EM Command Center / Neurology & Psychiatry • 3 min read • 4 sections

Acute Neuromuscular Weakness & Cord Syndromes

Critical emergency reference for rapid neuromuscular respiratory failure and compressive myelopathies. Covers bedside spirometry thresholds (the 20/30/40 rule: FVC < 20 mL/kg, NIF < -30 cmH2O), distinguishing myasthenic from cholinergic crises, Guillain-Barré management (IVIG vs. plasmapheresis, avoiding succinylcholine), and cauda equina syndrome.

Resuscitation Quick Ribbon (First 2 Minutes)

High-Acuity

Monitor respiratory mechanics:Check bedside NIF and FVC every 2-4 hours; do not rely on pulse oximetry.

Intubation triggers (The 20/30/40 Rule):FVC < 20 mL/kg, NIF worse than -30 cmH2O (e.g. -20 cmH2O), or Single Breath Count < 20.

Guillain-Barré Syndrome:Ascending symmetric weakness + areflexia; start IVIG 0.4 g/kg/day for 5 days or Plasmapheresis; NEVER give corticosteroids (ineffective); NEVER use succinylcholine (fatal hyperkalemia).

Myasthenic Crisis:Fatigable weakness, ptosis, diplopia, bulbar dysphagia; start IVIG or Plasmapheresis; treat underlying infection; stop/hold oral acetylcholinesterase inhibitors if copious secretions.

Cauda Equina / Cord Compression:Saddle anesthesia, urinary retention (PVR > 200 mL), decreased rectal tone; STAT MRI spine + emergency neurosurgery consultation.

Bottom-Line Clinical Pearl

In acute neuromuscular weakness (GBS or Myasthenia Gravis), pulse oximetry and arterial blood gases remain deceptively normal until catastrophic diaphragm fatigue occurs. Monitor bedside Negative Inspiratory Force (NIF < -30 cmH2O) and Forced Vital Capacity (FVC < 20 mL/kg) to predict the need for elective endotracheal intubation.

1. Bedside Respiratory Mechanics: The 20/30/40 Rule

Respiratory failure in neuromuscular disease is ventilatory (pump failure), not hypoxemic. PaO2 and SpO2 only fall at the point of terminal collapse:

Respiratory Parameter	Normal Value	High-Risk ICU Threshold	Definitive Intubation Trigger
Forced Vital Capacity (FVC)	60 - 70 mL/kg	< 30 mL/kg (transfer to ICU, prepare for RSI)	< 15 - 20 mL/kg (elective intubation)
Negative Inspiratory Force (NIF / MIP)	-70 to -100 cmH2O	Worse than -30 cmH2O (e.g., -25 cmH2O)	Worse than -20 cmH2O (inability to generate diaphragmatic negative pressure)
Single Breath Count	Able to count > 40 in one breath	Count 15 - 20	Count < 15 (correlates with FVC < 15-20 mL/kg)

2. Guillain-Barré Syndrome (GBS)

Clinical Presentation: Symmetrical, ascending flaccid motor weakness starting in lower extremities, accompanied by hyporeflexia or areflexia. Antecedent event (1-4 weeks prior): Campylobacter jejuni diarrhea, CMV, EBV, or respiratory virus.
Miller Fisher Variant: Triad of (1) Ophthalmoplegia; (2) Ataxia; (3) Areflexia (associated with anti-GQ1b antibodies).
Autonomic Instability: Up to 50% experience severe labile blood pressure, cardiac dysrhythmias (sinus bradycardia, asystole, VT), urinary retention, and ileus. Place on continuous cardiac telemetry.
Lumbar Puncture: Albuminocytological Dissociation (elevated CSF protein with normal WBC count); may be normal in the first 7 days.
Definitive Treatment: Intravenous Immunoglobulin (IVIG 0.4 g/kg/day IV for 5 days) OR Plasmapheresis (PE, 5 sessions over 10-14 days). Both have equal efficacy.
CONTRAINDICATIONS: (1) Systemic Corticosteroids have no benefit and may delay recovery; (2) Succinylcholine is STRICTLY CONTRAINDICATED due to up-regulated extrajunctional acetylcholine receptors causing massive, fatal hyperkalemic cardiac arrest. Use Rocuronium (1.2 mg/kg).

3. Myasthenia Gravis Crisis

Feature	Myasthenic Crisis	Cholinergic Crisis (Rare)
Underlying Mechanism	Severe worsening of autoimmune blockade against nicotinic acetylcholine receptors (anti-AChR or anti-MuSK antibodies)	Excessive acetylcholinesterase inhibitor (pyridostigmine) administration causing depolarization block
Clinical Signs	Fatigable weakness, ptosis, diplopia, bulbar weakness (dysarthria, dysphagia), weak neck flexion, respiratory failure	Profound weakness PLUS muscarinic toxidrome: Salivation, Lacrimation, Urination, Defecation, GI cramping, Emesis (SLUDGE), miosis, fasciculations
Treatment	IVIG (2 g/kg over 2-5 days) or Plasmapheresis + treat underlying trigger (infection, surgery, medications). Withhold or reduce pyridostigmine temporarily to decrease bronchial secretions.	Immediately stop pyridostigmine; administer Atropine 0.5-1 mg IV prn muscarinic symptoms; supportive ventilation.

Clinical Caution

Medications to AVOID in Myasthenia Gravis: (1) Antibiotics: Aminoglycosides, Fluoroquinolones, Macrolides, Clindamycin; (2) Cardiovascular: Beta-blockers, Calcium channel blockers; (3) Neuromuscular: Magnesium sulfate, Succinylcholine, Vecuronium/Rocuronium (patients are exquisitely sensitive to non-depolarizing paralytics; reduce dose by 50-75% if intubation is required).

4. Cauda Equina Syndrome (CES)

Pathology: Massive central lumbar disc herniation (most commonly L4-L5 or L5-S1), epidural hematoma, tumor, or abscess compressing the multi-nerve cauda equina roots.
Cardinal Red Flags: (1) Saddle anesthesia (perineal/perianal numbness in S3-S5 distribution); (2) Bladder dysfunction: Urinary retention with overflow incontinence (Post-Void Residual volume > 200 mL); (3) Bowel incontinence / loss of anal sphincter resting tone; (4) Bilateral lower extremity radicular pain and weakness.
Post-Void Residual (PVR) Measurement: Perform bedside bladder ultrasound. PVR < 100 mL strongly argues against CES (99% negative predictive value). PVR > 200 mL in a patient with sciatica/back pain mandates immediate MRI.
Emergency Disposition: Stat MRI of the lumbosacral spine and immediate emergency spine surgery consultation for decompression within 24-48 hours of onset to prevent permanent bladder/bowel/sexual paralysis.

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