Lambert-Eaton myasthenic syndrome.

Lambert-Eaton myasthenic syndrome (LEMS) is an idiopathic or paraneoplastic syndrome producing antibodies against presynaptic voltage-gated P/Q calcium channels. This decreases calcium entry into the presynaptic terminal, which prevents binding of vesicles to the presynaptic membrane and acetylcholine release. LEMS is most often associated with small cell lung cancer, although idiopathic presentations comprise approximately 40% of the cases. The most common initial complaint is proximal muscle weakness involving the lower extremities more than the upper extremities. Depressed deep tendon reflexes and autonomic dysfunction are frequently present. Involvement of the bulbar or respiratory muscles is rare. Diagnosis is confirmed by electrophysiological testing, which demonstrates small compound muscle action potentials and facilitation with exercise or 20-Hz repetitive stimulation. A serum test for voltage-gated calcium channel antibodies is commercially available. Treatment involves removing the cancer associated with the disease. If cancer is not found, immunosuppressive medications and acetylcholinesterase inhibitors are used with moderate success. Patients with idiopathic LEMS should be screened every 6 months with chest imaging for cancer.