myotonia congenita
"I have been using this for a couple of months and have been able to start a wave of exercise that I have never been able to before.  I can run without my body seizing up and punishing me.  As this is relatively new as an option for me, I will continue to use it although I have found that my ankles are regularly puffy and i get more visual migraines.  Still do not know if this is actually connected to the use of Mexiletine though."