Dystrophia myotonica and myotonia congenita concurring in one family. A clinical and genetic study.

In a unique sibship of 5, 2 siblings were found to have dystrophia myotonica and 3 had myotonia congenita. A study was made of the paternal and maternal families and of the offspring of the 5 siblings. Eighty relatives were examined clinically, by slit-lamp and by electromyography. In the relatives of the mother of the sibship only dystrophia myotonica was found and in the relatives of the father only myotonia congenita. In the offspring of the sibship the two diseases were transmitted independently in the successive generations. All patients showed a clinical picture of either one or the other disease and transitional clinical forms were not found. A linkage study was performed in the family with myotonia congenita and in the family in which both diseases occurred. The myotonia congenita gene is probably not linked to the secretor and lutheran genes. The hypothesis that there is no linkage between myotonia congenita and the secretor-lutheran linkage group was found to be 25 times more probable than the alternative hypothesis that myotonia congenita is linked to this group. This family study therefore provides evidence that dystorphia myotonica and myotonia congenita are both clinically as well as genetically distinct.