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Myasthenia Gravis

What is Myasthenia Gravis?

The name myasthenia gravis is Greek and Latin meaning "grave muscle weakness." It's an autoimmune disease characterized by weakness and the rapid fatigue of voluntarily controlled muscles. The weakness is caused by antibodies that block muscle cells from receiving chemical messages to move the body. As a result, muscles don't function properly and become easily fatigued.

Common muscle groups that are affected include those involved in the eyes, facial movement, swallowing, speaking, lifting up the arms, rising to stand and walking up stairs. When the disorder affects muscles needed for breathing, the results can be life-threatening. Myasthenia gravis affecting multiple muscle groups throughout the body is called generalized myasthenia gravis.

For more information about myasthenia gravis, or to schedule an appointment at Virginia Mason's Neuroscience Institute, call (206) 341-0420.

What Causes Myasthenia Gravis?

Myasthenia gravis is an autoimmune disease, which means the body's antibodies mistake something in the body as foreign and destroy it. In myasthenia gravis, antibodies attack receptor sites on the muscle cells, preventing them from receiving sufficient chemical messages from nerve cells to function normally.

It is thought that the thymus gland, an immune system gland located beneath the breastbone, may be responsible for the production of these antibodies. In some people with myasthenia gravis, the thymus is abnormally large. About 15 percent of all myasthenia gravis patients are found to have a thymoma, a tumor of the thymus. Although most thymomas are benign, the thymus is usually removed. Even if no tumor is present, removal of the thymus seems to improve symptoms in some patients. 

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Symptoms of Myasthenia Gravis

Because muscles do not receive sufficient signals from nerve cells, myasthenia gravis causes muscle weakness that worsens with repeated use. Resting can improve muscle performance causing the weakness to come and go. The disorder can affect any voluntary muscles, but specific muscle groups are frequently involved, including:

  • Eye muscles - Effects include the drooping if one or both eyelids and double or blurred vision.
  • Face and throat muscles - Muscle weakness in these areas affects facial expressions, chewing, swallowing and speaking.
  • Arm and leg muscles - Weakness in the arms and legs usually happens in conjunction with other affected muscle groups. Arms are typically affected more often than legs, but weakness in the legs can seriously impair walking.

Diagnosing Myasthenia Gravis

Sometimes myasthenia gravis is diagnosed long after onset due to mild symptoms, or because symptoms are attributed to another condition.

A diagnosis begins with a complete physical and neurological examination to evaluate symptoms and muscle function. Eye movement is observed for impairment, as is any additional muscle weakness. If myasthenia gravis is suspected, there are specific tests to help confirm the diagnosis, including:

  • Blood test - A blood test can show the presence of abnormal antibodies known to disrupt the receptor sites of impulses that signal muscles to move.
  • Endrophonium test - In this test the drug endrophonium chloride is injected intravenously, which blocks the breakdown of acetylcholine, a chemical necessary for the transmission of messages to muscle cells. People with myasthenia gravis will experience a sudden, but temporary improvement in muscle strength due to increased levels of acetylcholine.
  • Repetitive nerve conduction study - Small pulses of electrodes are repeatedly sent through nerves and monitored for their ability to transmit signals to the muscles. Muscles that weaken when nerves are repetitively stimulated is an indication of myasthenia gravis.
  • Single-fiber electromyography (EMG) - EMG tests measure the electrical activity of nerve-to-muscle signal transmission. In this type of EMG a single muscle fiber is tested for its response to electrical stimulation.
  • CT or MRI - A CT or MRI scan may be used to identify an abnormal thymus gland or the presence of a tumor (thymoma).
  • Pulmonary function test - This test measures breathing strength and may indicate possible future problems with respiration from weakened muscles.

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Treatment for Myasthenia Gravis

There is no cure for myasthenia gravis, but treatment can result in prolonged periods without any symptoms. The choice of treatment can depend on several factors, including the patient's age, overall health, severity of symptoms and rate of disease progression.

Types of medications used to relieve symptoms include:

  • Cholinesterase inhibitors - Drugs such as pyridostigmine and neostigmine work by enhancing communication between nerves and muscles. As a result muscles perform better and strength is improved.
  • Immunosuppressants - Corticosteroids and other drugs such as cyclosporine and azathioprine help by suppressing the activity of the immune system and increasing muscle strength. Because these treatments can cause serious side effects, they are generally used for more severe cases or when other medications are not effective.

Other treatments for myasthenia gravis include:

  • Plasmapheresis - If symptoms are very serious or life-threatening, a procedure called plasmapheresis can filter the blood of the muscle-weakening (autoimmune) antibodies by replacing the patient's plasma with donor plasma or with other intravenous fluids. The procedure may reduce symptoms for several weeks, but can have serious side effects including low blood pressure, blood clots or infection.
  • Intravenous immune globulin - In this technique, a mixture of helpful antibodies is given directly into the bloodstream to blunt the effect of the autoimmune antibodies. The abnormal immune system response of myasthenia gravis is altered, and symptoms generally improve within one to two weeks. Benefits of this procedure are temporary, lasting anywhere from three to eight weeks.

Surgery for myasthenia gravis

Thymectomy, the surgical removal of the thymus gland (which often is abnormal in myasthenia gravis patients), can reduce or eliminate symptoms in some patients. About 15 percent of the people who have myasthenia gravis have a tumor in their thymus and seem to benefit most from the thymectomy procedure.
Due to the risks involved in any surgical procedure, people with myasthenia gravis who don't have a tumor in the thymus should discuss the pros and cons of surgery with their doctor. In general, a thymectomy may not be recommended if symptoms are generally mild, only involve the eyes or if the patient is over 60 years old.

For more information about myasthenia gravis, contact Virginia Mason's Neuroscience Institute at (206) 341-1900.

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