Lambert-Eaton Myasthenic Syndrome

Lambert-Eaton Myasthenic Syndrome results from autoantibodies against voltage-gated calcium channels (VGCCs) at the presynaptic terminal of the neuromuscular junction. These antibodies prevent calcium influx necessary for acetylcholine release, leading to inefficient neuromuscular transmission. Most LEMS patients (80-85%) are seropositive for anti-VGCC antibodies, while seronegative cases involve antibodies against other presynaptic proteins like SOX1 or syntaxin-1A. LEMS is strongly associated with underlying malignancy in 50% of cases, most commonly small cell lung cancer (SCLC), but also other cancers. Seronegative patients have higher malignancy association. The pathophysiology involves both complement-mediated destruction of the presynaptic terminal and antibody-mediated interference with calcium channel function. Patients characteristically demonstrate improved strength with repetitive muscle activity due to calcium accumulation at the neuromuscular junction. Weakness predominantly affects proximal leg muscles, though it may progress to involve arms, trunk, and bulbar muscles in severe cases. Autonomic dysfunction is a distinctive feature, reflecting VGCC involvement in autonomic neurons. Management requires identifying and treating any underlying malignancy, as this often improves LEMS symptoms, along with symptomatic treatment with 3,4-diaminopyridine or immunosuppressive therapy.