Neuromuscular Emergencies
Neuromuscular Emergencies
What You'll Learn
- GBS: ascending paralysis + areflexia + albuminocytologic dissociation; IVIg or PLEX (NOT both, NOT steroids); monitor FVC every 4–6 hrs — intubate using the GBS 20/30/40 rule: FVC <20 mL/kg, NIF less negative than −30 cmH2O, or MEP <40 cmH2O (distinct from the MG 20/20/20 rule)
- Myasthenic crisis: 20/20/20 rule (FVC <20 mL/kg, NIF <−20 cmH2O, PaCO2 >50) → intubate; IVIg or PLEX equally effective; know the medication avoidance list
- Rhabdomyolysis: CK >5× ULN; aggressive IV NS (target UO >200–300 mL/hr); complications = AKI, hyperkalemia, DIC, compartment syndrome
- Critical illness neuromyopathy: failure to wean + flaccid quadriparesis; CIM (normal SNAPs, better prognosis) vs CIP (reduced SNAPs, worse prognosis); prevention > treatment
- Acute flaccid paralysis DDx: GBS vs botulism vs tick paralysis vs MG crisis vs CINM vs periodic paralysis vs transverse myelitis vs AFM — reflexes, pupils, sensory exam, and NCS pattern differentiate
- NM respiratory failure: FVC is the best single bedside test, but clinical trajectory (rate of decline, work of breathing, bulbar dysfunction, secretion management) and ability to protect the airway matter more than any single FVC value. A patient with a "normal" FVC who is fatiguing, swallowing poorly, or trending down rapidly should be intubated. Conversely, a stable patient just below threshold may not need immediate intubation. Do not rely on SpO2 alone (late finding); diaphragm weakness → orthopnea, paradoxical breathing.
- Toxin-related emergencies: botulism = descending + dilated pupils; organophosphates = SLUDGE + nicotinic (atropine + 2-PAM); black widow = excessive ACh release; tetrodotoxin = Na+ channel block
HighYield Pearls
- GBS 20/30/40 rule: FVC <20 mL/kg, MIP less negative than −30 cmH2O, or MEP <40 cmH2O → intubate. MG crisis uses the 20/20/20 rule (FVC <20, NIF <−20, PaCO2 >50).
- GBS & MG crisis treatment: IVIG or PLEX (equally effective, NOT both). AVOID steroids in GBS (no benefit, may delay recovery). Steroids are standard in MG.
- MG crisis drug triggers: aminoglycosides, fluoroquinolones, β-blockers, IV magnesium, telithromycin, procainamide, neuromuscular blockers — always reviewed on test day.
- Botulism: descending paralysis + bulbar onset + dilated/fixed pupils + dry mouth + autonomic dysfunction → equine antitoxin (adults), BabyBIG (IV human botulism Ig) for infant botulism (constipation, poor feeding, descending hypotonia).
- NMS vs serotonin syndrome: NMS = neuroleptic + lead-pipe rigidity + hyperthermia + autonomic + CK markedly elevated → stop neuroleptic, dantrolene + bromocriptine. Serotonin syndrome = serotonergic drug + clonus (esp lower extremity) + hyperreflexia + tremor → stop drug, cyproheptadine + benzo.
- Malignant hyperthermia: volatile anesthetic or succinylcholine in RYR1 / CACNA1S patient → masseter spasm + rigidity + hyperthermia + hyperkalemia + rhabdo → DANTROLENE + active cooling.
- Organophosphate poisoning: SLUDGE-M + miosis + fasciculations + bradycardia + respiratory failure → ATROPINE + PRALIDOXIME (2-PAM). Watch for intermediate syndrome at 24–96 hr (neck flexor, proximal, respiratory weakness) and OPIDN delayed neuropathy 1–3 wk later.
- Rhabdomyolysis labs: CK >5× ULN + myoglobinuria; expect HYPERKALEMIA, hyperphosphatemia, HYPOCALCEMIA, elevated uric acid; aggressive IVF; urinary alkalinization controversial; watch for compartment syndrome.
- Periodic paralysis attack: hypokalemic → cautious oral/IV KCl + glucose-free fluids; hyperkalemic → oral glucose / IV insulin-dextrose; monitor airway and telemetry.
- Tick paralysis: ascending flaccid paralysis mimicking GBS but with normal CSF + reduced CMAP amplitudes — find and remove the tick → rapid resolution. Anti-MDA5 amyopathic DM with rapidly progressive ILD → early aggressive triple immunosuppression.
🔍 Quick ReferenceClinical · Labs / monitoring · Treatment pearls
Clinical phenotype
- Ascending paralysis + areflexia + recent GI/URI infection → GBS (AIDP)
- Descending paralysis + dilated pupils + dry mouth + bulbar onset → Botulism
- Constipation + poor feeding + floppy infant + descending weakness → Infant botulism
- Neuroleptic + lead-pipe rigidity + hyperthermia + autonomic instability → NMS
- Serotonergic drug + lower-extremity clonus + hyperreflexia + diaphoresis → Serotonin syndrome
- Masseter spasm during induction with volatile anesthetic / succinylcholine → Malignant hyperthermia (RYR1)
- SLUDGE-M + miosis + fasciculations + garlic odor in a farmer → Organophosphate poisoning
- Ascending flaccid paralysis in a child after a hike, normal CSF → Tick paralysis
- Amyopathic DM rash + rapidly progressive hypoxemic ILD → Anti-MDA5 RP-ILD
Labs / monitoring / studies
- FVC <20 mL/kg, MIP >−30, MEP <40 cmH2O → GBS intubation threshold (20/30/40 rule)
- FVC <20 mL/kg, NIF <−20, PaCO2 >50 → Myasthenic crisis (20/20/20 rule)
- Albuminocytologic dissociation in CSF → GBS
- CK >5× ULN + myoglobinuria + hyperK + hypoCa + hyperphosphatemia + elevated uric acid → Rhabdomyolysis
- Markedly elevated CK + leukocytosis + transaminitis with rigidity/fever → NMS
- Reduced CMAP amplitudes with preserved sensory + clinical tick discovery → Tick paralysis
- Low red-cell & plasma cholinesterase activity → Organophosphate exposure
- Anti-GQ1b antibody → Miller Fisher / Bickerstaff; Anti-GT1a → Pharyngeal-cervical-brachial GBS
Treatment pearls
- IVIG or PLEX (NOT both, NOT steroids) → GBS
- IVIG or PLEX equally effective → Myasthenic crisis (steroids OK, often started after PLEX/IVIG begins)
- Equine antitoxin (adults), BabyBIG (infants) → Botulism
- Stop neuroleptic + cooling + IVF + DANTROLENE + BROMOCRIPTINE → NMS
- Stop offending drug + CYPROHEPTADINE + benzodiazepines → Serotonin syndrome
- DANTROLENE + active cooling + stop volatile anesthetic → Malignant hyperthermia
- ATROPINE + PRALIDOXIME (2-PAM); watch for intermediate syndrome & OPIDN → Organophosphate poisoning
- Aggressive IVF (UO >200–300 mL/hr); urinary alkalinization controversial → Rhabdomyolysis
- Find and remove the tick → Tick paralysis; early aggressive triple immunosuppression → Anti-MDA5 RP-ILD
Guillain-Barré Syndrome (GBS)
GBS Subtypes
| Subtype | Antibody | Pathology | NCS Pattern | Prognosis |
|---|---|---|---|---|
| AIDP | None consistently identified | Macrophage-mediated segmental demyelination | Prolonged distal latencies, slow CV, prolonged/absent F-waves, conduction block, temporal dispersion | Good; ~80% walk independently at 6 mo |
| AMAN | Anti-GM1, anti-GD1a | Antibody + complement attack at nodes of Ranvier → axonal degeneration (motor only) | Low CMAPs, normal SNAPs, normal CV & distal latencies; may show transient conduction block / prolonged latencies in first days (reversible conduction failure) before classic axonal pattern emerges | Variable; rapid recovery if reversible conduction failure; poor if severe axonal loss |
| AMSAN | Anti-GM1, anti-GD1a | Motor + sensory axonal degeneration | Low CMAPs AND low SNAPs, normal CV | Poor; prolonged recovery, severe residual deficits |
| Miller Fisher syndrome | Anti-GQ1b (>90%) | Cranial nerve & dorsal root ganglia involvement | Reduced/absent sensory potentials; motor studies often normal | Excellent; self-limited over weeks |
| Bickerstaff brainstem encephalitis | Anti-GQ1b | Brainstem inflammation + overlap with Fisher | Variable; may show CNS abnormalities on MRI | Good with treatment; altered consciousness distinguishes from Fisher |
| Pharyngeal-cervical-brachial | Anti-GT1a (most specific); also anti-GQ1b cross-reactivity | Oropharyngeal + neck + arm weakness (descending pattern) | Abnormal in upper limb + cranial nerves; legs spared | Generally good; may be confused with botulism |
Diagnostic Workup
| Test | Findings | Timing / Notes |
|---|---|---|
| CSF | Albuminocytologic dissociation: elevated protein (>0.45 g/L) with <10 WBC/μL | May be normal in first week; if >50 cells → consider HIV, Lyme, CMV, sarcoid, leptomeningeal disease |
| NCS/EMG (early) | Prolonged/absent F-waves (earliest finding); prolonged distal motor latencies; sural sparing pattern | First 1–2 weeks; may be normal day 1–3 |
| NCS/EMG (late) | Conduction block, temporal dispersion (AIDP); low CMAPs (AMAN/AMSAN); fibrillations at 2–4 weeks if axonal | Repeat at 2–3 weeks if initial study equivocal |
| MRI spine | Enhancing nerve roots (especially cauda equina) on post-contrast T1 | Supports diagnosis; helps exclude myelopathy |
| Antibodies | Anti-GQ1b (Fisher/Bickerstaff); anti-GM1/GD1a (AMAN) | Not required for diagnosis; useful for subtype classification |
🎯 Clinical Pearl
- Sural sparing pattern: sural sensory nerve is normal but median/ulnar sensory nerves are abnormal — highly specific for GBS (AIDP); reflects distal-predominant demyelination affecting longer nerve trunks
- F-wave abnormalities (absent or prolonged) are often the earliest NCS finding because proximal nerve roots are affected first
Treatment
| Therapy | Regimen | Notes |
|---|---|---|
| IVIg | 0.4 g/kg/day × 5 days (total 2 g/kg) | Equally effective as PLEX; easier to administer; risk: aseptic meningitis, renal failure, thrombosis |
| PLEX | 5 exchanges over ~10–14 days | Most effective if started within 7 days; requires central line; risk: hemodynamic instability, line infection |
| Steroids | — | NOT effective in GBS — do not use; may actually delay recovery |
| IVIg + PLEX | — | Do NOT combine — IVIg after PLEX just gets removed; no added benefit |
Respiratory Monitoring & ICU Management
- FVC every 4–6 hours — single most important bedside test
- Intubation criteria (GBS 20/30/40 rule): FVC <20 mL/kg, NIF less negative than −30 cmH2O, MEP <40 cmH2O, >30% decline in FVC from baseline, or inability to protect airway — note: distinct from the MG 20/20/20 rule (FVC <20, NIF <−20, PaCO2 >50)
- Autonomic dysfunction: sinus tachycardia (most common), bradycardia (may require pacing), BP lability, ileus, urinary retention — continuous telemetry required
- VTE prophylaxis: immobilized patients require LMWH or UFH
- Pain management: neuropathic pain is common (gabapentin, pregabalin); opiate-sparing approach preferred
Prognostic Factors
| Poor Prognosis | Good Prognosis |
|---|---|
| Age >60 years | Young age |
| Rapid onset (<7 days to nadir) | Slow progression |
| Need for mechanical ventilation | Preserved walking |
| Preceding Campylobacter jejuni diarrhea | No need for mechanical ventilation |
| AMAN/AMSAN subtype with severe axonal loss | AIDP with demyelinating pattern |
| Low distal CMAP amplitude (<20% of LLN) | Preserved CMAP amplitudes |
Hughes GBS Disability Scale
| Grade | Description |
|---|---|
| 0 | Normal |
| 1 | Minor symptoms, able to run |
| 2 | Walks ≥10 m without assistance but unable to run |
| 3 | Walks ≥10 m with assistance (walker/cane) |
| 4 | Bed- or chair-bound |
| 5 | Requires mechanical ventilation |
| 6 | Dead |
Treatment-Related Fluctuations (TRF) vs Acute-Onset CIDP (A-CIDP)
| Feature | Treatment-Related Fluctuation (TRF) | Acute-Onset CIDP (A-CIDP) |
|---|---|---|
| Frequency | ~10% of treated GBS patients | ~5% of patients initially diagnosed with GBS |
| Timing | Re-worsening within 8 weeks of treatment after initial improvement or stabilization | Deterioration >8 weeks after onset, OR ≥3 deteriorations in the course |
| Pathophysiology | Ongoing immune-mediated injury outlasts effect of single IVIg/PLEX course | Chronic demyelinating polyneuropathy presenting with an acute onset (mimics GBS) |
| Treatment | Repeat IVIg or PLEX (same course); patients usually re-respond | Start chronic immunotherapy (maintenance IVIg, steroids, or steroid-sparing agents) — steroids ARE effective in A-CIDP (unlike GBS) |
| Clinical clue | Single re-worsening episode ≤8 weeks; otherwise typical GBS course | Multiple relapses or late deterioration; revisit diagnosis |
💎 Board Pearl
- Anti-GQ1b = Miller Fisher triad (ophthalmoplegia + ataxia + areflexia); same antibody in Bickerstaff brainstem encephalitis (add altered consciousness)
- AMAN is the most common subtype in Asia and post-Campylobacter — axonal, may have reversible conduction failure mimicking demyelination early on
- Steroids do NOT work in GBS — one of the few autoimmune diseases where steroids are ineffective; this is a classic board question
- CSF pleocytosis >50 cells in suspected GBS → reconsider diagnosis: HIV polyradiculopathy, Lyme, CMV, sarcoidosis, leptomeningeal carcinomatosis
- Re-worsening after initial improvement? Within 8 weeks → TRF (repeat IVIg/PLEX); >8 weeks or ≥3 deteriorations → A-CIDP (start chronic immunotherapy)
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