Pre‐symptomatic detection and genetic counselling in myotonic dystrophy

A study of patients with myotonic dystrophy and their first degree relatives has shown that 17.6 % of asymptomatic individuals at risk were definitely affected on the basis of neurological and slit lamp examination, while a further 13.7 % showed suspicious but not diagnostic changes. In those individuals who were definitely affected neurologically, a high incidence of lens opacities was shown by the slit lamp, except during the first decade of life. Analysis of penetrance of the myotonic dystrophy gene in the offspring of affected individuals suggested that penetrance was largely complete by adulthood. No instance of mutation was encountered in the study, and it is suggested that undetected transmission is much more frequently responsible for the apparently isolated case than is mutation. The use in genetic counselling and prenatal diagnosis of the various methods of detecting the myotonic dystrophy gene is discussed.