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Causes
By Mayo Clinic staff
Antibodies
Your nerves communicate with your muscles by releasing chemicals (neurotransmitters) that fit precisely into receptor sites on the muscle cells at the nerve-muscular junction.
In myasthenia gravis, your immune system produces antibodies that block or destroy many of your muscles' receptor sites for a neurotransmitter called acetylcholine (as-uh-teel-KOH-leen). With fewer receptor sites available, your muscles receive fewer nerve signals, resulting in weakness.
Antibodies may also block the function of a protein called a muscle-specific receptor tyrosine kinase (TIE-roh-seen KIE-nays). This protein is involved in forming the nerve-muscular junction. When antibodies block the function of this protein, it may lead to myasthenia gravis. Research continues to study how the antibodies inhibiting this protein are related to the development of myasthenia gravis.
Thymus gland
Researchers believe that the thymus gland, a part of your immune system situated in the upper chest beneath your breastbone, may trigger or maintain the production of the antibodies that block acetylcholine.
Large in infancy, the thymus is small in healthy adults. In some adults with myasthenia gravis, however, the thymus is abnormally large. Some people with myasthenia gravis also have tumors of the thymus (thymomas). Usually, thymomas aren't cancerous (malignant).
Other causes
Some people may have myasthenia gravis that isn't caused by antibodies blocking acetylcholine or the muscle-specific receptor tyrosine kinase. This type of myasthenia gravis is called antibody-negative myasthenia gravis. Antibodies against another protein, called lipoprotein-related protein 4, may play a part in the development of this condition.
Genetic factors also may be associated with myasthenia gravis.
Rarely, mothers with myasthenia gravis have children who are born with myasthenia gravis (neonatal myasthenia gravis). If treated promptly, children generally recover within two months after birth.
Some children are born with a rare, hereditary form of myasthenia, called congenital myasthenic syndrome.
Factors that can worsen myasthenia gravis
- Fatigue
- Illness
- Stress
- Extreme heat
- Some medications — such as beta blockers, quinidine gluconate, quinidine sulfate, quinine (Qualaquin), phenytoin (Dilantin), certain anesthetics and some antibiotics
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- Bird SJ. Thymectomy for myasthenia gravis. http://www.uptodate.com/home. Accessed Feb. 5, 2013.
- Limmer KK, et al. Minimally invasive and robotic-assisted thymus reduction. Thoracic Surgery Clinics. 2011;21:69.
- Barbara Woodward Lips Patient Education Center. Myasthenia gravis: A guide for patients. Rochester, Minn.: Mayo Foundation for Medical Education and Research; 2003.
- How can I provide emotional support for a relative who has MG? Myasthenia Gravis Association of America. http://www.myasthenia.org/LivingwithMG/InformationalMaterials.aspx. Accessed Feb. 7, 2013.
- U.S. News best hospitals 2012-2013. U.S. News & World Report. http://health.usnews.com/best-hospitals/rankings. Accessed Jan. 25, 2013.
- Crum BA (expert opinion). Mayo Clinic, Rochester, Minn. April 1, 2013.
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