A disorder characterized by chronic weakness of voluntary muscles, which improves with rest and worsens with activity.
Myasthenia gravis involves fluctuating levels of weakness of commonly used muscles.
The cause of autoimmune disorders such as myasthenia gravis is unknown. It also may be associated with thyrotoxicosis , rheumatoid arthritis , systemic lupus erythematosus , and other immune system disorders. In many cases, no other disorder is identified. Myasthenia gravis affects about 3 out of 10,000 people. Symptoms may worsen with pregnancy or menstrual periods.