 7 days — NO proven benefit for Bell palsy when added to steroids (but use for Ramsay Hunt syndrome); NICE does not recommend routine antiviral for Bell palsy

Eye Protection (Essential)

• Artificial tears (hypromellose drops): during the day for corneal dryness
• Eye ointment (Lacri-Lube): at night
• Tape eyelid closed at night (micropore tape)
• Moisture chamber (cling film over eye): if severe lagophthalmos
• Urgent ophthalmology referral if corneal symptoms (pain, redness, visual change)

Physiotherapy

• Facial exercises (gentle) once some recovery begins
• Avoid aggressive electrical stimulation (may worsen synkinesis)

Surgical (Rare)

• Tarsorrhaphy: if persistent lagophthalmos and corneal risk
• Facial nerve decompression: controversial; not standard practice
• Facial reanimation surgery: for severe permanent paralysis (nerve grafts, muscle transfers)

Referral Criteria

• ENT/neurology: atypical features, no improvement by 3 months, complete paralysis at onset (HB grade VI)
• Ophthalmology: corneal complications
• Plastic surgery: if permanent severe paralysis requiring reanimation

~70% make full recovery without treatment. With prednisolone within 72 hours: >90% recover completely. Incomplete paralysis: ~95% full recovery. Complete paralysis: ~60-70% full recovery. Recovery usually begins within 3 weeks; most recovery by 3 months. ~15% have permanent residual weakness. ~5% develop synkinesis (involuntary movement of one facial region during voluntary movement of another — e.g., eye closure when smiling). Ramsay Hunt syndrome: worse prognosis than Bell palsy (~50% full recovery).

House-Brackmann Facial Nerve Grading

UMN vs LMN Facial Weakness