Myasthenia Gravis (pronounced My-as-theen-ee-a Grav-us) comes from the Greek and Latin words meaning "grave muscular weakness." The most common form of MG is a chronic autoimmune neuromuscular disorder that is characterized by fluctuating weakness of the voluntary muscle groups. The prevalence of MG in the United States is estimated to be about 20/100,000 population. However, MG is probably under diagnosed and the prevalence may be higher. Myasthenia Gravis occurs in all races, both genders, and at any age. MG is not thought to be directly inherited nor is it contagious. It does occasionally occur in more than one member of the same family.
In the most common cases, muscle weakness is caused by circulating antibodies that block acetylcholine receptors. Alternatively, in rarer forms, muscle weakness is caused by a genetic defect in some portion of the neuromuscular junction that is inherited at birth, as opposed to developing through autoimmunity later in life or through passive transmission from the mother's immune system at birth.