Facial Nerve Palsy (Bell Palsy)
A boards-focused lesson on Bell palsy: acute idiopathic peripheral (LMN) CN VII paralysis, keyed to the forehead-sparing UMN-vs-LMN distinction, classic vignette buzzwords, and the next-best-step management chain (clinical dx → early steroids + eye protection, antivirals reserved for Ramsay Hunt/severe cases).
Bell palsy is acute, idiopathic, unilateral lower motor neuron (LMN) paralysis of cranial nerve VII — the most common cause of acute facial paralysis. Reactivation of HSV-1 in the geniculate ganglion is the leading proposed mechanism: nerve edema and compression within the narrow bony facial canal produce dysfunction. Onset is rapid (hours to <72 h), often preceded by retroauricular pain. Because the lesion is peripheral, weakness involves the entire hemiface, including the forehead — the single most tested feature separating it from a central (UMN) lesion, in which the forehead is spared by bilateral cortical innervation. Classic risk factors: diabetes, pregnancy (3rd trimester), and recent viral URI. Diagnosis is clinical (essentially one of exclusion); routine imaging and labs are not required in typical cases. Most patients recover, but early corticosteroids meaningfully improve the odds of complete recovery.
- LMN sign: entire half of face weak — cannot wrinkle forehead, close eye, or smile on the affected side
- Bell phenomenon: eyeball rolls up and out on attempted eye closure (incomplete closure = lagophthalmos)
- Extra features localize the lesion within the facial canal: hyperacusis (stapedius denervation), loss of taste on anterior 2/3 of tongue (chorda tympani), decreased lacrimation/salivation
- Other signs: drooping mouth corner, loss of nasolabial fold, drooling, food pocketing
- Forehead SPARED → think central (stroke/tumor) → get neuroimaging; forehead involved → peripheral → Bell
- Bilateral facial palsy is NOT typical Bell — think Lyme disease, sarcoidosis (Heerfordt), GBS
- Severity graded by the House–Brackmann scale (I–VI)
- ~70–85% recover fully; worse prognosis with complete paralysis, older age, diabetes, and hyperacusis
| Feature | UMN (central) | LMN (peripheral / Bell) |
|---|---|---|
| Forehead | Spared (bilateral cortical input) | Weak — whole hemiface |
| Typical cause | Stroke, tumor (contralateral) | Bell palsy, VII-canal lesion (ipsilateral) |
| Eye closure | Relatively preserved | Impaired → lagophthalmos |
| Other clues | Arm/leg weakness, dysarthria | Hyperacusis, taste loss, dry eye |
| Next best step | Neuroimaging (stroke workup) | Clinical dx → steroids + eye protection |
Vignette: A 34-year-old woman 32 weeks pregnant wakes with right-sided facial droop. She cannot raise her right eyebrow, close her right eye, or smile on the right; there is drooling, and she reports sounds seem abnormally loud in the right ear. No limb weakness, no rash, sensation intact. Symptoms peaked over ~2 days.
Diagnosis: Bell palsy (peripheral CN VII palsy) — forehead involvement + hyperacusis localize to the facial nerve; intact limbs make stroke unlikely.
Next best step: This is a clinical diagnosis — no imaging needed. Start oral corticosteroids within 72 h of onset (prednisone is considered safe in pregnancy) plus aggressive eye protection — artificial tears by day, lubricating ointment + taping/patch at night — to prevent exposure keratopathy. Antivirals are not required for typical Bell palsy.
Vignette: A 60-year-old man has left facial paralysis, ear pain, and vertigo. Exam reveals painful vesicles in the left ear canal and on the auricle plus left-sided sensorineural hearing loss.
Diagnosis: Ramsay Hunt syndrome (herpes zoster oticus) — VZV reactivation at the geniculate ganglion, NOT idiopathic Bell palsy. The vesicular rash + hearing loss/vertigo is the giveaway; prognosis is worse than Bell.
Next best step: Corticosteroids PLUS an antiviral (e.g., valacyclovir or acyclovir) — antivirals are clearly indicated here, unlike in typical Bell palsy. Protect the eye as well.
Board rule of thumb: facial palsy + ear vesicles = Ramsay Hunt = add antiviral.
Facial nerve (CN VII) motor branches, superior → inferior — "To Zanzibar By Motor Car":
- Temporal
- Zygomatic
- Buccal
- Marginal mandibular
- Cervical
(Equally common variant: "Ten Zebras Bit My Cookies.")
Forehead rule: a stroke spares the brow, Bell spares nothing — a peripheral (LMN) CN VII lesion weakens the whole side of the face, while a central (UMN) lesion spares the forehead because the frontalis receives bilateral cortical input.
- Corticosteroids are first-line: start within 72 h of onset (e.g., prednisone 60 mg/day for ~1 week, or prednisolone). Improves the rate of complete recovery.
- Antivirals (valacyclovir/acyclovir): NOT needed for typical Bell palsy; add to steroids only in severe/complete palsy (modest benefit) or when VZV / Ramsay Hunt is suspected.
- Eye protection is mandatory (incomplete closure → exposure keratitis / corneal ulceration): artificial tears by day, lubricant + patch at night.
- When to image or reconsider the diagnosis: gradual onset, bilateral or recurrent palsy, other cranial nerves involved, forehead spared, or no improvement by 3–4 months → work up tumor, stroke, Lyme, sarcoidosis.
- Prognosis: most improve within 3 weeks; 70–85% recover fully. Residual synkinesis can occur.
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