Synopsis
Recognise impending neuromuscular respiratory failure before oxygen saturation falls, avoid medicines and procedures that worsen transmission, and coordinate ventilation with rapid immunomodulatory treatment.
- Myasthenic crisis is severe myasthenia gravis causing respiratory failure or unsafe bulbar function; it may be the first presentation rather than a complication of known disease.
- Infection is a frequent precipitant, but surgery, pregnancy, aspiration, undertreatment, abrupt immunotherapy change and medicines that impair neuromuscular transmission must also be sought.
- Ask the patient to count aloud on one breath and assess speech, swallowing, neck flexion, cough and accessory-muscle use, then obtain serial forced vital capacity through experienced respiratory staff.
- A normal oxygen saturation does not exclude ventilatory failure because neuromuscular weakness first causes falling tidal volume and carbon-dioxide retention rather than primary gas-exchange failure.
- Arterial or venous blood gas showing hypercapnia is a late and worrying sign; do not postpone airway planning until carbon dioxide rises or the patient becomes drowsy.
- Bulbar weakness, copious secretions and inability to protect the airway may require intubation even when the measured vital capacity is not yet profoundly reduced.
- Treat the trigger and give plasma exchange or intravenous immunoglobulin through a specialist plan; onset, vascular access, haemodynamic risk and local availability influence the choice.
- Pyridostigmine is often withheld after intubation because secretions complicate ventilation, then reintroduced before extubation under neurological and critical-care direction.
- Avoid or carefully review magnesium, aminoglycosides, fluoroquinolones, macrolides, beta-blockers, sedatives and neuromuscular blockers; the clinical need and alternatives determine action rather than an unqualified blacklist.
- Steroids can transiently worsen weakness when introduced or escalated, so immunosuppression changes during crisis require monitored specialist planning.
Key red flags
Exhaustion, drowsiness, silent weak cough, rising carbon dioxide or inability to clear secretions mandates immediate definitive airway action.
Investigation priorities
Measure the direction and severity of respiratory-muscle decline.
Management branches
A patient with known or possible myasthenia develops breathlessness, dysphagia or rapidly worsening weakness.
- Call critical care and neurology, position safely, give suction and oxygen only as needed, and assess cough, secretions, speech, swallow, neck flexion and breathing pattern.
- Measure forced vital capacity serially and obtain a blood gas without allowing either result to delay intubation when exhaustion or airway protection is failing.
- Prepare difficult-airway and aspiration precautions, choosing non-invasive ventilation only for an alert, cooperative patient with manageable secretions and close monitoring.
Key medicines
Intravenous immunoglobulinFor life-threatening myasthenic crisis with intensive-care respiratory or bulbar failure, current NHS England criteria specify a total 2 g/kg over 2–5 days using the commissioned weight and infusion policy.Assess thrombosis, renal impairment, fluid load, haemolysis, headache and aseptic meningitis; record product and batch and slow or stop infusion for significant reaction. The 1 g/kg-first regimen applies to a hospitalised non-life-threatening exacerbation, not to myasthenic crisis.
PyridostigmineUsual oral dosing is individualised through the current BNF; during intubated crisis it may be withheld then cautiously reintroduced by neurology before extubation.Excess causes salivation, diarrhoea, abdominal cramp and bradycardia, while secretions impede ventilation; renal impairment may increase exposure.