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Myotonic dystrophy type 1 is the commonest neuromuscular disease affecting adults. It is inherited in an autosomal dominant manner, and is linked to a dynamic expansion of a CTG triplet repeat localised to chromosome 19q13.3. The phenotype can be divided into four main groups: mild, juvenile, classical, and congenital. The most severe form of the condition is observed in congenitally affected infants usually born to classically affected mothers. Recently, the nomenclature has been revised and myotonic dystrophy is referred to as DM1.1
Congenital myotonic dystrophy (CDM) was first described in 19602 and is the most severe phenotypic expression of DM1. It represents the final stage in the typical three generation anticipation cascade observed in this condition.3 The symptoms may present late in pregnancy with reduced fetal movements, polyhydramnios, or hydrops fetalis.4–6 Often the birth of a severely affected child identifies an extensive DM1 pedigree. The reasons for the almost exclusive maternal transmission of CDM are not clearly understood. There are no particular clinical features in the mothers of CDM children to account for this, but, from earlier studies, all the women exhibited clinical myotonia7 and CDM cases were confined to the offspring of clinically affected women.8 Koch et al9 found that only women with multisystem signs of DM1 at the time of pregnancy and delivery were likely to have congenitally affected offspring and that the chance of having a more severely affected child increased with maternal disease severity. These observations have been given support by more recent molecular studies; infants …