Survey of patients with early-onset myotonic dystrophy in the San-in district, Japan

We epidemiologically and genetically analyzed early-onset and adult forms of myotonic dystrophy in patients from the San-in district, Japan. Patients were collected from all available sources and re-examined by our group. Of a total 49 patients from 20 families, 38 were alive at the time of this study (June, 1980). The prevalence of this disease, including early-onset myotonic dystrophy, in the general population was 2.73×10−5. The incidence of early-onset myotonic dystrophy in patients under 30 years was 1.50×10−5. All patients with the early-onset form (4 males, 5 females) had an affected parent. The disease in the affected parent was mild or asymptomatic at the birth of the affected child. We detected no clear difference between the characteristics of the disease in the affected parent and those commonly seen in the adult form of myotonic dystrophy. Analysis of the pedigree patterns of families in which one or more members were affected suggested that the existence of the gene for myotonic dystrophy produced the early-onset form, while neonatal symptoms may be ascribable to an intra-uterine environmental factor.