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Myotonic dystrophy (dystrophia myotonica, DM) is the most frequently inherited neuromuscular disease of adult life. DM is a multisystem disease with major cardiac involvement. Core features of myotonic dystrophy are myotonia, muscle weakness, cataract, and cardiac conduction abnormalities. Classical DM (first described by Steinert and called Steinert’s disease or DM1) has been identified as an autosomal dominant disorder associated with the presence of an abnormal expansion of a CTG trinucleotide repeat on chromosome 19q13.3 (the DM 1 locus). A similar but less common disorder was later described as proximal myotonic myopathy, caused by alterations on a different gene on chromosome 3q21 (the DM2 locus). This article will mainly focus on DM1. It will provide an insight into the epidemiology and genetic alterations of the disease and provide up-to-date information on postmortem and clinical findings and on diagnostic and therapeutic options in patients presenting cardiac involvement.
EPIDEMIOLOGY AND CLASSIFICATION OF DM1
The incidence of DM1 is estimated to be 1 in 8000 births and its worldwide prevalence ranges from 2.1 to 14.3/100 000 inhabitants.1 Based on the age of onset and on its clinical features, DM1 can be divided into three forms: congenital, classical, and minimal, which may occur in the same kindred.
Congenital DM1 presents at birth or during the first year of life in a severe form. It is characterised by neonatal hypotonia, facial diplegia, joint contractures, frequent and often fatal respiratory failure, feeding difficulties, and developmental delay. The risk of dying from congenital DM1 in the neonatal period is high.1 Patients who survive exhibit non-progressive psychomotor retardation and may subsequently exhibit the features of the adult-type, classical form of DM1.
In the classical form, which is the most common, symptoms become evident between the second and the fourth decade of life, showing a slow progression over time (table 1⇓). The …