You can still have Lems and respond to mestinon. Like Annie said, it needs to be ruled out as soon as possible because of the cancer link.
I take mestinon and 3,4 Dap for Lems and sometimes take extra mestinon when I am especially droopy.
Before I got diagnosed with Lems my two neuro's would have bet the farm on an mg diagnosis. I think I may have it along with the Lems because I have symptoms of both and have a hard time sustaining any benefit from treatment.