The exact cause of myasthenia gravis is not fully understood, but abnormalities in the thymus gland are often implicated. In many cases, the thymus, which plays a role in immune system development, is found to be unusually large (thymic hyperplasia) or may have a tumour (thymoma).
MG affects all ages, sex and ethnic groups. It is most common in young adult women under 40 and older men over 60. It may affect newborns when a mother with MG passes the abnormal antibodies to the foetus. In adults, it is usually not hereditary. MG is not contagious.
