Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disease - a disease in which the immune system attacks the body's own tissues. The attack occurs at the connection between nerve and muscle (the neuromuscular junction) and interferes with the ability of nerve cells to send signals to muscle cells.

Specifically, the immune system attacks the calcium channels on nerve endings that are required to trigger the release of chemicals (acetylcholine). With fewer calcium channels, the nerve ending releases less acetylcholine. Acetylcholine is a chemical messenger that triggers muscle contraction. In people with LEMS, the lowered levels of acetylcholine are not sufficient to cause normal muscle contractions, causing muscle weakness.

The disease is named for Edward Lambert and Lee Eaton, neurologists at the Mayo Clinic in Rochester, Minn., who first described myasthenic syndrome in the 1950s and '60s.