Myasthenia gravis

The exact cause of myasthenia gravis (MG) is unknown, but it is likely that a number of factors contribute to the risk of developing this disorder.

MG is a type of autoimmune disorder. Autoimmune disorders occur when the immune system mistakenly attacks healthy tissue. In MG, the body produces antibodies that block the muscle cells that receive messages (neurotransmitters) from the nerve cells.

Normally when impulses travel down a nerve, the nerve endings release a neurotransmitter substance called acetylcholine. Acetylcholine travels from the neuromuscular junction - the place where nerve cells connect with the muscles they control - and binds to acetylcholine receptors which are activated and generate a muscle contraction. In myasthenia gravis, antibodies block, alter, or destroy the receptors for acetylcholine at the neuromuscular junction, which prevents a muscle contraction from occurring. These antibodies are produced by the body's own immune system.

In some cases, MG is linked to tumors or other abnormalities of the thymus (a gland the plays an important role in the immune system). Researchers also believe that variations in certain unidentified genes may increase the risk to develop MG.

Myasthenia gravis is not directly inherited, nor is it contagious. However, a genetic predisposition to autoimmune disease can run in families. Occasionally, myasthenia gravis may occur in more than one member of the same family.

The association of heart problems with myasthenia gravis (MG) has been suspected for decades, but heart problems are not believed to be a common feature or complication of MG. Many medical researchers believe some people with MG may be at an increased risk for several kinds of heart conditions, ranging from asymptomatic changes on electrocardiography (ECG) to ventricular tachycardia, myocarditis, Takotsubo cardiomyopathy, conduction disorders (heart does not beat normally), heart failure, and sudden cardiac death. However other researchers feel that myocarditis (including giant cell myocarditis) and Takotsubo cardiomyopathy are the only heart conditions increased in people with MG, and the risk for other cardiac problems is no different from the general population risk. In addition, in many descriptions of the signs and symptoms of MG, cardiac issues are not mentioned.

Most medical researchers agree that although the antibodies normally associated with MG (AChR) do not attack the heart muscle, some people with MG, especially those with thymoma, may develop antibodies against the heart muscle causing myocarditis, including giant cell myocarditis. The possible cause of the increased risk for developing Takotsubo cardiomyopathy is less clear. Takotsubo cardiomyopathy does not appear to be related to thymoma, but instead usually occurs during a myasthenia crisis. Abnormal heart rate and rhythm may be linked to complications of myocarditis, or as some medical researchers suggest, may be related to a problems within the autonomic nervous system.

Medical researchers continue to call for better studies that could determine which types of cardiac issues may be caused by MG and how to detect those at an increased risk. One concern is that the symptoms of cardiac problems including fatigue, shortness of breath and exercise intolerance are similar to symptoms of MG, so cardiac problems may be missed.

Treatment of cardiac problems in people with MG must be carefully considered because some heart medications may make other symptoms of MG worse.

Myasthenia gravis (MG) is a chronic autoimmune neuromuscular disease characterized by weakness of the skeletal muscles. Common symptoms include weakness of the muscles that control the eye and eyelid, facial expressions, chewing, talking, and swallowing. Weakness tends to increase during periods of activity and improve after periods of rest. The condition results from a defect in the transmission of nerve impulses to muscles, which is due to the presence of antibodies against acetylcholine. The exact reason this occurs is not known. Some cases have been linked to tumors in the thymus gland. Researchers believe that variations in certain genes may increase a person's risk to develop MG, but other factors likely also play a role. There is no cure for MG at this time, but treatment can significantly improve muscle weakness. Some cases may go into remission temporarily, and muscle weakness may disappear so that medications can be discontinued.

MedlinePlus offers a comprehensive list of resources for myasthenia gravis. MedlinePlus is a Web site designed by the National Library of Medicine to help you research your health questions. The Myasthenia Gravis Foundation of America also has information related to this topic.

You can also use the navigation menu on the left hand side of this page to view many additional resources on myasthenia gravis.

In the US, the estimated annual incidence of myasthenia gravis (number of newly diagnosed cases) has been reported as 1 in 500,000 people. To our knowledge, estimates of prevalence (total number of cases that exist at a given time) in the US range considerably, from 1 in 2,500 to 1 in 200,000 people. The prevalence appears to have risen over the past 2 decades, mostly due to earlier diagnosis and increased lifespan of affected people.

While myasthenia gravis can occur at any age, the female incidence is highest in the third decade of life, and the male incidence is highest in the sixth or seventh decade. The average age of onset is 28 years in females and 42 years in males.

The Muscular Dystrophy Association provides additional information on myasthenia gravis. Click on the link to view this information page.