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Saturday, May 25, 2013
The initial diagnostic exam for Myasthenia Gravis includes the following:
An additional test is:
In rare patients all these tests are normal, but examination by a neurologist suggest Myasthenia Gravis. If the disease is mild or purely ocular (symptoms of the eye muscles), then the tests are more frequently negative then in the case of the generalized disease.
The Tensilon test is done by injecting the drug Tensilon into a vein and watching for rapid improvement of strength, usually of eye muscles. Improvement in strength of speech may also be considered a positive test. A patient feeling better after Tensilon does NOT necessarily mean that the patient has MG. Blood pressure and pulse need to be monitored during the test. A patient may have MG and a negative Tensilon test (no improvement with the administration of the medication), but then the diagnosis should be supported by other tests, such as the acetylcholine receptor antibodies in the blood, or by a positive EMG test. False positive (the Tensilon test suggests that the MG is present, but it actually is not) and false negatives (the Tensilon test does not improve strength even if MG is present) occur. False negative tests are more frequent. Possible side effects need to be explained to each patient.
A CT (computed tomography) or MRI (magnetic resonance imaging) of the brain does not make a diagnosis of MG, but may help rule out other diagnoses. It is important to have a CT or MRI of the chest to makes sure a tumor of the thymus is not present. PET scans are not useful to diagnosis of MG. These test brain function, which is normal in MG.
The acetylcholine receptor antibody test is a blood test to measure the amount of antibody directed against the acetylcholine receptor. The test is measured in nanomoles (nM) per liter. Individual laboratories establish particular ranges of normal. These are always listed with the test result. For the acetylcholine receptor antibody binding test the normal range generally is 0-.03 nM/liter. Antibody tests are normal in 10-20% patients with the generalized disease and in about 50% of the patients with ocular (eyes only) myasthenia gravis.
There is a rough correlation between the level of the antibody test and the severity of weakness, but patients may have little weakness or be in remission but have increased acetylcholine antibodies.
The single-fiber EMG evaluates how well the nerve and muscle communicate. It involves insertion of a recording needle electrode into the area of the nerve-muscle communication point (the neuromuscular junction). There may be some discomfort with insertion of the electrodes (similar to an intramuscular injection, or a shot that goes right into the muscle). A single fiber EMG is often done on an eye, forehead, or forearm muscle. If a person is in remission, the SFEMG is normal, but if there is weakness from MG, the SFEMG is usually abnormal. Patients with ocular myasthenia will have an abnormal SFEMG. The diagnosis of MG is made after accounting for all the clinical symptoms and signs as well as the tests performed. Most patients tolerate the procedure well. A single-fiber EMG is a more technically difficult procedure than the standard EMG. Studies of this method suggest that it may detect abnormalities in over 95% of MG patients. However, other diseases may produce similar abnormalities. The test should be done by someone with extensive experience.
Last Reviewed: Dec 29, 2003
Henry J Kaminski, MD
Formerly, Professor of Neurology
School of Medicine
Case Western Reserve University