Myasthenia gravis is an autoimmune disorder that impairs communication between nerves and muscles, resulting in episodes of muscle weakness.
Myasthenia gravis results from malfunction of the immune system.
People usually have drooping eyelids and double vision, and muscles become unusually tired and weak after they are used and recover when they are rested.
Tests using an ice pack or rest to see whether these measures make eyelids droop less, electromyography, and blood tests can help confirm the diagnosis.
Some medications can improve muscle strength rapidly, and others can slow progression of the disorder.
If myasthenic crisis occurs, IV immune globulin or plasma exchange is used.
(See also Overview of Neuromuscular Junction Disorders.)
Myasthenia gravis usually develops in women between the ages of 20 and 40 and in men between the ages of 50 and 80. However, the disorder may affect men or women at any age. Rarely, it begins during childhood.
Nerves communicate with muscles by releasing a chemical messenger (neurotransmitter), which interacts with receptors on muscles (at the neuromuscular junction) and stimulates muscles to contract. In most people with myasthenia gravis, the immune system produces antibodies that attack one type of receptor on the muscle side of the neuromuscular junction—the receptors that respond to the neurotransmitter acetylcholine. As a result, communication between nerve cells and muscles is disrupted.
What causes the body to attack its own acetylcholine receptors—an autoimmune reaction—is unknown. According to one theory, malfunction of the thymus gland may be involved. In the thymus gland, certain cells of the immune system learn how to differentiate between the body and foreign substances. For unknown reasons, the thymus gland may instruct the immune system cells to produce antibodies that attack the acetylcholine receptors. People may inherit a predisposition to this autoimmune abnormality. About 65% of people who have myasthenia gravis have an enlarged thymus gland, and about 10% have a tumor of the thymus gland (thymoma). About half of these thymomas are cancerous (malignant).
Myasthenia gravis also often occurs in people with other autoimmune disorders, such as rheumatoid arthritis, systemic lupus erythematosus, and disorders that cause the thyroid gland to become overactive (autoimmune hyperthyroidism), as occurs in Hashimoto thyroiditis.
Some people with myasthenia gravis do not have antibodies to acetylcholine receptors, but they have antibodies to an enzyme involved in the formation of the neuromuscular junction instead. These people may require different treatment.
Myasthenia gravis may be triggered by
Infections
Surgery
Rarely, myasthenia gravis is inherited as an autosomal (not sex-linked) recessive trait. That is, two genes for the disorder, one from each parent, are required for a child to have the disorder.
Neonatal myasthenia
Neonatal myasthenia develops in 12% of babies born to women who have myasthenia gravis. Antibodies against acetylcholine receptors, which circulate in the blood, may pass from a pregnant woman through the placenta to the fetus. In such cases, the baby has muscle weakness that disappears several days to a few weeks after birth. The remaining 88% of babies are not affected.
Symptoms of Myasthenia Gravis
Episodes of worsened symptoms (exacerbations) are common. At other times, symptoms may be minimal or absent.
The most common symptoms of myasthenia gravis are
Weak, drooping eyelids
Weak eye muscles, which cause double vision
Excessive weakness of affected muscles after they are used
When people with myasthenia gravis use a muscle repetitively, the muscle usually becomes weak. For example, people who once could use a hammer well become weak after hammering for several minutes. The weakness disappears when the muscles are rested but recurs when they are used again.
Muscle weakness varies in intensity from minute to minute, hour to hour, and day to day, and the course of the disease varies widely. Weakness is less severe in cooler temperatures.
In about 40% of people with myasthenia gravis, the eye muscles are affected first, but 85% eventually have this problem. In 15% of people, only the eye muscles are affected, but in most people, the whole body is affected.
Difficulty speaking and swallowing and weakness of the arms and legs are common. Neck muscles may become weak. Sensation is not affected.
Myasthenia crisis
About 15 to 20% of people with myasthenia gravis have a severe episode (called myasthenia crisis) at least once in their life. It is sometimes triggered by an infection. The arms and legs may become extremely weak, but even then, they do not lose sensation.
In some people, the muscles needed for breathing weaken (causing respiratory failure). This condition is life threatening.
Diagnosis of Myasthenia Gravis
Tests using an ice pack or rest
Electromyography
Blood tests
Doctors suspect myasthenia gravis in people with episodes of weakness, especially when the eye or facial muscles are affected or when weakness increases with use of the affected muscles and disappears with rest.
Because coolness and rest lessen weakness due to myasthenia, doctors may use the ice pack test or the rest test to check for myasthenia gravis.
The ice pack test is done only if the person's eyelids are obviously drooping. Doctors apply an ice pack to the person's eyes for 2 minutes, then remove it. If drooping resolves, doctors suspect myasthenia gravis.
The rest test is done if a person has trouble moving the eyeball (typically causing double vision). The person is asked to exercise the eye muscles until they tire. Then doctors ask the person to lie quietly in a dark room for a few minutes with eyes closed. If the eyes then move normally and double vision resolves, doctors suspect myasthenia gravis.
Other diagnostic tests are needed to confirm the diagnosis. They include
Electromyography (inserting a needle into a muscle to record its electrical activity)
Blood tests to detect antibodies to acetylcholine receptors and sometimes the other antibodies present in people with the disorder
Blood tests are also done to check for other disorders, including thyroid disorders.
After myasthenia gravis is diagnosed, computed tomography (CT) or magnetic resonance imaging (MRI) of the chest is done to assess the thymus gland and to determine whether a thymoma is present.
Other tests are done to check for autoimmune disorders that are common among people with myasthenia gravis, such as pernicious anemia, rheumatoid arthritis, and systemic lupus erythematosus (lupus).
Pulmonary function tests are done when weakness is severe enough to affect breathing. These tests help determine whether help with breathing is needed (mechanical ventilation).
Treatment of Myasthenia Gravis
A corticosteroid or a medication that inhibits the immune system
Sometimes intravenous immune globulin or plasma exchange
Often removal of the thymus gland
In people with myasthenia gravis, medications may be used to
Help improve strength quickly
Suppress the autoimmune reaction and slow progression of the disorder
Medications to help improve strength quickly
Doctors must periodically adjust the dose, which may have to be increased during episodes of weakness. However, doses that are too high can cause weakness that is difficult to distinguish from that caused by the disorder. This effect is called cholinergic crisis. If cholinergic crisis occurs, the medication must be stopped for several days. In cholinergic crisis (as in myasthenic crisis), the muscles needed for breathing may weaken. In such cases, people are treated in an intensive care unit.
Also, the effectiveness of these medications may decrease with long-term use. Increasing weakness, which may be due to a decrease in the medication’s effectiveness, must be evaluated by a doctor with expertise in treating myasthenia gravis.
respiratory failure develops, mechanical ventilation may be needed.
Treatments to suppress the autoimmune reaction
To suppress the autoimmune reaction, doctors may also prescribe the following:
A medication that inhibits the immune system (immunosuppressant
These medications are taken by mouth.
When these medications do not provide relief, monoclonal antibodies
Efgartigimod, another type of monoclonal antibody, may be used to treat myasthenia gravis.
Intravenous immune globulin (IVIG) or plasma exchange may be used when a myasthenic crisis occurs. They may also be used when medications do not provide relief and the thymus gland is going to be removed (before that operation).
Immune globulin (a solution containing many different antibodies collected from a group of donors) may be given by vein (intravenously) once a day for 5 days. It contains antibodies that help control the immune system and stop the autoimmune reaction that causes myasthenia gravis. Over two thirds of people improve in 1 to 2 weeks, and effects may last for up to 2 months.
In plasma exchange, toxic substances (in this case, abnormal antibodies) are filtered from the blood. Improvement after plasma exchange is similar to that after taking immune globulin.
Other treatments
If a thymoma is present, the thymus gland must be surgically removed to prevent the thymoma from spreading. Removal may result in remission or enable doctors to reduce the dose of the corticosteroid.
Sometimes doctors remove the thymus even if there is no thymoma. This treatment appears to lessen symptoms of myasthenia gravis over the long term.
