A. Myasthenia gravis (MG) is an autoimmune disorder characterized by muscle weakness and exaggerated muscle fatigue.

B. Prevalence is between 10 and 20 per 100,000 with a 3:2 female-to-male predominance.

C. Incidence in women peaks in the third decade and in men in the fifth to sixth decades.

D. Oculomotor and bulbar muscles are most commonly affected; proximal muscles are usually more affected than distal muscles; variability over time in both severity and distribution of weakness is common.

E. About 20% of patients with generalized MG experience myasthenic crisis, which reflects severe bulbar and respiratory muscle weakness, constituting a medical emergency and requiring intensive care.

F. Factors commonly associated with myasthenic crisis include systemic infection, electrolyte imbalance, anesthesia, and drugs that impair neuromuscular transmission (Tables 126-1 and 126-2).

A. Circulating autoantibodies bind with acetylcholine receptors in postsynaptic muscle membrane.

B. Receptors may be blocked, and receptor density decreases due to accelerated degradation and turnover.

C. Miniature end-plate potentials are normal in number (indicating normal quantal release of acetylcholine) but decreased in amplitude.

A. Available therapies offer effective treatment for the vast majority of patients.

B. Aggressive airway management and respiratory support during myasthenic crisis minimize morbidity and mortality with current mortality rate <5%.

C. Elective thymectomy favorably alters the natural history of the disease.

1. MG should be considered in any patient with unexplained weakness, especially when weakness fluctuates or involves bulbar muscles.

2. Normal sensation, tendon reflexes, pupillary reflexes, and mental status distinguish MG from most other acute and subacute paralytic illnesses.