Myotonia congenita and myotonic dystrophy: descriptive epidemiological investigation in Turin, Italy (1955-1979).

A descriptive epidemiological survey of Myotonia Congenita (MC) and Myotonic Dystrophy (MD) was carried out on the resident population of the City of Turin, Italy. Cases were collected from the Archives of the Neurological Clinic, University of Turin, and from other neurological departments in the city's hospitals. Every patient (and the "healthy" relatives of MD subjects) underwent clinical and EMG re-examination, in order to confirm the initial diagnosis and to investigate the familial distribution of the diseases. The point prevalence rate was 0.9 (+/- 0.6) X 10(-5) for MC and 2.1 (+/- 0.8) X 10(-5) for MD. During the period 1955-1979 the mean annual incidence was 0.3 (+/- 0.2) X 10(-6) for MC and 0.7 (+/- 0.3) X 10(-6) for MD. The incidence-at-birth rate was 1.4 (+/- 1.2) X 10(-5) for MC and 2.9 (+/- 1.8) X 10(-5) for MD. The modalities of inheritance and sex distribution of MC and MD were also studied. Knowledge of epidemiological features of these myopathies is of vital importance for genetic counselling.