Myasthenia gravis - emergency medicine revision
Concise summary
Myasthenia gravis (MG) is an autoimmune, antibody‑mediated disorder of the postsynaptic neuromuscular junction, typically caused by anti‑AChR or anti‑MuSK antibodies. It produces fluctuating, fatigable weakness that worsens with exertion and can progress to life‑threatening respiratory failure (myasthenic crisis).
In the ED the priorities are early recognition of bulbar or respiratory compromise, airway/ventilatory support, identification and reversal of precipitants (including drugs), and rapid liaison with anaesthesia/ICU and neurology to arrange rescue therapy (plasmapheresis or IVIG).
Epidemiology and thymic associations
- Incidence approximately 7-23 new cases per million per year; affects all ages and ethnicities.
- Bimodal peaks: young adult women (≈30s) and older men (≈60s). MG is often underdiagnosed in older patients.
- Thymic abnormalities are common and relevant to longer‑term management: lymphoid hyperplasia ≈70%, thymoma ≈10%.
Pathophysiology
- Normal neuromuscular junction (NMJ): presynaptic motor neuron releases acetylcholine (ACh), which binds postsynaptic ACh receptors (AChR) to trigger muscle contraction.
- In MG: autoantibodies reduce functional AChR and disrupt the postsynaptic membrane, lowering the safety margin for neuromuscular transmission and producing fatigable weakness.
- Contrast: Lambert‑Eaton syndrome is a presynaptic disorder with reduced ACh release and facilitation with repetitive activity.