I've been looking for the actual biochemical reason for avoiding beta blockers. Because they inhibit the release of renin and the production of aldosterone, I suspect they cause potassium to increase. For most people that's a good thing. If you try low doses and don't see an increase in myotonia, then it may not be a problem for you.
Here's a very good article written by Professor Hanna, a myotonia expert in London, which mentions various drugs that can cause muscle problems:
I don't know if taking a prescription normally used for myotonia would cancel out the side effects of a beta blocker. It would probably need to be a loop diuretic which would cause an increase in potassium excretion.
Dr. Tawil would be the expert I would recommend to sort it all out.
Thanks so much Jan. As usual you provide a wealth of knowledge. I look forward to reading the article, but my new computer won't let me access it for some reason at the moment.
I tried mexiletine 150mg on Sat with no problem, but when I took a second dose on Sunday morning with my thyroid medication my HR became very elevated for several hours. Nothing seems to be very easy...
I see you have added the SCN4A mutation to your "type of "myotonia". I have done so as well. I find myself wondering if that combo might have some added medical impact...