MG is characterized by a fluctuating weakness of skeletal muscle with remissions and exacerbations.In 85% of patients with MG, the disease is caused by antibodies against the AChR at the post-synaptic side of the neuromuscular junction that cause transmission failure and produce destruction of the endplate. Of the 15% of generalized MG patients without AChR anti-
bodies, 20–50% have antibodies against another synaptic antigen, muscle-specific tyrosine kinase [MuSK]. The remaining patients probably have antibodies against unknown antigens at the neuromuscular junction or low level/affinity antibodies against AChR or MuSK that are not detectable by standard assays. MG is closely associated with thymic pathology. Fifteen per
cent of patients with MG have a thymoma and often have antibodies against additional striated muscle antigens such as titin and ryanodine receptors.These antibodies are more common in thymoma and severe MG and are considered as useful markers.A hypertrophic thymus is found in 60%of patients with MG, typically young women, whilst most patients with debut after 50 years of age have a normal or atrophic thymus.