In 1909 Steinert and colleagues first clearly described the “classic”
type of myotonic dystrophy which was called Steinert’s disease (OMIM 160900). The gene defect responsible for myotonic dystrophy described by Steinert was discovered in 1992 and found to be caused by expansion of a CTG repeat in the 3′ untranslated region of DMPK, a gene encoding a protein kinase (2-4). Subsequently, in 1994, a different chemical structure multisystemic disorder was described with Inhibitors,research,lifescience,medical dominantly inherited myotonia, proximal greater than distal weakness, and cataracts but lacking the gene defect responsible for Steinert’s disease (5-7). In Europe, the disease was termed proximal myotonic myopathy (PROMM, OMIM*160900) (6) or proximal myotonic dystrophy (PDM) (7) while in the United States was termed myotonic dystrophy with no CTG repeat expansion or myotonic dystrophy type 2 (DM2) (5). Later studies demonstrated that many of the families identified as having myotonic dystrophy type 2, PROMM or PDM had the same disease, a disorder caused by an unstable tetranucleotide CCTG Inhibitors,research,lifescience,medical repeat expansion in intron 1 of Zinc finger protein 9 gene (ZNF9) mapped to 3q21.3 (8, 9). Due to the existence of different types of myotonic dystrophy, the International Myotonic Dystrophy Consortium developed a new nomenclature and guidelines for DNA testing
(10). The Steinert’s disease that results from an unstable trinucleotide repeat expansion on chromosome 19, is now termed myotonic dystrophy Inhibitors,research,lifescience,medical type 1 (DM1). Patients with the clinical picture of myotonic dystrophy type 2/proximal myotonic Inhibitors,research,lifescience,medical myopathy, who have positive DNA testing for the unstable tetranucleotide repeat expansion on chromosome 3, are now classified as having myotonic dystrophy type 2 (DM2) (5, 11-12). Although DM1 and DM2 have similar symptoms, they also present a number of very dissimilar features making them clearly separate diseases (Table 1). Table 1. Comparison of clinical manifestations between DM1 and DM2. Myotonic Dystrophy type 1 Clinical features Myotonic dystrophy type 1 is the most common inherited muscular dystrophy Inhibitors,research,lifescience,medical in adults with
an estimated prevalence of 1/8000. DM1 is characterized by the phenomenon of anticipation, by which the disease has an earlier onset and more severe course in subsequent generations. Patients with DM1 can be divided into four main categories, each presenting specific clinical features and Astemizole management problems: congenital, childhood-onset, adult-onset, and late-onset/asymptomatic. Table 2 summarises these subtypes. Table 2. Summary of myotonic dystrophy type 1 phenotypes, clinical findings and CTG length. Congenital DM1 Congenital DM1 (CDM) shows a distinct clinical phenotype with distinct clinical features, therefore it is to be considered a severe early form of ‘classical’ DM1. CDM often presents before birth as polyhydramnios and reduced fetal movements. After delivery, the main features are severe generalized weakness, hypotonia and respiratory involvement.