Definition of Myasthenia gravis:

Myasthenia gravis is a neuromuscular disorder characterized by variable weakness of voluntary muscles, which often improves with rest and worsens with activity. The condition is caused by an abnormal immune response.

Causes, incidence, and risk factors:

In myasthenia gravis, weakness occurs when the nerve impulse to initiate or sustain movement does not adequately reach the muscle cells. This is caused when immune cells target and attack the body's own cells (an autoimmune response). This immune response produces antibodies that attach to affected areas, preventing muscle cells from receiving chemical messages (neurotransmitters) from the nerve cell.

The cause of autoimmune disorders such as myasthenia gravis is unknown. In some cases, myasthenia gravis may be associated with tumors of the thymus (an organ of the immune system). Patients with myasthenia gravis have a higher risk of having other autoimmune disorders like thyrotoxicosis, rheumatoid arthritis, and systemic lupus erythematosus.

Myasthenia gravis affects about 3 of every 10,000 people and can affect people at any age. It is most common in young women and older men.