Clinical evidence for heterogeneity in myotonic dystrophy.

In a study of 35 index patients who developed myotonic dystrophy between birth and 30 years (neonatal cases aware excluded), 30 could be categorised into two clinical types. The 13 type 1 patients had a more severe limb weakness, of patchy distribution, associated with proportional facial weakness. The 17 type 2 patients had a milder and more diffuse limb weakness; their facial weakness, however, was very pronounced and preceded the limb weakness by several years. All but one of the 25 affected relatives who were examined belonged to the same category as their index relative, providing evidence that the cause of the clinical heterogeneity was genetic. Subsequent observations showed that mental retardation, male infertility, and neonatally affected offspring were commoner in type 2 patients. Congenital myotonic dystrophy could occur among the offspring of either affected males or affected females, but neonatal symptoms were confined to the offspring of affected women. The overall risk for having neonatally affected offspring for this prospective study of young adult patients was 7 in 38, and for the offspring of affected females 7 in 27. The risk for having a surviving child whose mental or physical handicap or both required special schooling was 1 in 12 for males and 4 in 27 for females.