ECG: Sinus rhythm with increased PR-interval (first degree AV-block), complete left bundle branch block (QRS-width > 120 ms, qS and rS morphology in V1-V2, positive complexes in V6 with delayed intrinsicoïd deflection, repolarisation abnormalities.

Commentary: Myotonic dystrophy, also known as Steinert disease (autosomal dominant inheritance) is the most common muscle dystrophy in the adults and is characterized by the existence of muscular weakness and progression into various organs with possible involvement of the occular, respiratory, endocrinology, gastro-intestinal and cardiac systems. In the classical form, the symptoms arise between the age of 10 and 40. Even though respiratory complications are the leading cause of death, cardiac involvement is also responsible for a large part of the mortality due to mutiple mechanisms such as heart failure, ischemic heart disease, pulmonary embolism and elevated risk of sudden cardiac death by rhythm or conduction disorders.

The ECG may show various aspecific abnormalities (repolarisation disorder, pseudo q-waves of necrosis, atrial arrhythmia) but the most frequently observed electrical sign is progressive conduction disease (on for example bi-annual registrations). A bundle branch block (complete or incomplete) or AV-block (first, second or third degree) is seen in approximately 40% of patients. During long PR-interval, such as seen in this patient, since it is extremely difficult to define the location of block (nodal or infranodal) on the surface ECG, an electrophysiological study is often performed, even in asymptomatic patients. Conduction delay may appear at various anatomical levels (atrial, AV-node, His-bundle or in bundle branches). In this patiets, the existence of a long PR-interval and a complete left bundle branch block is in favour of infranodal conduction disease, confirmed with an HV-interval of 95 ms (normally below 70 ms). In such cases, there exists an absolute indication for implantation of a device, leading to the discussion whether it should be a pacemaker or a defibrillator. It was previously believed that sudden cardiac death in patients with Steinert disease was predominantly caused by bradycardia and sustained asystole. However, mortality remained elevated after pacemaker implantation indicating underestimated importance of ventricular arrhythmias, also confirmed by device interrogation.

Various mechanims seem responsible for the development of ventricular arrhythmias such as regional heterogeneity of ventricular repolarisation with increased QTinterval and dispersion of repolarisation, banch-to-branch re-entry, re-entry around fibrotic areas). While implanting a defibrillator may enable treatment of ventricular arrhythmias, it comes with the price of inappropiate shocks and elevated risk of lead dysfunction.

Conclusion: In Steinert disease, the most frequently observed ECG abnormality is the existence of conduction abnormalities both located above and below the His-bundle.