Myotonic dystrophy type 1 (DM1) and 2 (DM2) are autosomal dominant inherited neuromuscular diseases with an estimated incidence of 1 in 120,000 in Europe. The genetic cause of DM1 is a CTG repeat expansion in the DMPK (dystrophia myotonia protein kinase) gene on chromosome19q13. Transcription of these repeats results in CUG expansion and accumulates in ribonuclear inclusions in the nucleus, finally resulting in altered splicing of multiple genes such as those encoding for the chloride channel ClC1, the insulin receptor, the cardiac troponin and the NR1 subunit of the N-methyl-D-aspartate receptor.
Primary manifestation and clinical progress depend on the amount of CTG repeats: there are usually no clinical signs in individuals with only 50-100 repeats of CTG, but there is a correlation between repeat size and age of onset of DM1 when the number of repeats is < 400. Clinically DM 1 is characterised by muscle wasting primarily of the distal, axial, facial, pharyngeal and respiratory muscles, accompanied by cataracts, cardiac conduction blockade and arrhythmia, cardiomyopathy, diabetes, dysthyroidism and drowsiness. The risk of general anaesthesia includes cardiac rhythm disturbances and increased risk of sudden cardiac arrest, even with implanted pacemakers or cardioconverters. Anaesthesia should be conducted only in cases of emergency or following multidisciplinary risk assessment. Retrospective data estimated a complication rate of about 8.2% in these patients which were mainly due to respiratory complications (atelectasis, pneumonia, acute ventilatory failure).
DM 2 is caused by untranslated CCTG repeat expansions in the ZNF9 (zinc finger protein 9) gene on chromosome 3q21 and is transmitted by autosomal dominance. Primary clinical manifestations are frequently present at the age of 40-50. Clinical characteristics are proximal limb weakness, myotonic stiffness and myalgias. Typical cardiac dysfunction include conduction abnormalities. Cardiomyopathy and sudden cardiac arrest are also observed in DM 2 but with lower incidence compared to DM 1.