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Lambert-Eaton Myasthenic Syndrome (LEMS)

Diagnosis

The diagnosis of LEMS is often difficult if the disease is not suspected, since weakness in the legs with or without arm weakness and fatigue are common complaints in general. The diagnosis is more likely to be suspected in people with cancer. Often, the initial suspected diagnosis is MG, due to an overlap in symptoms. The presence of a dry mouth is also a helpful clue.

A neurologist will ask many questions and conduct a physical exam to determine the extent of weakness. The tendon (muscle stretch) reflexes are usually decreased in LEMS and that is a helpful finding. In addition, some patients will have improvement in reflexes after 10 seconds of activating a muscle, and this finding is also a clue that LEMS may be present.

Often, the diagnosis is made in the electromyography (EMG) laboratory, where patients may undergo testing for various causes of weakness. The nerve responses are usually low, and the examiners should then have the person exercise the muscle for 10 seconds and stimulate it again. In LEMS, the response usually increases by more than 60 percent and this is called an increment. When positive, especially at >100 percent increment, this test is diagnostic of LEMS. In addition, if the physical exam is consistent with LEMS and if the EMG is suggestive of LEMS, the neurologist usually orders a blood test designed to detect antibodies to calcium channels on the nerve side of the nerve-muscle junction. Positive test results support a diagnosis of MG, but some individuals lack these antibodies. In some instances, the antibody test is performed before the EMG.

If a diagnosis of LEMS is confirmed, a CT scan of the chest should be obtained to look for a small cell lung cancer. If negative, a PET-CT should be considered. If negative, follow-up chest CTs are recommended for up to five years to monitor for an underlying cancer. A 2010 taskforce recommended repeating this screening at three to six months, and then every six months for two years in people with LEMS, if testing remains unrevealing. Most cancers, if present, are detected in the first year.

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