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 exercise.
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 drugs 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 Overview of Neuromuscular Junction Disorders Nerves connect with muscles at the neuromuscular junction. There, the ends of nerve fibers connect to special sites on the muscle’s membrane called motor end plates. These plates contain receptors... read more .)
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 Overview of the Peripheral Nervous System The peripheral nervous system refers to the parts of the nervous system that are outside the central nervous system, that is, those outside the brain and spinal cord. Thus, the peripheral nervous... read more communicate with muscles by releasing a chemical messenger (neurotransmitter), which interacts with receptors on muscles (at the neuromuscular junction Overview of Neuromuscular Junction Disorders Nerves connect with muscles at the neuromuscular junction. There, the ends of nerve fibers connect to special sites on the muscle’s membrane called motor end plates. These plates contain receptors... read more ) and stimulates muscles to contract. In myasthenia gravis, the immune system produces antibodies Antibodies One of the body's lines of defense (immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more 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 the nerve cell and the muscle is disrupted.
What causes the body to attack its own acetylcholine receptors—an autoimmune reaction Autoimmune Disorders An autoimmune disorder is a malfunction of the body's immune system that causes the body to attack its own tissues. What triggers autoimmune disorders is not known. Symptoms vary depending on... read more —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 Rheumatoid Arthritis (RA) Rheumatoid arthritis is an inflammatory arthritis in which joints, usually including those of the hands and feet, are inflamed, resulting in swelling, pain, and often destruction of joints.... read more , systemic lupus erythematosus Systemic Lupus Erythematosus (SLE) Systemic lupus erythematosus is a chronic autoimmune inflammatory connective tissue disorder that can involve joints, kidneys, skin, mucous membranes, and blood vessel walls. Problems in the... read more , and disorders that cause the thyroid gland to become overactive (autoimmune hyperthyroidism), as occurs in Hashimoto thyroiditis Hashimoto Thyroiditis Hashimoto thyroiditis is chronic, autoimmune inflammation of the thyroid. Hashimoto thyroiditis results when the body attacks the cells of the thyroid gland—an autoimmune reaction. At first... read more .
Some people with myasthenia gravis do not have antibodies to acetylcholine receptors but 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
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.
Episodes of worsened symptoms (exacerbations) are common. At other times, symptoms may be minimal or absent.
The most common symptoms of myasthenia gravis are
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.
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. This condition is life threatening.
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. These tests are usually done only if the person's eyelids are obviously drooping. The person is asked to exercise the eye muscles until they tire. Then doctors apply an ice pack to the person's eyes or ask the person to lie quietly in a dark room for a few minutes with eyes closed. If the eyelids droop less after the test, doctors suspect myasthenia gravis.
Other diagnostic tests are needed to confirm the diagnosis. They include
Electromyography Electromyography Diagnostic procedures may be needed to confirm a diagnosis suggested by the medical history and neurologic examination. Electroencephalography (EEG) is a simple, painless procedure in which... read more (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.
In people with myasthenia gravis, drugs may be used to
Drugs that increase the amount of acetylcholine, such as pyridostigmine (taken by mouth), may improve muscle strength. Long-acting tablets are available for nighttime use to help people who experience severe weakness or difficulty swallowing when they awaken in the morning.
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 drug 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 drugs may decrease with long-term use. Increasing weakness, which may be due to a decrease in the drug’s effectiveness, must be evaluated by a doctor with expertise in treating myasthenia gravis.
Common side effects of pyridostigmine include abdominal cramps and diarrhea. Drugs that slow the activity of the digestive tract, such as atropine or propantheline, may be needed to counteract these effects.
To suppress the autoimmune reaction, doctors may also prescribe the following:
A drug that inhibits the immune system (immunosuppressant Suppression of the Immune System Transplantation is the removal of living, functioning cells, tissues, or organs from the body and then their transfer back into the same body or into a different body. The most common type of... read more ), such as mycophenolate mofetil, cyclosporine, or azathioprine
These drugs are taken by mouth.
Most people need to take a corticosteroid indefinitely. If the dose is too high when the corticosteroid is started, symptoms may worsen initially, but improvement occurs within several weeks. The dose is then reduced to the minimum that is effective. Corticosteroids, when taken for a long time, can have moderate or severe side effects. Thus, azathioprine may be given so that the corticosteroid can be stopped or its dose reduced. With azathioprine, improvement may take many months.
When these drugs do not provide relief, monoclonal antibodies Monoclonal Antibodies Immunotherapy is the use of drugs that mimic or modify components of the immune system (such as tumor antigens and immune checkpoints—see also Overview of the Immune System) to fight disease... read more , such as rituximab or eculizumab, may help. Monoclonal antibodies are manufactured antibodies that target and suppress specific parts of the immune system.
Intravenous immune globulin (IVIG) or plasma exchange may be used when a myasthenic crisis occurs. They may also be used when drugs 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 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 Plateletpheresis (platelet donation) In addition to normal blood donation and transfusion, special procedures are sometimes used. In plateletpheresis, a donor gives only platelets rather than whole blood. Whole blood is drawn from... read more , toxic substances (in this case, abnormal antibodies) are filtered from the blood. Improvement after plasma exchange is similar to that after taking immune globulin.
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.