Myasthenia gravis is an acquired autoimmune disorder characterised by weakness, typically of the periocular, facial, bulbar, and girdle muscles. It is associated with serum IgG antibodies to acetylcholine receptors in the postsynaptic membrane of the neuromuscular junction. Classically, the muscles are easily fatigued.

It affects 5 people in every 100 000. Non-thymoma cases have a peak incidence at 10-30 years and again, at 60-70 years of age; those associated with thymoma have a peak incidence at 40-50 years of age. Females are affected more often in the under 40 year old age group whereas men predominate in cases which develop in older age groups. About 10% of cases develop during childhood.