Myasthenia gravis crisis and neuromuscular respiratory failure

Pathophysiology — the disease at the junction

Myasthenia gravis is a T-cell-dependent, antibody-mediated, postsynaptic disorder of the neuromuscular junction (NMJ). The motor nerve terminal releases acetylcholine (ACh) into the synaptic cleft; ACh binds nicotinic ACh receptors (AChR) on the motor end-plate, depolarising the muscle fibre. In MG, pathogenic autoantibodies attack the postsynaptic membrane, reducing the available receptor reserve. Because the normal junction has a large safety factor (a single nerve impulse releases far more ACh than is needed to trigger contraction), the patient is asymptomatic at rest. As transmission fatigues with sustained effort, weakness emerges — the hallmark fatigability. The respiratory and bulbar muscles, which must sustain high-frequency firing, are the first to fail in crisis. [1]

The antibody target defines the phenotype, the response to treatment, and the prognosis. [1]

The distinction matters at the bedside: MuSK MG tends to present in crisis with severe bulbar and respiratory weakness, responds poorly to pyridostigmine (which may even worsen it) and very well to plasmapheresis and rituximab, and is not helped by thymectomy. AChR MG is the prototype that drives the monitoring thresholds, the immunosuppression ladder, and the thymectomy decision.[2]

Triggers of myasthenic crisis

Crisis rarely arises de novo — it is almost always precipitated. Identifying and treating the trigger is as important as treating the weakness, because failure to remove the precipitant predicts relapse and prolonged ventilation. In a typical series, infection accounts for around 30-40% of crises, aspiration and surgery each around 15-20%, and medication change or immunosuppression taper another 15%; a substantial minority remain idiopathic.[1][7]

Clinical presentation and bedside diagnosis

The crisis is a clinical diagnosis made on the background of known (or occasionally newly presenting) MG. Weakness is fatigable — worse with sustained or repeated effort, better with rest — and progresses over hours to days. The danger signs are respiratory and bulbar. [1]

Confirmatory investigations

Bedside tests establish the crisis; laboratory tests confirm the underlying MG and exclude mimics. In a patient with established MG, treatment must not be delayed while awaiting antibody results — they take days. [1]

Monitoring

Treatment

Respiratory support — NIV first, then intubation

The respiratory muscles fail because the fatigued NMJ cannot sustain the high-frequency firing the diaphragm and intercostals require. The aim is to rest the fatigued junction while IVIG or plasma exchange takes effect, intubating before the patient arrests. [1]

Myasthenic vs cholinergic crisis — the exam-defining distinction

This is the most commonly examined distinction in MG crisis, because the treatments are opposite. Both produce weakness; the rest of the picture tells them apart. The Tensilon (edrophonium) test is now avoided because it can precipitate bradycardia, bronchospasm and worse weakness if the diagnosis is wrong — but historically a brief improvement pointed to myasthenic crisis. [1]

Immunotherapy and thymectomy

Beyond the acute short-term therapy (IVIG/PLEX), most MG patients need long-term immunosuppression to maintain remission, and selected patients benefit from thymectomy. Steroids are first-line but have an early-worsening caveat; steroid-sparing agents take months to work. [1]

Differential diagnosis of acute neuromuscular respiratory failure

Not every patient with weak breathing has myasthenia. The pattern of weakness, the reflexes and the time course separate the NMJ disorders from pre- and post-synaptic mimics. [1]

Landmark trials

Clinical pearls

Red flags

Regional practice