| Myasthenia Gravis - The Resource
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TESTING FOR MYASTHENIA GRAVIS 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. (The acetylcholine receptor antibody attacks receptors for the neurotransmitter acetylcholine, which sends signals from nerves to muscles and from nerve to nerve in the brain. The antibody prevents transmission of the signal and causes muscle weakness.)
Acetylcholine Receptor (AChR) Binding Antibodies Striational (Striated Muscle) Antibodies Acetylcholine Receptor (AChR) Modulating Antibodies (Will be performed if AChR Binding Antibodies is negative.) Of particular value in the initial investigation of: 1) a patient older than age 20 with recent onset of Myasthenia Gravis symptoms (<1 year), particularly if weakness is mild or restricted to ocular muscles; 2) any patient with suspected or proven thymoma, whether or not symptoms or signs of Myasthenia Gravis are present. (also of value for serially monitoring patients after removal of thymoma; a rising titer of striational antibodies may herald tumor recurrence); and 3) a bone marrow transplant recipient with suspected graft-vs.-host disease, particularly if evidence of weakness. CPT Codes Reference Range Another test is called the edrophonium test. This approach requires the intravenous administration of edrophonium chloride or Tensilon(r), 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. According to the Merck Manual, the test is as follows:
Other methods to confirm the diagnosis include a version 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. Low-rate repetitive nerve stimulation also is used to demonstrate problems with neuromuscular transmission. A decremental pattern (>10% difference in compound muscle action potential amplitude between the first and fourth or fifth stimulus) is the usual finding. Single-fiber electromyography produces the most sensitive data to support the diagnosis. Findings are abnormal in more than 90% of patients. This test shows abnormality in neuromuscular transmission as increased jitter and blocking. 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 compared 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.
The hottest topic in MG testing right now is the MuSK antibody test. Muscle specific tyrosine kinase (MuSK) is a surface membrane enzyme that is essential in aggregating AChR during the development of the neuromuscular junction. Its role in mature muscle is not yet clear. Recent studies have shown that antibodies to MuSK are present in patients with generalized seronegative MG. Anti-MuSK antibodies have been described in 40% to 70% of patients with generalized SN-MG, but not in patients with ocular myasthenia or antibodies to the acetylcholine receptor. Circulating antibodies to the acetylcholine receptor (AChR) are found in up to 80% of patients with autoimmune MG. For this reason, researchers believe that there may be TWO kinds of myasthenia gravis and each responds to treatment, differently. Those that test positive for MuSK oftentimes don't respond as well, if at all, to the conventional medications currently used. Most reported MMG patients are female. The disease may begin from childhood to the seventh decade of life. Some MMG patients have clinical features indistinguishable from non-MuSK MG, but others differ from the usual clinical pattern of MG. Oropharyngeal muscles may be predominantly involved and become severely atrophied in some MMG patients. Others have weakness predominantly in neck, shoulder and respiratory muscles, with little or no ocular muscle involvement. Results of electrodiagnostic testing may be atypical for MG: EMG findings suggest myopathy in some, although muscle biopsy has not shown myopathic features; repetitive nerve stimulation studies are frequently normal in limb muscles; and, unlike most MG, jitter may be abnormal only in muscles that are severely involved. Cholinesterase inhibitors produce a variable response in MMG, and may actually exacerbate weakness. Thymectomy does not appear to benefit these patients, but they have consistently improved after plasma exchange, and many respond well to early, aggressive and selected immunotherapy. For those that are seronegative, don't be afraid to suggest the MuSK test. It is still fairly new so it could take a few weeks but well worth knowing the results. Insurance will normally pay for it. General cost of the test is between $350 - $500. Athena Diagnostics is the main lab that handles MuSK testing. If you are interested in getting a copy of the requisition form for this test to take to your doctor, you may download it HERE (.pdf)
There are other labs that do this testing but they are hard to find. Many of them are university or hospital related labs. I've provided what contact information I could. If one of these labs is near you and you'd like to find out more, I'd suggest calling them and finding out their particular procedures for testing.
Washington University School of Medicine
CONCLUSION The blood test for myasthenia gravis (acetylcholine receptor antibody test) is often negative in patients with myasthenia gravis. About half of patients with only eye muscle weakness (ocular myasthenia) will not have antibodies detected. Early in the disease or when it is relatively mild perhaps 1 of every 5 patients may not have antibodies. The next step in confirming the diagnosis a standard electromyogram (EMG) with repetitive stimulations. This test may also be negative and you could have myasthenia. If so, then a single-fiber EMG (performed by an expert) should be done. This test is the most sensitive test for myasthenia.
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