Katja,

I know the two US labs (Athena and Fullerton) do a full sequencing of both genes, but I don't know if the Canadian lab does. So you can be pretty sure you don't have myotonia congenita, but there might be a possibility that you still have an SCN4A mutation if the lab only checked for known mutations.

There are a few other types of myotonia. This is an excellent paper on the various non-dystrophic conditions that cause it:

Redefining the Non-Dystrophic Myotonic Symptoms

I'm assuming your doctor ruled out myotonic muscular dystrophy. I believe that is the myotonic condition that is so prevalent in the province of Quebec that you mentioned. It can be very mild in the first generation of females with the disease. Even in milder cases it is almost always associated with a particular type of cataract. In fact optometrists are often the ones to initially diagnose it.

You might want to have your doctor send a sample to Dr. Lehmann-Horn's lab in Germany. If you go to the Periodic Paralysis Association and join (free), there is a menu tab for genetic testing that has all the paperwork. His lab is always looking for novel mutations related to myotonia. It can take several months, but it would be a good option for you.

Jan

Type of Myotonia: Thomsen's

Country: USA

Thanks jan for this paper, it was one I had no yet read and it was pretty clear although still very technical. I have to say the story in clinical box 2 of the 35 year old construction worker sounds a lot like me. I had been misdiagnosed for years with carpal tunnel in my hands because i had the classic locking of the hand muscles when opening jars or door handles. My legs often freeze when i get up form a chair and try to walk and it was amplified 10-fold during my pregnancies or when i was stressed. I think that i probably fit somewhere in the hyperkalemic PP. Myotonia has definitely been confirmed through 2 different EMGs done by neurologists looking for MC. They have thankfully ruled out dystrophic myototonia. Are there many people whose genetic sequencing has not found a mutation.
By the way Athena sequenced 23 exons, 3093 bp of the CLCN1 gene and the canadian lab sequenced only the exons known to have mutations for PMC (exons 13,19,22,23 and 24).
thanks kd

Type of Myotonia: thought i was beckers

Country: canada

If you didn't have sequencing on SCN4A, then you're probably right about it most likely being hyperkalemic periodic paralysis or paramyotonia congenita. Although new mutations are being discovered all the time, it can take 6 months to a year for them to show up on the international databases.

I would encourage you to send a sample to Dr. Lehmann-Horn's lab along with the report so that they know which exons were already checked.

The testing for myotonia congenita (CLCN1) is almost 100% accurate now. But the sodium channel testing is still improving and I believe the last estimate I saw was about 60% accurate.

Jan

Type of Myotonia: Thomsen's

Country: USA

katja d:

There have been instances of people having myotonia with only one recessive mutation found during sequences and there are a few reasons that might be, such as the mutation not really being recessive, or copy number variation. There could also have been a lab error.

When genetic testing fails to turn up CLCN1 mutations or SCN4A mutations, in the presence of electrical or clinical "myotonia", and your doctor's have no other ideas, it's probably time to contact one of the ion channel specialists who deal with the more exotic forms of myotonia, like Dr. Tawil or Dr. Ptáček.

Type of Myotonia: Becker

Country: USA