Myotonic dystrophies (MD1 also known as Steinert disease and MD2/PROMM) are genetic disorders, dominantly inherited and showing multisystemic features.

Main features include progressive muscle weakness (distal muscles in MD1; proximal muscles in MD2), and myotonia, which means difficult relaxation of muscles after contraction.

Indeed, skeletal muscles (fatigability and general hypostenia), myocardium (dilatative cardiomyopathy, conduction defects and arrhythmias), smooth muscles (abnormalities in gastroenteric motility and constipation), crystalline lens (cataract), endocrine system (diabetes mellitus and hypothyroidism, gonadic atrophy and sterility), and the central nervous system are the organs and the tissues affected. The age of disease onset is variable, even congenital in MD1. Generally, the earlier is the onset the more severe is the clinical phenotype.

MD pathogenesis is in the expansion of repeated nucleotide triplets and quadruplets: CTG in the DMPK gene for DM1 and CCTG in the ZNF9 gene for DM2. In the former, the size of the expansion is related to the severity of the disease. The variability in the number of repeats among different tissues accounts for the different clinical involvement.

The prognosis is usually more favourable for DM1. Molecular diagnosis is fundamental for both the forms, in order to prompt symptomatic treatment and to avoid the improper use of anaesthetics in case of surgical interventions.