Causes and Risk Factors for Myasthenia Gravis
MG usually is caused by a malfunction of the immune system. The causative factor is unknown, but the disorder may have a genetic link. Causes include a genetic defect, which results in congenital MG, and the circulation of maternal antibodies through the placenta, which result in transient neonatal MG.
Acetylcholine (ACh) is a neurotransmitter that is involved in the transfer of information to muscle tissue. In myasthenia gravis, cells that bind to other cells to neutralize or destroy them (called antibodies) destroy acetylcholine receptor sites (AChR) in areas of muscle tissue that receive nerve impulses (called neuromuscular junctions), preventing nerve impulses from reaching the muscles. This results in weakness and rapid fatigue in affected muscles.
MG may be associated with other autoimmune diseases. Patients with family members who suffer from disorders such as rheumatoid arthritis, scleroderma, and lupus may have an increased risk for myasthenia gravis.
The thymus is an organ that produces cells involved in immune responses. It is located below the larynx and above the heart. Approximately 15 percent of MG patients have a tumor of the thymus (thymoma) and 6080 percent have abnormal enlargement (hyperplasia) of the thymus.