Myasthenia gravis (MG) affects the skeletal or voluntary muscles of the body. Functions affected by MG include the ability to control movement of the eyes and eyelids, facial expression, chewing, talking, swallowing, neck movement, and movement of the arms and legs. The muscles that control breathing can also be affected. The disease causes muscle weakness that increases during activity and is relieved by periods of rest.
MG affects approximately 20 in every 100,000 people in the United States.
MG is an autoimmune disease * affecting the transmission of impulses in the neuromuscular system. Normally, nerves connect with the muscles they control at a place called the neuromuscular junction * . Messages from the muscles to the nerves and back again go through the neuromuscular junction because of a substance called acetylcholine (uh-SEE-til-ko-leen). Acetylcholine occurs throughout the nervous system and functions as a transmitter of nerve impulses. It is released by the nerve endings in the neuromuscular junction and travels to acetylcholine receptors, which are activated and cause the muscle to contract. In MG, there is a defect with the acetylcholine receptors that interferes with the transmission of messages/impulses through the neuromuscular junction. This defect is related to the immune system as antibodies * block, change, or destroy the acetylcholine receptors at the neuromuscular junction. The result is the inability of the muscle to contract normally, thus affecting movement.
The thymus gland * may be involved in MG. According to some reports, 85 percent of people diagnosed with MG have an enlarged thymus gland, and about 10 percent have a tumor of the thymus.
Drugs, such as some antibiotics, lithium, magnesium, prednisone, and others, may induce symptoms of MG or may make the symptoms of MG worse.
MG affects people of all races, genders, and age. It occurs more frequently in young adult women under age 40 and older adult men over age 60. While not believed to be an inherited trait, MG may occur in more than one member of the same family.
One of the first signs of MG (in up to 50 percent of cases) is eye muscle weakness or a drooping eyelid, which is called ptosis (TOE-sys). This is referred to as ocular myasthenia. As many as 90 percent of people affected by MG will experience ocular myasthenia during the course of their illness.
Other signs of MG include:
The healthcare provider obtains a complete medical history, focusing on the signs and symptoms that the person describes. Often the person with signs of MG is referred to a physician who has special knowledge and experience in neuromuscular disorders. This physician is called a neurologist (new-RHAL-eh-jist). The neurologist performs a physical examination to determine the extent of weakness. A laboratory examination of a blood sample is done to detect antibodies to the acetylcholine receptor. This blood test is highly specific in diagnosing people with MG but is more sensitive in diagnosing people with generalized MG as compared to those cases of purely ocular MG. Electromyogram * (EMG) tests may be done to determine the pattern of muscle response to electrical stimulation both when the muscle is at rest and when it is contracting. Repetitive nerve stimulation (RNS) is the most frequently performed test to determine the effectiveness of an individual's neuromuscular transmission function. Other diagnostic studies may be done to better understand the person's problem.
There is no known cure for MG. However, there are treatments that enable people with MG to carry out normal daily activities and live a fulfilling life. Treatment decisions are made based on the extent of weakness, the degree of impairment, and the person's age and sex.
Intravenous administration of immunoglobulin (IVIG) may be used to treat MG in the elderly and in those people who have other complex diseases other than MG. IVIG is most effective in the treatment of people with moderate or severe forms of MG.
Removal of the thymus gland, known as a thymectomy, is indicated for all MG patients with a thyoma, and for patients between the ages of 10 and 55 years without a thyoma but who are experiencing generalized MG. Remission * may be achieved by some patients after thymectomy. Individuals with ocular MG are not offered thymectomy until at least two years after diagnosis in order to determine if the MG will resolve on its own or if the patient will progress to the generalized form of MG. Thymectomy in individuals younger than 10 and in those older than 55 years remains controversial.
People with MG may have a severe episode called a myasthenic crisis. Myasthenic crisis is a medical emergency. When myasthenic crisis occurs, the muscles of the respiratory system that control breathing are weakened and the person has extreme difficulty breathing. Infection, fever, or an adverse reaction to a medication can cause a myasthenic crisis. The person may be put on a ventilator * to assist their breathing while the problem that caused the crisis is treated and resolved. IVIG may be administered to patients experiencing myasthenic crisis.
Some people with MG have periods of remission when the effects of the disease decrease. During periods of remission, the treatment plan may be modified or reduced. When in remission, the person with MG continues to be monitored by his or her physician. Research to find a cure and/or treatments to help people with MG to live a better life continues.
MG cannot be prevented, but there are strategies that can be implemented to decrease the risk of exacerbation * of the disease or myasthenic crisis. These strategies include:
See also Autoimmune Disorders: Overview • Chronic Illness • Disability • Immune System and Other Body Defenses: Overview
Engel, Andrew G. (ed.). Myasthenia Gravis and Myasthenic Disorders, Second Edition. Oxford, UK: Oxford University Press, 2012.
Henderson, Ronald E. Attacking Myasthenia Gravis. Montgomery, AL: NewSouth Books, 2013.
MedlinePlus. “Myasthenia Gravis.” U.S. National Library of Medicine, National Institutes of Health. https://www.nlm.nih.gov/medlineplus/myastheniagravis.html (accessed March 30, 2016).
Merck Manual: Consumer Version. “Myasthenia Gravis.” http://www.merckmanuals.com/home/brain-spinal-cord-and-nerve-disorders/peripheral-nerve-disorders/myasthenia-gravis (accessed March 30, 2016).
Muscular Dystrophy Association. “Myasthenia Gravis.” https://www.mda.org/disease/myasthenia-gravis (accessed March 30, 2016).
National Institute of Neurological Disorders and Stroke. “Myasthenia Gravis Fact Sheet.” http://www.ninds.nih.gov/disorders/myasthenia_gravis/detail_myasthenia_gravis.htm (accessed March 30, 2016).
National Organization for Rare Disorders. “Myasthenia Gravis.” https://rarediseases.org/rare-diseases/myasthenia-gravis/ (accessed March 30, 2016).
American Autoimmune Related Diseases Association. 22100 Gratiot Ave., Eastpointe, MI 48021. Toll-free: 800-598-4668. Website: www.aarda.org (accessed March 30, 2016).
Muscular Dystrophy Association. 222 S. Riverside Plaza, Suite 1500, Chicago, IL 60606. Toll-free: 800-572-1717. Website: https://www.mda.org (accessed March 30, 2016).
Myasthenia Gravis Foundation of America, Inc. 355 Lexington Ave., 15th Floor, New York, NY 10017. Toll-free: 800-541-5454. Website: http://www.myasthenia.org (accessed March 30, 2016).
National Organization for Rare Disorders. 55 Kenosia Ave., PO Box 1968, Danbury, CT 06813. Telephone: 203-744-0100. Website: http://www.rarediseases.org (accessed March 30, 2016).
* neuromuscular system (NUR-Omusk-u-lar SIS-tem) is the system made up of the muscles of the body and the nerves that supply and control them.
* immune disorder (ih-MYOON) is a disorder of the immune system, the system of the body composed of specialized cells and the substances they produce that help protect the body against disease-causing germs.
* autoimmune disease is a disease or disorder in which the body's immune system attacks a person's own normal body tissues. The cause of most autoimmune disorders or diseases is unknown.
* neuromuscular junction (NURO-musk-u-lar JUNK-shun) is a synapse or meeting place between a motor neuron or nerve fiber and the skeletal muscle it controls.
* antibodies (AN-tih-bah-deez) are protein molecules produced by the body's immune system to help fight specific infections caused by microorganisms, such as bacteria and viruses.
* thymus gland (THY-mus GLAND) is an organ located in the neck that produces cells for the immune system.
* dysarthria (dis-ARTH-ree-yah) is a condition in which the muscles used for speech are weakened or the person has difficulty in controlling them, causing slowed or slurred speech.
* electromyogram (ee-lek-tro-MY-eh-gram) (EMG) is a visual record made by an electromyograph, which measures the electrical activity associated with functioning muscle.
* corticosteroids (KOR-ti-ko-STIRoyd) are chemical substances made by the adrenal glands that have several functions in the body, including maintaining blood pressure during stress and controlling inflammation. They can also be given to people as medication to treat certain illnesses.
* remission means an easing of a disease or its symptoms for a prolonged period.
* ventilator (VEN-tuh-lay-ter) is a machine used to support or control a person's breathing.
* exacerbation (EX-as-er-BAYshun) refers to a worsening, aggravation, or increase in severity of a disease or its signs and symptoms.