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HOW IS THE DIAGNOSIS OF MG CONFIRMED?

Weakness and fatigue are such common complaints from a variety of causes that it is not surprising that the diagnosis of myasthenia gravis is often missed in people in whom the weakness is mild or restricted to only a few muscles. Once the possibility occurs to a doctor, however, there are several approaches to confirming the diagnosis.

One way is to test for specific muscle fatigue by repetitive movements of the eyes, arms or legs. This can be done without equipment, or it can be done electrically by recording a weakening muscle response when a nerve to that muscle is electrically stimulated repetitively. Not every person with the disease will show a characteristic response to this repetitive nerve stimulation. A much more sensitive electrical test called single fiber electromyography is more likely to show evidence of neuromuscular malfunction (not specific for MG), but it requires special equipment and skills which are not widely available.

A very specific test for MG is a blood test to look for serum antibodies to acetylcholine receptor. Eighty percent of all patients with MG have abnormally elevated serum levels of these antibodies, but positive test results are less likely in patients with mild or purely ocular forms. The chance of receiving a falsely positive test result from a reputable laboratory is small, although borderline tests should be repeated.

A third approach to MG diagnosis is pharmacological, using drugs which may worsen or improve the weakness. At one time the native South American poison curare was used in very small doses to test for worsening of MG, but this can be dangerous and has fallen out of fashion. Nowadays the short-acting drug edrophonium chloride (brand-name "Tensilon/') is used intravenously to try to make the diagnosis of MG by reversing some obvious and measurable weakness, such as a drooping eyelid or a 10W breathing capacity.

Sometimes all these tests are negative or equivocal in someone whose story and examination still seem to point to a diagnosis of myasthenia gravis. The positive clinical findings should probably take precedence over negative confirmatory tests, each of which has its weaknesses. Some people, therefore, have to be followed by their doctors with a diagnosis of "possible MG" or "probable MG" until the situation clarifies itself. Under such circumstances both the patient and the doctor have to keep an open mind.  

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