Detection of heterozygotes for myotonic dystrophy

In 34 families the effectiveness of various methods (clinical examination, electromyography and slit lamp examination of the lens) for detecting heterozygotes for myotonic dystrophy has been assessed. In those families in which the index patient developed symptoms in infancy or childhood, most heterozygotes were recognised by early adult life. However, in the families of index patients who developed symptoms after the age of 20, the expected 50% incidence of clinically affected heterozygotes among fight degree relatives was not reached before the age of 40. Only in the early onset families may it be assumed that a relative who is clinically unaffected by age 20 years is at small risk of transmitting the disorder.