Lambert-Eaton myasthenic syndrome (LEMS) is a rare condition in which the immune system attacks the neuromuscular junctions – the areas where your nerves communicate with muscles – specifically the voltage-gated calcium channels (VGCC). Normally, the nerve cells send signals to muscle cells to help muscles move. When the neuromuscular junctions are attacked, this interferes with the ability of nerve cells to send signals to muscle cells resulting in muscle weakness, especially of the muscles in the legs and arms. LEMS is commonly associated with long-term smokers and patients with small-cell lung cancer (SCLC), but it is also seen in other autoimmune conditions such as type 1 diabetes, thyroid disorders, rheumatoid arthritis, and systemic lupus erythematosus. Sometimes the cause of LEMS is unknown.