Re: Is the a screening test for the gene that causes MC?
Stephen, it sounds like your cousin might have had myotonic dystrophy, not myotonia congenita. There is a test for that which checks for both types, DM1 and DM2. There is a congenital form that shows up at birth and can be fatal.
Myotonia congenita does not directly cause death in children or adults. It can make you susceptible to anesthesia reactions which may be fatal.
There are over 100 documented mutations in the CLCN1 gene which can cause myotonia congenita. There are two labs in the US that do the testing, Athena and Fullerton Genetics. You usually have to get a referral to a genetics counselor who can order the test.
Does anyone else in your family have stiffness/myotonia? If your cousin did have myotonic muscular dystrophy, then probably your aunt was the affected parent since it is much more severe when it comes through the mother. That is not the case with myotonia congenita.
So the first step would be to confirm which condition your cousin had and how it was diagnosed. Then you can talk to your doctor about ordering a test for that particular condition. If you don't have insurance to cover myotonia congenita testing, there is a lab in Germany that will do it for free with a doctor's order.
Re: Is the a screening test for the gene that causes MC?
Chris, I have never known someone with myotonia congenita whose muscle biopsy was not inconclusive. It's painful, leaves a scar, and is totally unnecessary these days unless all the DNA tests for myotonic disorders come back negative. Then it can help confirm something like a glycogen storage disease.