Hi Richard:

Non-dystrophic myotonias can usually be classified as sodium channel myotonias or chloride channel myotonias.

Becker and Thomsen MC are chloride channel myotonias and are caused by mutations on the gene that makes skeletal muscle chloride ion channels, CLCN1.

Paramyotonia, and Potassium Aggravated Myotonia are sodium channel myotonias and are caused by mutations on the gene that makes skeletal muscle sodium ion channels, SCN4A.

These two ion channels are essentially two sides of the same coin. The way muscles work from a very overly simplified ion channel perspective is, an action potential comes down the nerve and eventually causes the sodium ion channels to open, which pump sodium ions into the cell. This ultimately causes the muscle fiber to contract. For the muscles to relax, the chloride ion channels have to open, and pump chloride ions into the cell to neutralize the sodium ions and turn the muscle "off".

If either the sodium ion channels can't close fast enough for well enough, or the chloride ion channels can't open fast enough or get enough chloride ions into the cell fast enough, you can have myotonia.

Mexiletine acts on the sodium ion channels. From my understanding, it binds to the sodium ion channels, though I'm not clear if it causes them to deactive faster or makes them less permeable to sodium ions. Either way, I think the end result is, less sodium ions get into the cell, so you don't need as much chloride ions to relax the muscles. I could be wrong.

But it usually works very well for people with chloride channel myotonias.

It seems to be less effective for people with sodium channel myotonias. I read one study which found that one possible reason is that the Mexiletine molecules didn't bind very well to some of these mutated sodium ion channels.
It literally could not dock on to these mutated channels properly.

Anyway, there are other medications, though I'm not very familiar with them. Acetazolamide is one of them, though it's more commonly used for sodium channel myotonias I think. The National Institute for the Health here also lists Phenytoin, Procainamide, Quinine, and Tocainide.

All of these medications can cause potentially serious side effects.

Type of Myotonia: Becker

Country: USA