The mean age at onset is 28 years in women and 42 years in men. Because MG affects the nicotinic AChR at the neuromuscular junction, only motor systems are affected. Classic autoimmune MG can also develop in childhood and responds well to thymectomy or anticholinesterases. MG can be associated with thymoma in as many as 15 percent of adult patients, or it may develop after removal of a thymoma. In addition, as many as five percent of patients with MG have other systemic autoimmune conditions, as many as 15 percent of patients have various autoantibodies, as many as 20 percent of patients have thyroid disease, and as many as five percent of patients have a strong family history of another autoimmune disease.
Pregnancy has a variable effect, but for most patients, weakness increases in the postpartum period.
About 12 percent of children born to mothers with MG develop transient neonatal MG as a result of transplacental transfer of circulating anti-AChR antibodies from the myasthenic mother to the fetus. These infants develop weakness, feeding difficulties, respiratory problems, a weak cry, and facial weakness within the first few hours after birth. Most of these patients have classic autoimmune MG, but the antibodies either have low affinity or are directed against epitopes not present in the soluble extracts of AChR used for standard assays. If the AChR binding assay is negative, a bioassay can be performed that uses modulating antibodies to detect cross-linking of AChR present in muscle cells in tissue culture. Other antibodies associated with MG include antistriational cell antibodies.