Myasthenia gravis is often diagnosed with a thorough medical history and physical exam. In most cases, specialized tests are performed to confirm that the diagnosis is correct.

During the exam, the doctor will ask about the patient's symptoms. The doctor will want to know when the symptoms occur and things that seem to make them worse. The doctor will also look for signs of muscle weakness. These signs could include drooping eyelids, double vision, and difficulty holding the arms in a certain position. The doctor will also note if the patient has noticed any changes in sensation. Changes in sensation suggest that a disorder other than myasthenia gravis may be causing the weakness.

Since muscle weakness is a common symptom in many disorders, certain tests will help confirm a diagnosis of myasthenia gravis. Sometimes all tests are negative even though the patient's history and physical exam suggest myasthenia gravis. In these cases, a patient may be diagnosed with "possible MG" or "probable MG."

Tensilon Test: During this test, a short-acting drug, edrophonium chloride (Tensilon) is injected through a vein. This drug quickly reverses some muscle weakness, such as drooping eyelids, in patients with myasthenia gravis. When the response to Tensilon is inconclusive, a longer acting drug, neostigmine, may be used. Neostigmine is injected into a muscle or given in pill form.

Acetylcholine Receptor Antibody Titer: This procedure is performed on a sample of the patient's blood. The blood is tested for the presence of antibodies to the acetylcholine receptor. About 70-80 percent of patients with general myasthenia gravis have high antibody concentrations, known as titers. High titers are less likely in patients with mild or ocular myasthenia gravis. Abnormal titers help confirm the diagnosis but do not predict the severity of the disorder. Normal titers, however, do not exclude a diagnosis of myasthenia gravis.

Electromyography Repetitive Nerve Stimulation (RNS): This study helps distinguish nerve disorders from muscle disorders. Many conditions, including myasthenia gravis, can cause this test to be abnormal. The test is performed by placing small electrodes on the skin over a muscle, often a muscle in the hand. A mild shock is used to make the muscle contract. A special machine records information about the muscle contraction. The doctor then evaluates the information to help make a diagnosis.

Single Fiber Electromyography: This is the most sensitive test for disorders of the neuromuscular junction (the area in which a nerve cell and muscle cell meet). The test is abnormal in almost all patients with myasthenia gravis and a normal result usually excludes this diagnosis. However, other disorders can also cause abnormal results. To perform the test, a thin needle is inserted into a muscle. The needle is connected to a machine that records the electrical change in muscle cells when the muscle contracts. The doctor evaluates this information to help make a diagnosis.

Other Tests Used for Patients with Myasthenia Gravis

Computed tomography (CT): This test helps doctors identify an abnormal thymus gland or the presence of a tumor, called a thymoma.

Pulmonary Function Test: This test measures breathing strength. The test can help predict problems with breathing that may lead to a myasthenic crisis.

References

First published April 1, 2000
Last updated November 25, 2002
Copyright © 2000 Accordant Health Services, Inc. All Rights Reserved.

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