In about 60 percent of cases, LEMS is associated with small-cell lung cancer (and more rarely with other types of cancer), which might be diagnosed at the same time as LEMS or years later.
There’s evidence that cancer cells inappropriately make a protein called voltage-gated calcium channel (VGCC), which triggers the immune system to make anti-VGCC antibodies. Some 85 to 90 percent of people with LEMS test positive for antibodies against VGCC, so even in cases where no cancer is present, LEMS can be confirmed by detection of anti-VGCC antibodies in the blood. For more on the molecular underpinnings of LEMS, see Causes/Inheritance.
The autonomic (involuntary) symptoms and predominant leg weakness of LEMS also help to distinguish it from myasthenia gravis (MG). Electrodiagnostic testing that shows an increased muscle response to repeated stimulation also favors LEMS rather than MG (in which the response decreases).