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Q1: Definitions and the UMN vs LMN discrimination (2 min)
- Trigeminal neuralgia (TN, tic douloureux): recurrent, unilateral, brief (fraction of a second to two minutes), paroxysmal, severe, electric-shock or stabbing pain in one or more trigeminal divisions, provoked by innocuous stimuli; no neurological deficit between attacks (ICHD-3 criteria).
- Bell palsy: acute, unilateral, peripheral (lower motor neurone) facial-nerve (CN VII) palsy of idiopathic cause, developing over less than 72 hours, with no other neurological signs.
- The decisive bedside rule: the upper face has bilateral cortical innervation, so a UMN lesion (stroke) spares the forehead (contralateral lower-face droop only), whereas an LMN lesion (Bell palsy) involves forehead AND eye AND mouth on one side. Forehead spared = UMN = stroke; forehead involved = LMN = Bell palsy.
Q2: Pathophysiology of each condition (3 min)
- TN — neurovascular conflict: a blood vessel (usually the superior cerebellar artery) compresses the trigeminal nerve at the root-entry zone — the transition between central (oligodendrocyte) and peripheral (Schwann) myelin — causing focal demyelination. This produces ephaptic transmission (demyelinated touch A-beta fibres cross-activate pain A-delta/C fibres) and ectopic firing (the ignition hypothesis), explaining why a light touch is felt as electric-shock pain, with a refractory period after each paroxysm.
- TN — secondary: an MS plaque or tumour (meningioma, schwannoma) at the root or ganglion; these patients more often have sensory loss and bilaterality.
- Bell palsy: inflammation and oedema of the facial nerve within the narrowest part of the bony fallopian canal (labyrinthine segment/meatal foramen), most likely viral (HSV-1), causing compression, ischaemia and conduction block (neuropraxia) or, if severe, Wallerian axonal degeneration.
- Branch anatomy localises the level: greater petrosal (tearing → Schirmer test), nerve to stapedius (hyperacusis), chorda tympani (taste/salivation) — 'tears, tastes and taps'.
Q3: Investigations (2 min)
- Both are CLINICAL diagnoses; no routine bloods or imaging for a typical case.
- MRI in TN for any atypical feature — sensory loss, onset under 40, bilaterality, non-response to carbamazepine, deafness/other cranial-nerve deficit — to show neurovascular conflict (CISS/FIESTA sequences) and exclude MS or tumour.
- Imaging/bloods in facial palsy for slow progression, no recovery by 3 to 6 months, recurrence, bilaterality or other signs — MRI; plus Lyme serology, glucose/HbA1c, HIV, ACE level where indicated; ESR/CRP if giant-cell arteritis is possible.
- House-Brackmann grade (I normal to VI complete paralysis) quantifies severity and predicts recovery; ENoG (>90 percent degeneration in 2 weeks predicts poor outcome) for severe palsy.
Q4: Management — definitive and stepwise (3 min)
- TN medical: carbamazepine 200 mg bid, titrate to 600 to 1200 mg/day (max 1600); ~70 percent respond. Alternatives/add-ons: oxcarbazepine, lamotrigine, gabapentin, pregabalin, baclofen (esp. MS), phenytoin. Monitor sodium (SIADH), rash, FBC.
- TN surgical (refractory): microvascular decompression (Jannetta) — best long-term relief (~70 percent pain-free at 5 years) but surgical risk (hearing loss, CSF leak, meningitis, stroke, death <1 percent); gamma knife radiosurgery — non-invasive, delayed, risk of numbness; percutaneous procedures (radiofrequency, balloon compression, glycerol) via foramen ovale — for frail/elderly, immediate relief but sensory loss and recurrence.
- Bell palsy: oral prednisolone 60 mg daily for 5 days then taper 5 days, WITHIN 72 HOURS (recovery 80 to 94 percent); antivirals add marginal benefit (essential in Ramsay Hunt); eye protection mandatory (lubricants by day, ointment + taping at night, tarsorrhaphy/botulinum ptosis for severe lagophthalmos); facial physiotherapy.
Q5: Complications, prognosis and red flags (2 min)
- TN complications: depression, weight loss/dehydration from food avoidance; carbamazepine SIADH hyponatraemia, Stevens-Johnson syndrome; surgical risks (above).
- Bell palsy complications: exposure keratopathy/corneal ulcer (preventable blindness), residual weakness, synkinesis/aberrant regeneration (crocodile tears — eye waters on eating), hemifacial spasm, contracture.
- Prognosis: Bell palsy ~85 percent recover fully untreated, rising to ~94 percent with early steroids; recovery begins ~3 weeks, complete by 3 to 6 months. Worse if complete paralysis at onset, age over 60, diabetes, pregnancy, severe pain, hyperacusis, no recovery by 3 weeks, ENoG >90 percent. Ramsay Hunt worse (~70 percent). TN is chronic relapsing-remitting; MVD gives the most durable relief.
- Red flags: forehead sparing = stroke; ear vesicles = Ramsay Hunt; TN with sensory loss/young/bilateral = MS or tumour; bilateral facial palsy = GBS/sarcoid/Lyme/HIV; no recovery by 3 months = reassess.