Bell palsy and cranial nerve palsies

Bell palsy is an acute, idiopathic, unilateral lower motor neurone paralysis of the facial nerve (CN VII), and is the commonest cause of acute unilateral facial weakness. The Fellowship decision the candidate must make at the bedside is single and load-bearing: is this a lower motor neurone (peripheral, usually Bell) palsy or an upper motor neurone (central, stroke) lesion? — because that distinction decides whether the patient is discharged on a steroid taper with eye care or sent to the stroke pathway. The second decision is eye protection, because the unprotected open eye ulcerates. Roughly 70 per cent of patients recover full facial function.[6]

Definition and classification

Bell palsy is defined as acute, unilateral, idiopathic paralysis of the facial nerve of lower motor neurone type, reaching maximal weakness within 72 hours, in the absence of an identifiable cause. It is a diagnosis of exclusion — the label applies only once stroke, infection (otitis media, Ramsay Hunt, Lyme), tumour and systemic causes have been ruled out at the bedside and by targeted testing.[7]

Classification runs along two axes. By lesion level: a lower motor neurone (LMN) lesion — Bell palsy, Ramsay Hunt, Lyme — denervates all ipsilateral facial muscles including frontalis; an upper motor neurone (UMN) lesion — cortical stroke — spares the forehead because frontalis receives bilateral cortical innervation while the lower face receives only contralateral input. By aetiology: idiopathic (Bell), viral reactivation (herpes simplex virus type 1 is the leading proposed trigger; varicella zoster virus causes Ramsay Hunt), bacterial (Lyme, otitis media), and structural (cholesteatoma, parotid or cerebellopontine angle tumour).[1][6]

Pathophysiology — the fallopian canal and the five facial nerve functions

The facial nerve is a mixed nerve carrying five functional components, and a Bell palsy can impair any combination of them: motor to the muscles of facial expression (forehead, eye closure, smile); parasympathetic to the lacrimal and submandibular/sublingual glands (lacrimation, salivation); special sensory taste from the anterior two-thirds of the tongue; and a small somatic sensory branch to the external ear. The clinical signature of a complete LMN palsy is therefore drooping of one entire hemiface, an eye that will not close (with eyeball rolling upwards on attempted closure — the Bell phenomenon), hyperacusis from stapedius paralysis, reduced taste, and reduced lacrimation.[6]

The mechanism is oedema of the nerve within the narrow bony fallopian canal, most critically at the narrowest segment near the geniculate ganglion. Swelling (proposed viral, ischaemic, or inflammatory) compresses the nerve against unyielding bone, producing conduction block and, in severe cases, axonal degeneration — which is why early anti-oedema therapy with corticosteroids improves recovery and why delay beyond 72 hours forfeits the benefit.[1][3]

Clinical presentation

The onset is rapid, over hours to a maximum of 72 hours, and the patient (or a mirror, or a photo) usually notices the asymmetry first. The face droops on one side: the forehead is smooth and will not wrinkle on attempted frowning, the palpebral fissure is widened and the eye cannot be closed, the nasolabial fold is flattened, and the corner of the mouth droops, causing drooling and a slurred quality to labial consonants. Associated features reflect the loss of the other facial nerve functions: hyperacussis (sounds seem loud and harsh on the affected side because the stapedius cannot dampen the ossicles), altered or reduced taste over the anterior two-thirds of the tongue, and reduced tearing. Pain around the ear or jaw, reduced hearing, tinnitus or vertigo may accompany the palsy but prominent ear pain with vesicles or hearing loss should redirect the diagnosis to Ramsay Hunt.[6][7]

The red flags that argue against simple Bell palsy are a forehead-sparing pattern (UMN stroke), vesicles in the ear or mouth or on the tympanic membrane (Ramsay Hunt), otitis media or mastoiditis on otoscopy, bilateral weakness (GBS, Lyme, sarcoid), slowly progressive weakness beyond 72 hours (tumour), recurrent palsy (rule out tumour), involvement of other cranial nerves (basilar meningitis, cerebellopontine angle tumour), and any limb weakness, dysarthria, or visual field defect (stroke).[7]

Differential diagnosis — LMN versus UMN, and the dangerous mimics

The differential is the heart of the Fellowship question, because every unilateral facial weakness must first be classified as LMN or UMN, and then the LMN group subdivided into the dangerous secondary causes. The single discriminating sign is the forehead. [1]

The broader cranial nerve lens adds the dangerous ocular motor palsies the ED candidate must not miss. A pupil-involving CN III palsy — ptosis, the eye deviated "down and out," and a dilated unreactive pupil — is a posterior communicating artery aneurysm compressing the nerve until proven otherwise, and demands emergency CT angiography. (The pupil is spared in a microvascular/diabetic third, because the parasympathetic pupilloconstrictor fibres run in the nerve's periphery and survive an ischaemic core lesion.) A CN VI palsy (failure of abduction, horizontal diplopia worse on lateral gaze) can be a false-localising sign of raised intracranial pressure. Any isolated ocular motor palsy with pain, a dilated pupil, trauma, or a stepwise progression is imaged.[7]

Bedside assessment

Examine all the cranial nerves, not just the face. On the facial nerve itself, grade severity by asking the patient to raise the eyebrows (frontalis), gently close the eyes (orbicularis oculi — observe for Bell phenomenon and incomplete closure), and smile, puff out the cheeks, and show the teeth (orbicularis oris and buccinator). The standard severity scale is House–Brackmann (below). Perform otoscopy on the affected ear — looking for vesicles (Ramsay Hunt), a bulging or erythematous tympanic membrane (otitis media), or a pearly mass (cholesteatoma). Examine the tongue and oral mucosa for vesicles. Perform a full neurological examination including the limbs (strength, tone, reflexes), speech, and visual fields to exclude stroke, and look for a rash (erythema migrans) and lymphadenopathy.[6][7]

House–Brackmann facial nerve grading

The House–Brackmann scale (1985) grades facial nerve function from normal to total paralysis and is the universal reporting and prognostic standard.[6]

Investigations — a clinical diagnosis, with targeted testing

Bell palsy is a clinical diagnosis. Routine bloods (full blood count, electrolytes, glucose, inflammatory markers) and an HbA1c screen for diabetes are reasonable baseline tests, because diabetes is both a risk factor and a cause of alternative cranial nerve palsies. Add Lyme serology in an endemic area or with bilateral disease, an HIV test when risk factors are present, and a pregnancy test in women of childbearing potential (pregnancy multiplies the risk of Bell palsy roughly three-fold). [7]

Imaging is not required for a typical first Bell palsy. Order CT or, preferably, MRI of the brain and temporal bone when the palsy is atypical — slow progression beyond 72 hours, recurrent, bilateral, involving other cranial nerves, with an abnormal ear, after head trauma, or with any stroke feature — to exclude stroke, tumour, cholesteatoma, or demyelination. Electroneurography (ENoG) and electromyography grade axonal degeneration and prognosticate, but these are outpatient investigations for the 10 to 15 per cent with no recovery by three weeks, not ED tests. A lumbar puncture is reserved when Guillain–Barre, Lyme neuroborreliosis, or basilar meningitis is suspected.[6][7]

Immediate management — steroids, antivirals, and the cornea

Three interventions define ED management: oral corticosteroid within the time window, antiviral therapy in the appropriate context, and immediate corneal protection. Intravenous access and resuscitation are rarely required unless vomiting or an alternative diagnosis dominates.[1][7]

Corticosteroid. Give oral prednis(ol)one to every patient seen within 72 hours of onset, at a dose of 50 mg once daily for 5 days, then tapered over the following 5 days (for example 50 mg daily on days 1–5, then 25 mg, 20 mg, 15 mg, 10 mg, 5 mg). The landmark Scottish trial (Sullivan 2007) used 60 mg daily for 5 days then tapered, and the AAO-HNS guideline recommends a total of approximately 1 mg/kg per day (typically 50–60 mg daily) for 5 to 7 days with a taper. The Cochrane meta-analysis confirms corticosteroids increase the rate of complete recovery at nine months (number needed to treat approximately 6 to 11).[1][3][7]

Antiviral therapy. Antivirals alone confer no benefit in idiopathic Bell palsy, and the addition of an antiviral to corticosteroid in Bell palsy shows no convincing added benefit over corticosteroid alone (Sullivan 2007 aciclovir; Engström 2008 valaciclovir; Cochrane 2009). Antivirals are therefore not routine for Bell palsy. They are reserved for Ramsay Hunt syndrome, where varicella zoster is the cause: combine high-dose antiviral — aciclovir 800 mg orally five times daily for 7 to 10 days, or valaciclovir 1 g three times daily — with corticosteroid. (The aciclovir 400 mg five-times-daily regimen used in the Bell palsy trials is sub-therapeutic for herpes zoster oticus; use the higher zoster dose.) Severe Ramsay Hunt or immunocompromise warrants intravenous aciclovir and ENT admission.[1][2][4][5][7]

Eye protection. Corneal exposure is the principal acute complication of an unprotectable eye and can progress to exposure keratitis, ulceration, and sight loss. Prescribe preservative-free lubricating eye drops hourly by day, lubricating ointment at night, and mechanical closure of the eyelid at night with surgical tape or a moisture chamber; advise the patient not to drive if binocular vision is impaired. If the eye cannot be closed at all (House–Brackmann IV–VI), refer urgently to ophthalmology — a temporary tarsorrhaphy or botulinum-induced ptosis may be required.[7]

Complications and pitfalls

The complications, in descending danger, are the missed stroke (a UMN lesion mislabelled as Bell palsy — the forehead test is the safeguard), corneal exposure keratitis and ulceration from the unprotected eye, residual facial weakness and synkinesis (aberrant regeneration causing eye closure on smiling, or "crocodile tears"), missed Ramsay Hunt (with consequent worse facial and hearing recovery), and missed bilateral or systemic disease (GBS, Lyme, sarcoid). The recurring pitfalls at the bedside are: failing to examine the forehead and so misclassifying a stroke; omitting otoscopy and missing Ramsay Hunt or otitis media; prescribing an antiviral without a steroid in routine Bell palsy; starting steroids too late (beyond 72 hours) or in inadequate dose; and forgetting eye care entirely.[1][5][7]

Prognosis and disposition

Prognosis is favourable for the majority. In Peitersen's 2500-patient natural-history series, 71 per cent recovered completely without treatment and 84 per cent attained a good functional outcome; recovery began within three weeks in most and was complete by six months.[6] Corticosteroid within 72 hours raises the complete-recovery rate further.[1][3] Poor prognostic markers are complete paralysis at onset (House–Brackmann V–VI), age over 60, severe pain, diabetes, hypertension, and Ramsay Hunt syndrome (only about half recover good facial function).[5][6]

Disposition is outpatient for a typical Bell palsy: steroid course, eye care, and a written safety-net for new neurological symptoms, with GP or ENT review at 3 to 4 weeks. Admit for Ramsay Hunt with severe pain or systemic features (for intravenous antiviral), any bilateral or atypical palsy requiring work-up (GBS, Lyme, meningitis), and any patient in whom a dangerous mimic cannot be confidently excluded. Refer to ophthalmology if the eye is unprotected, and to ENT/neurology for the 10 to 15 per cent without recovery by three weeks.[7]

Special populations

Pregnancy multiplies the risk of Bell palsy roughly three-fold, particularly in the third trimester and early puerperium; oral prednisolone is used, with ophthalmology oversight of the eye, and the prognosis is similar to non-pregnant adults. Diabetes is both a risk factor and a prognostic marker — screen for it, and remember that an isolated CN III palsy in a diabetic is usually a pupil-sparing microvascular palsy that recovers over months (still image if painful or the pupil is involved). Children have an excellent prognosis; exclude otitis media as the cause. Recurrent Bell palsy (roughly 7 per cent) and familial cases warrant imaging to exclude a structural lesion. Immunocompromised patients are at higher risk of Ramsay Hunt and disseminated zoster — use intravenous antivirals.[6][7]

The systematic cranial nerve examination in the ED (CN I–XII)

Every facial-weakness patient deserves a full cranial nerve screen, both to confirm the lesion is isolated (Bell palsy) and to surface the dangerous companion signs of a cerebellopontine angle tumour, basilar meningitis, or brainstem stroke. Examine cranially to caudally, in order, so nothing is omitted.[6][7]

The reflex trio — pupillary, corneal, and gag

These three reflexes interrogate pairs of cranial nerves at the bedside and are disproportionately high-yield in the Fellowship viva because they localise the lesion precisely.[9]

The other cranial nerve palsies — the complete ED reference

Beyond the facial nerve, the cranial nerve palsies that present to the emergency department fall into a small number of patterns the candidate must recognise instantly, because each carries a specific dangerous diagnosis.[9]

Corticosteroid dosing across the landmark Bell palsy trials

The dose and duration of oral corticosteroid vary slightly between the major trials and guidelines, but all converge on roughly 1 mg/kg per day (50–60 mg) started within 72 hours, with a taper.[1][2][3][7]

Trial cards — the evidence base for Bell palsy and cranial nerve palsies

ED disposition pathway — Bell palsy and the dangerous mimics

High-yield Fellowship viva pearls

SAQs

Red flags