Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disease that is often associated with small-cell lung carcinoma. It is characterized by fatigability, hyporeflexia, and autonomic dysfunction. The neuromuscular junction appears normal in morphology, and postjunctional receptor function is unchanged. However, particles at the active zones of nerve terminals that correspond to Ca++ channels are reduced in number and disorganized, and patients with LEMS have high titers of antibodies against the prejunctional P/Q-type Ca++ channel. Thus, muscle weakness results from impaired influx of Ca++ and diminished release of ACh. Diagnosis is confirmed by an incremental increase in electromyographic recordings upon repetitive stimulation.
Treatment with guanidine may produce clinical improvement within 3 to 4 days. Side effects include pares-thesia, gastrointestinal distress, renal tubular necrosis, and hyperirritability. The most serious effect is bone marrow depression, which is dose-related and potentially fatal. Aminopyridines have been used in clinical studies with some positive results. Corticosteroids and plasmapheresis may also be of some benefit, whereas anticholinesterase agents are only marginally effective.
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