Electrical myotonia in heterozygous carriers of recessive myotonia congenita

We investigated electrophysiologically the unaffected parents of patients w ith recessive myotonia congenita. We studied 18 families, in nine of which the diagnosis was confirmed by molecular genetics. Brief myotonic discharge s were present in at least one parent in 67% of the families. Fathers were more likely than mothers to show these discharges. The difficulty in distin guishing very mildly affected parents with dominant myotonia congenita from the heterozygous carriers of recessive myotonia congenita is stressed. (C) 1999 John Wiley & Sons, Inc.