Yes, I am seronegative; however, I was started on the potassium supplements several months after my MG diagnosis (arrived at through a dramatic improvement on Mestinon trial).

I had what they THOUGHT was a heart attack last December (angiogram showed NO blockage in any arteries; nobody knows why I tested "positive" for a heart attack); my internist put me on a different BP medication which is known to deplete potassium in some patients, so she prescribed a supplement at the same time. It was a month or so after that - when bloodwork showed that my levels had dropped despite supplementation - that she put me on a higher dosage.

Thanks for the info on "Potassium-aggravated myotonia" - I'll have to add that to my list of "you learn something knew every day"....