LEMS is another neurotransmitter disease...similar in many ways to MG. In LEMS the problem is presynaptic and in MG it's post. LEMS is sometimes secondary to a carcinoma (usually - but not always - small cell lung cancer).

I only know this because it was suspected when I was clinically dx'd and they tested for LEMS in addition to the MG antibodies. When they did the thoracic CT they focused on lung and thymus just to be sure.

LEMS is more rare than MG and is an autoimmune disease that may (or may not) be triggered by a cancer in the body.