Congenital myotonic dystrophy type 1 (Steinert disease, congenital-onset DM1)

Myotonic dystrophy type 1 (DM1, Steinert disease) has an overall prevalence between ~1/2,300 and 1/8,000 in many populations, with regional clusters [WEB-02, WEB-09]. Congenital-onset DM1 represents the most severe end of the spectrum and is relatively rare but captured in pediatric DM1 registries [PAPER-01, WEB-09]. Congenital cases typically arise from maternal transmission of very large CTG expansions (>2000 repeats) [WEB-03, BIOMNI-02, BIOMNI-05].