Myasthenia gravis (MG) is usually a chronic autoimmune disease of fluctuating muscle weakness. But sometimes it presents dramatically as a myasthenic crisis — life-threatening respiratory failure due to weakness of the diaphragm and bulbar muscles. Rookies must learn to spot it early, because timely airway management saves lives.
What Is a Myasthenic Crisis?
- Definition: Severe MG exacerbation requiring intubation or non-invasive ventilation.
- Caused by respiratory muscle or bulbar weakness.
- Mortality has fallen with modern care, but still carries major morbidity if not recognized early.
Common Triggers
- Infections (respiratory, urinary, sepsis).
- Medications: aminoglycosides, fluoroquinolones, macrolides, beta-blockers, magnesium, neuromuscular blockers.
- Surgery, stress, pregnancy.
- Rapid steroid escalation.
Clinical Features
- Respiratory: dyspnea, orthopnea, weak cough, difficulty clearing secretions.
- Bulbar: dysphagia, dysarthria, nasal speech, choking.
- Ocular: ptosis, diplopia (often precede crisis).
- Generalized weakness worsens with exertion, improves with rest.
Rookie pearl: Unlike GBS, weakness in MG fluctuates and worsens with fatigue.
Step 1: Airway & Breathing Assessment
- Bedside spirometry:
- FVC <15 mL/kg → intubation needed.
- Negative inspiratory force (NIF) <–20 cmH₂O = poor reserve.
- Monitor SpO₂, respiratory rate, ability to count to 20 in one breath.
- Bulbar weakness = high aspiration risk.
Step 2: ED Stabilization
- Admit all suspected crisis to ICU.
- Prepare for early intubation — don’t wait until patient crashes.
- Avoid paralytics if possible; if needed, use short-acting non-depolarizers at reduced dose. Succinylcholine contraindicated (risk of hyperkalemia).
- Oxygen and non-invasive ventilation may help early but are unreliable if bulbar involvement severe.
Step 3: Definitive Therapy (Neurology/ICU Driven)
- Plasmapheresis or IVIG (2 g/kg over 5 days) = first-line treatments.
- Corticosteroids often started but may initially worsen weakness → give with specialist input.
- Stop offending drugs if identified.
- Treat infections or other triggers aggressively.
Step 4: Medications to Avoid in Crisis
- Aminoglycosides, fluoroquinolones, macrolides.
- Magnesium (IV or oral).
- Beta-blockers, calcium channel blockers (can worsen weakness).
- Neuromuscular blockers (profound, prolonged paralysis).
ED Investigations
- ABG/VBG: look for hypercapnia, hypoxia.
- Chest X-ray: pneumonia often triggers crisis.
- Basic labs: CBC, electrolytes, infection markers.
Common Rookie Mistakes
- Missing early respiratory distress — waiting for hypoxia rather than measuring FVC/NIF.
- Giving contraindicated antibiotics (e.g., levofloxacin, azithromycin).
- Using succinylcholine in rapid sequence intubation.
- Assuming crisis is only “weakness” — forgetting bulbar and respiratory involvement.
- Not consulting ICU/neurology early.
Rookie Pearls
- FVC <15 mL/kg = intubate.
- Always ask MG patients about new meds or infections.
- Avoid paralytics if possible; if unavoidable, dose carefully.
- Crisis is reversible with IVIG/plasmapheresis — airway and supportive care buy time.
- Admit to ICU even if they “look okay” — deterioration can be rapid.
Take-Home Message
Myasthenic crisis is a neuromuscular emergency. For rookies:
- Look for respiratory and bulbar weakness.
- Admit to ICU, monitor FVC/NIF.
- Prepare for intubation early — don’t wait for a crash.
- Avoid contraindicated drugs.
- Definitive treatment = IVIG or plasmapheresis with neurology input.
Remember: In MG crisis, the difference between life and death is early recognition of impending respiratory failure.
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