Bell's Palsy - A Complete Teaching

1. Definition and Naming

1. Paralysis or paresis of all muscle groups of one side of the face
2. Sudden onset
3. Absence of signs of CNS disease
4. Absence of signs of ear or cerebellopontine angle disease

2. Epidemiology

• Annual incidence: 23-37 per 100,000 population (approximately 1 in 60 persons over a lifetime)
• Higher in those >65 years (59/100,000); lower in children <13 years (13/100,000)
• Equal sex ratio overall; slight female predominance in those <20 years; slight male predominance in those >40 years
• Left and right sides equally affected
• ~30% present with incomplete paralysis; ~70% with complete paralysis
• Bilateral paralysis: only 0.3% of cases (bilateral should prompt search for systemic disease)
• Recurrence: ~7-8% of cases (average 10-year interval between episodes)
• Family history: 8% of patients
• Risk factors: pregnancy (especially 3rd trimester, peripartum), diabetes mellitus, and possibly hypertension

3. Anatomy of the Facial Nerve (CN VII)

• Nucleus in the caudal pons (facial motor nucleus)
• Loops around the abducens nucleus (forms the facial colliculus)
• Exits at the cerebellopontine angle (CPA)

1. Labyrinthine segment - narrowest, most vulnerable segment; site of greatest nerve swelling and compression in Bell's palsy
2. Geniculate ganglion - location of the HSV/VZV reactivation; gives off the greater superficial petrosal nerve (lacrimation, salivation)
3. Tympanic (horizontal) segment
4. Mastoid (vertical) segment - gives off the nerve to stapedius, then the chorda tympani (taste anterior 2/3 tongue; submandibular/sublingual salivation)

4. Etiology and Pathophysiology

• HSV-1 reactivation from the geniculate ganglion is the leading cause. Murakami et al. (1996) using PCR detected HSV type 1 DNA in endoneurial fluid surrounding CN VII in 11 of 14 Bell's palsy cases
• VZV (varicella-zoster virus) reactivation accounts for up to one-third of cases (sometimes without skin vesicles - "zoster sine herpete")
• Other implicated agents: SARS-CoV-2, HIV (at seroconversion), EBV, CMV

• Microcirculatory failure of the vasa nervorum (dilated engorged veins, hemorrhage in the IAC - Fowler's autopsy findings)
• Ischemic neuropathy
• Autoimmune reaction

• Diffuse demyelination throughout the intratemporal course
• Lymphocytic infiltration and phagocytosis of myelin by macrophages
• Most severe changes at the labyrinthine segment and meatal foramen
• Engorged veins, hemorrhage within the IAC
• Osteoclastic bone resorption around the geniculate ganglion (in severe cases)

5. Clinical Features

• Retroauricular/post-auricular pain - present in ~50%; may precede paralysis by 1-2 days (common, can be intense)
• Taste impairment (chorda tympani involvement) - anterior 2/3 tongue; present in most but rarely persists beyond 2nd week
• Hyperacusis (stapedius muscle paralysis) - present in up to 30%; ipsilateral ear
• Decreased lacrimation (greater petrosal nerve) - dry eye; clinically important for eye care
• Numbness/fullness of the face - reported by patients; formal sensory testing usually normal (pure motor nerve)
• Mild CSF lymphocytosis - in a small number of cases

6. House-Brackmann Grading Scale

7. Investigations

1. Typical presentation (acute-onset, unilateral, peripheral pattern)
2. No risk factors for other causes
3. No skin vesicles in the ear canal
4. Normal neurologic exam (other than CN VII)

• Bilateral palsy → Lyme titer, HIV, ACE, CXR (sarcoidosis), LP (Guillain-Barré)
• Slowly progressive → MRI (tumor)
• Recurrent palsy → consider Melkersson-Rosenthal syndrome
• Signs of herpes zoster → confirm Ramsay Hunt

• Electroneuronography (ENoG): Evoked compound muscle action potential; compares affected to normal side. Used for prognosis in complete palsy (performed 4-14 days after onset)

  • 90% degeneration on ENoG: poor prognosis; may consider surgical decompression

• Electromyography (EMG): Detects fibrillation potentials (denervation) or voluntary motor unit potentials (regeneration)
  • Evidence of denervation after 10 days = axonal degeneration → expect 3+ months before recovery, may be incomplete
  • Voluntary motor units = ongoing reinnervation, good sign
• Nerve conduction (blink reflex): Assesses conduction across the geniculate ganglion

8. Differential Diagnosis

9. Treatment

A. Eye Care (IMMEDIATE - most important)

• Artificial tears (lubricating eye drops) during the day
• Lubricating eye ointment at night
• Tape eyelid shut or moisture chamber goggles during sleep
• If lagophthalmos is severe: ophthalmology referral; consider temporary tarsorrhaphy

B. Corticosteroids (First-line treatment)

• Prednisone 60-80 mg/day for the first 5 days, then taper over 5 days (Harrison's) OR
• Prednisolone 25 mg twice daily for 10 days (Sullivan et al. RCT protocol)
• Must be started within 72 hours of onset for maximum benefit (ideally within 48 hours)
• Mechanism: reduces nerve edema/inflammation, decreases compression within the fallopian canal
• Evidence: Multiple RCTs (Sullivan 2007, Engstrom 2008) showed statistically significant benefit; NNT approximately 6

C. Antiviral Agents (Adjunct - contested)

• Acyclovir 400 mg 5× daily × 10 days OR Valacyclovir 1000 mg daily × 5-7 days (better bioavailability)
• Evidence is mixed:
  • Major RCT (Sullivan et al.): antiviral alone = no benefit over placebo; combination = no significant additional benefit over steroids alone
  • Hato et al.: suggested additive benefit of valacyclovir + prednisolone particularly in complete palsy
  • Cochrane review: low-quality evidence for combination benefit; moderate-quality evidence for reduction of synkinesis with combination therapy
• Current consensus: Antivirals should be added if vesicular lesions are seen (ear canal, palate) - confirming HSV/VZV involvement. In pure Bell's palsy, steroids alone are standard; antivirals remain optional but are commonly co-prescribed in severe/complete palsy

D. Surgical Decompression

• Highly controversial and not routinely recommended
• Considered in severe cases with >90% degeneration on ENoG within 14 days, no recovery by 4-6 weeks
• Approach: middle cranial fossa decompression of the labyrinthine segment (highest-yield site)
• Risk: sensorineural hearing loss
• Most guidelines and systematic reviews do NOT support routine decompression
• Adams & Victor: "There is no evidence that surgical decompression of the facial nerve is effective, and it may be harmful."

E. Physical Therapy

• Facial exercises, neuromuscular re-education
• Most useful during recovery phase and for sequelae (synkinesis rehabilitation)
• 2025 systematic review (PMID 39647183) supports physical therapy for idiopathic facial paralysis

F. Surgical Rehabilitation (Late/Permanent Paralysis)

• Static procedures: Gold weight implant (upper eyelid loading for lagophthalmos), facial slings
• Dynamic reinnervation: Hypoglossal-facial nerve anastomosis; cross-facial nerve graft
• Gracilis free muscle flap: For long-standing complete paralysis
• Botulinum toxin: For synkinesis management

10. Prognosis

• ~80-90% recover completely
• ~70% recover completely within 1-2 months
• ~85% achieve near-normal function
• Incomplete paralysis at presentation: 95-100% complete recovery (most favorable)
• Evidence of denervation on EMG after 10 days: long delay (3+ months) before recovery; may be incomplete; proceeds via axonal regeneration (up to 2 years)
• Most favorable prognostic sign: some voluntary motor recovery in the first 5-7 days; early return of taste (first week)

• Complete paralysis at onset
• Age >60 years
• Diabetes mellitus / hypertension
• Hyperacusis + decreased tearing (high lesion level)
• Severe pain at onset
• Late onset of recovery

11. Sequelae

1. Synkinesis - aberrant regeneration; voluntary movement of one muscle group triggers involuntary movement in another (e.g., eye closes when smiling - "crocodile tears" variant); managed with botulinum toxin + physiotherapy
2. Contracture - persistent hemifacial hypertonicity
3. Crocodile tears (Bogorad syndrome) - gustatory lacrimation; aberrant regeneration of greater petrosal nerve fibers to lacrimal gland (tears when eating/salivating)
4. Hemifacial spasm (rare late sequela)

12. Special Situations

Pregnancy

• 3× increased incidence; highest risk in 3rd trimester and first 2 weeks postpartum
• Managed identically to non-pregnant patients (corticosteroids are safe in pregnancy)
• Collaborative management with obstetrician

Children

• Less common; 8% of Bell's palsy patients are children
• Evidence for corticosteroids less clear in children (some studies show no benefit)
• Generally favorable prognosis

Bilateral Bell's Palsy (0.3% of cases)

• Must actively exclude: Lyme disease, Guillain-Barré syndrome, sarcoidosis, HIV, bilateral acoustic neuromas

13. Key Teaching Points Summary

• Harrison's Principles of Internal Medicine 22e, p. 3597-3598
• Adams and Victor's Principles of Neurology, 12th Ed., p. 1371-1372
• Cummings Otolaryngology Head and Neck Surgery (Bell's Palsy chapter)
• Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol 2 (Grading Systems)
• Recent systematic reviews: physical therapy (PMID 39647183), photobiomodulation (PMID 39627559), laser therapy (PMID 39546047)

Bell's Palsy for 1st Year MBBS

Level: Anatomy + basic Physiology foundation | Tone: Simple, story-like, exam-ready

🧠 Start With a Story

Part 1: The Facial Nerve (CN VII) - The "Star" of the Story

Functions of CN VII (memorize these)

Mnemonic for CN VII branches at the parotid (terminal motor branches): "Two Zebras Bit My Cat" → Temporal, Zygomatic, Buccal, Marginal mandibular, Cervical

Course of the Facial Nerve (the "Long Journey")

BRAINSTEM (Pons)
     ↓
Facial nucleus + Superior salivatory nucleus
     ↓
Internal Acoustic Meatus (with CN VIII - the hearing nerve)
     ↓
INTRATEMPORAL (inside the temporal bone = "fallopian canal")
     ├─ Labyrinthine segment → GENICULATE GANGLION
     │        └── Greater Petrosal Nerve → lacrimal gland (tears)
     ├─ Tympanic (horizontal) segment
     │        └── Nerve to Stapedius
     └─ Mastoid (vertical) segment
              └── Chorda tympani → taste + sublingual/submandibular saliva
     ↓
STYLOMASTOID FORAMEN (exits the skull here)
     ↓
PAROTID GLAND (divides into 5 terminal branches)
     ↓
5 branches to muscles of facial expression

Key exam point: The facial nerve exits the skull at the stylomastoid foramen. In Bell's palsy, the nerve gets swollen and trapped inside the bony canal (specifically at the narrowest part - the labyrinthine segment).

Part 2: What is Bell's Palsy?

What causes it?

Part 3: The #1 Anatomy Lesson in Bell's Palsy

Upper Motor Neuron (UMN) vs. Lower Motor Neuron (LMN) facial palsy

Exam shortcut: "Forehead spared = Central lesion." "Forehead affected = Peripheral (Bell's palsy)."

Part 4: Clinical Features (What the Patient Shows)

Motor symptoms (you can SEE these):

1. Cannot wrinkle the forehead (frontalis muscle paralyzed)
2. Cannot close the eye (orbicularis oculi paralyzed) → the eye stays open, even at night → lagophthalmos
3. Bell's phenomenon - when the patient tries to close the eye, the eyeball rolls UPWARD (you can see the white of the eye). This is a normal protective reflex that only becomes visible because the lid won't close.
4. Drooping of the mouth on the affected side
5. Cannot smile symmetrically - mouth pulled to the normal side
6. Cannot puff cheeks / whistle - air leaks from the weak side
7. Food falls out of the corner of the mouth
8. Flat nasolabial fold on the affected side

Non-motor symptoms (from branch involvement):

Simple rule: The more symptoms the patient has (taste + dry eye + hyperacusis), the higher the lesion and the worse the prognosis.

Part 5: Why the Eye is the Emergency

• The cornea (transparent front of the eye) is exposed day and night
• It dries out
• It can get infected or develop a corneal ulcer
• This can lead to permanent vision loss

Part 6: Remembering All the Features - A Mnemonic

Part 7: How the Diagnosis is Made

1. Sudden onset (within hours to a few days)
2. All facial muscles on one side are weak (including forehead - this rules out a stroke)
3. No other neurological signs
4. No ear disease, no tumor

A simple bedside test: Ask the patient to raise their eyebrows. In Bell's palsy, the eyebrow on the affected side does NOT move. In a stroke, it does.

Part 8: Treatment (Simple Version)

Part 9: Prognosis (Outcome)

• 80-90% of patients recover completely, usually within 2-3 months
• The most favorable sign: if some facial movement returns within the first 5-7 days
• If taste comes back in the first week - also a good sign
• A small group (~10-15%) have incomplete recovery with some residual weakness or synkinesis (abnormal linked movements, e.g., eye closes when smiling)

Part 10: Comparison Table - Bell's Palsy vs. Stroke (UMN palsy)

Part 11: Quick Exam Revision Summary

What is Bell's palsy? Acute idiopathic unilateral LMN facial nerve (CN VII) palsy due to HSV-1 reactivation causing inflammation and entrapment of the nerve in the fallopian canal.

What is the key clinical sign? Unilateral facial weakness involving the FOREHEAD (distinguishes it from central/UMN palsy).

What is Bell's phenomenon? Upward rolling of the eyeball when attempting eye closure (normal reflex unmasked by orbicularis oculi paralysis).

What is the most urgent complication? Corneal exposure keratopathy due to lagophthalmos → treat with artificial tears + eyelid taping.

What is the treatment? Eye protection (immediate) + Oral corticosteroids (within 72 hours of onset).

What is the prognosis? 80-90% complete recovery.

Lesion site? Labyrinthine segment of CN VII inside the fallopian canal (temporal bone), near the geniculate ganglion - the narrowest and most vulnerable segment.

Anatomy Diagram Summary