Risk estimates for neonatal myotonic dystrophy.

Children who inherit the autosomal dominant gene for myotonic dystrophy from their mother rather than their father may develop the severe neonatal type rather than the late onset type. The families of 22 neonatal type probands and 59 late onset type probands were studied to determine the risk of occurrence and recurrence of the neonatal type. The frequency of the neonatal type in sibs of neonatal type probands was 29%, or 37% if cases of neonatal deaths are counted as affected. This is significantly higher than the 6% of the neonatal type found in the offspring of affected women not ascertained through a child with the neonatal type. These data suggest that certain women carrying the gene for myotonic dystrophy are predisposed to have children affected with the neonatal type rather than the late onset type. The female near relatives of these women do not seem to share this predisposition. The data should be useful for genetic counseling.