The utility of the determination of CTG trinucleotide repeat length in hypotonic infants.

A 9-month-old male infant was floppy from birth with nonprogressive facial and distal limb weakness and apparently normal mother and father. The facial characteristics and distribution of involvement suggested congenital myotonic dystrophy and the infant, but not the mother, had insertional myotonia in one of four muscles tested. Had the number of CTG trinucleotide repeats been tested when the presence of a congenital myotonic dystrophy-like clinical picture was first appreciated, the proper diagnosis could have been made several months earlier. The application of new molecular genetic techniques is changing the usual sequence of studies performed in the evaluation of the hypotonic infant.