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Re: What causes the weakness with MC?

Linda:

One study reported that they observed that some individuals with Becker MC and transient weakness experienced a "de-polarization conduction block" when the muscle was stimulated at certain frequencies. That's just a fancy way of saying, the muscles began to have difficulty responding to electrical stimulus, I think because the muscle membrane couldn't de-polarize.

I don't think they had speculated on a physiological model at that point.

You are the first person I've heard from who reports weakness with Thomsen MC. But MC has a high phenotype variability in general. I wouldn't be able to say if the SCN4A mutation is contributing to anything. If you post the mutation here maybe Jan or I could see if we could find anything on it for you.

Have you talked about the weakness with your doctors? There is always a chance it could be caused by something else.

Type of Myotonia: Becker

Country: USA

Re: What causes the weakness with MC?

Hi Jenna,

Thanks for your reply. Based on a recent EMG, my doctor said that my deeper muscles were "spent" or "burned out" and that the outer muscles were over working. The nerve conduction study was fine. He said the EMG results showed a chronic (long term) myopathy. His interpretation seemed to be that the constant firing of muscles from the MC burned the muscles out. A muscle biopsy done in 1978 when the MC was first diagnosed by EMG (of unknown etiology at that point) reported "Skeletal muscle with mild atrophy of both fiber groups, equivocal fiber type grouping and minor nonspecific myofiber changes . No pathologic diagnosis."

How do I determine which mutation I have on the SCN4A gene from the Athena report? There is something that says SCN4A c.999 C>T. is that the mutation? The report says that "there is no known published research on this variant."

Regarding others with MC experiencing weakness: as I have tried to search the issue in this forum, it seems that a number of other people have reported weakness. Perhaps this is just not a well reported or studied possible effect of MC for Thomsen's as well as Becker's MC. I also have a low tolerance for very hot weather and can become unduly affected and weakened under such conditions. Also, I find that if I eat too much table salt I can get muscle cramps. I saw that on another post by Jan she reported the same thing but attributed the cramping to the iodine in the salt. I have switched to sea salt. I don't know if I had too much sea salt if it would cause the same result. So for so good for now, however.

Thanks,

Linda

Type of Myotonia: Thomsen's

Country: USA

Re: What causes the weakness with MC?

Jenna, I have just finished looking over some of your previous posts on weakness that you associate with Becker's MC, e.g., March 18, 2013. What you describe sounds exactly like what I have and am expereincing. The mutation I have on the CLCN1 is the same as another person diagnosed with Thomsen's MC. My mother reported mild stifness when she was pregnant. I have a cousin my age that reports myotonia sounding symptoms. It was only because of noticing the stiffness in my cousin that I asked for MC testing. Prior to that I had been tested for MDI and MDII and hyper-PP. Neither have had the degreee of difficulty that I have had. Both my mother and cousin have a more muscular appearance than do I.

Can you send or post the article you mentioned regarding depolarization in Becker's MC?

Thanks,

Linda

Type of Myotonia: Thomsens

Country: USA

Re: What causes the weakness with MC?

Linda:

I also have a problem with weakness sometimes when it gets hot.

I have one of the more recent studies bookmarked on my computer but I'm not at home right now. However, if you go to scholar.google.come and type in "myotonia congenita" and weakness, you should pull a lot of studies up. I recommend using the date filer to the left of the page and limiting your results to no older than the year 2000 because in the older studies they did not have genetic testing and could not always distinguish the different myotonic disorders from one another by symptoms.

Type of Myotonia: Becker

Country: USA