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Myasthenia Gravis

What is myasthenia gravis?

Myasthenia gravis is a chronic autoimmune neuromuscular disease characterized by varying degrees of weakness of the skeletal (voluntary) muscles of the body. Myasthenia gravis is caused by a defect in the transmission of nerve impulses to muscles. It occurs when normal communication between the nerve and muscle is interrupted at the neuromuscular junction, the place where nerve cells connect with the muscles they control.

 

What are the symptoms of myasthenia gravis?

Although myasthenia gravis may affect any voluntary muscle, the muscles that are most frequently affected are those that control eye and eyelid movement, facial expression, and swallowing. The onset of the disorder may be sudden. Symptoms often are not immediately recognized as myasthenia gravis.


In most cases, the first noticeable symptom is weakness of the eye muscles. In others, difficulty in swallowing and slurred speech may be the first signs. The degree of muscle weakness involved in myasthenia gravis varies greatly among patients, ranging from a localized form, limited to eye muscles (ocular myasthenia), to a severe or generalized form in which many muscles, sometimes including those that control breathing, are affected. Symptoms, which vary in type and severity, may include a drooping of one or both eyelids(ptosis), blurred or double vision(diplopia) due to weakness of the muscles that control eye movements, unstable or waddling gait, weakness in arms, hands, fingers, legs, and neck, a change in facial expression, difficulty in swallowing and shortness of breath, and impaired speech (dysarthria).

 

How is myasthenia gravis diagnosed?

The first steps of diagnosing myasthenia gravis include a review of the individual's medical history, and physical and neurological examinations. The signs a physician must look for are impairment of eye movements or muscle weakness without any changes in the individual's ability to feel things. If the doctor suspects myasthenia gravis, several tests are available to confirm the diagnosis.


A special blood test can detect the presence of immune molecules or acetylcholine receptor antibodies. Most patients with myasthenia gravis have abnormally elevated levels of these antibodies. However, antibodies may not be detected in patients with only ocular forms of the disease.


Another test is call the edrophonium test. This approach requires the intravenous administration of edrophonium chloride or Tensilon, a drug that blocks the degradation (breakdown) of acetylcholine and temporarily increases the levels of acetylcholine at the neuromuscular junction. In people with myasthenia gravis involving the eye muscles, edrophonium chloride will briefly relieve weakness.


Other methods to confirm the diagnosis include a type of nerve conduction study which tests for specific muscle fatigue by repetitive nerve stimulation. This test records weakening muscle responses when the nerves are repetitively stimulated, and helps to differentiate nerve disorders from muscle disorders. Repetitive stimulation of a nerve during a nerve conduction study may demonstrate decrements of the muscle action potential due to impaired nerve-to-muscle transmission.


A different test called single fiber electromyography (EMG), in which single muscle fibers are stimulated by electrical impulses, can also detect impaired nerve-to-muscle transmission. EMG measures the electrical potential of muscle cells. Muscle fibers in myasthenia gravis, as well as other neuromuscular disorders, do not respond as well to repeated electrical stimulation compaired to muscles from normal individuals.


Computed tomography (CT) or magnetic resonance imaging (MRI) may be used to identify an abnormal thymus gland or the presence of a thymoma.


A special examination called pulmonary function testing - which measures breathing strength - helps to predict whether respiration may fail and lead to a myasthenic crisis.

 

What is the treatment for myasthenia gravis?

Myasthenia gravis can be controlled. There are several therapies available to help reduce and improve muscle weakness. Medications used to treat the disorder include anticholinesterase agents such as neostigmine and pyridostigmine, which help improve neuromuscular transmission and increase muscle strength. Immunosuppressive drugs such as prednisone, cyclosporine, and azathioprine may also be used. These medications improve muscle strength by suppressing the production of abnormal antibodies. They must be used with careful medical follow-up because they may cause major side affects.


Thymectomy, the surgical removal of the thymus gland (which is abnormal in myasthenia gravis patients), improves symptoms in more than 50 percent of patients without thymoma and may cure some individuals, possibly by re-balancing the immune system.


Other therapies used to treat myasthenia gravis include plasmapheresis, a procedure in which abnormal antibodies are removed from the blood, and high-dose intravenous immune globulin, which temporarily modifies the immune system and provides the body with normal antibodies from donated blood. These therapies may be used to help individuals during especially difficult periods of weakness.

 

A neurologist, along with the primary care physician, will determine which treatment option is best for each individual depending on the severity of the weakness, which muscles are affected, and the individual's age and other associated medical problems.