Myotonic dystrophy associated with Down syndrome (trisomy 21).

A 27-year-old man had both autosomal dominant myotonic muscular dystrophy and classic trisomy 21 Down syndrome. Down syndrome had been recognized many years before the myotonic dystrophy. Although he had the typical stigmata of Down syndrome, he functioned socially with an I.Q. of 50. There was no evidence that either of the two diseases adversely affected the manifestations of the other. Although the occurrence of these two disorders in the same patient could be coincidental, this is at least the fifth report of myotonic dystrophy associated with chromosomal aneuploidy; the previous foud patients all had Klinefelter syndrome (47, XXY).