Lambert-Eaton syndrome is a condition in which the body's immune system attacks the connections between nerves and muscles. It is most often seen in people with small cell lung cancer or other cancers, but it can also occur in people without cancer. What causes LEMS? In about 50 to 60 percent of cases, LEMS is associated with an underlying disease, particularly small cell lung cancer. It is thought that the body's attempt to fight the cancer inadvertently causes it to attack nerve endings because cancer cells share some of the same proteins as nerve endings.