Epidemiological surveillance of myotonic dystrophy type 1: a 25-year population-based study.

This epidemiological study investigates the evolution of the prevalence and the phenotypes in a large cohort of myotonic dystrophy type 1 (DM1) patients living in the Saguenay-Lac-Saint-Jean (SLSJ) region (Quebec, Canada) over a 25-year period (1985-2010). In 1985, 406 patients with DM1 were known. From 1985 to 2010, 352 new DM1 patients were diagnosed and 321 patients died. During this period, we observed a significant ageing of the DM1 population, from a median age of 34.5years to 49.0years. The proportion of patients with an adult phenotype decreased significantly from 78% to 53% while the proportion of patients with a mild phenotype increased from 6% to 26%. Reasons for the ageing of the DM1 population and the changes in the distribution of DM1 phenotypes include the progressive lowering of total fertility rates over the last decades, a reduction of births at risk as a result of genetic counselling and prenatal diagnosis, and an increase in the number of mildly affected patients often recognised at an older age by predictive testing and molecular analysis. Although the DM1 prevalence remains very high in 2010 (158/100,000) in the SLSJ region, we observe a trend toward a decline in point prevalence rates over the last 10years.