Arrhythmias are common in patients with MD with infrahisian conduction abnormalities. The prophylactic implantation of a pacing system when the HV interval is > or =70 ms seems appropriate. The PM protects the patient against the clinical consequences of paroxysmal profound bradycardia and facilitates the diagnosis and management of frequent paroxysmal tachyarrhythmias.

Prolongation of the HV interval is the most common conduction abnormality in MD and can be reliably recognized only by invasive EP testing. It raises the issue of prophylactic pacing to limit the incidence of sudden death in MD. Atrial and ventricular arrhythmias are often inducible, although their predictive value remains to be determined. Young age emerged as the most powerful predictor of inducible ventricular tachyarrhythmias. Conversely, we found no relationship between ECG or EP abnormalities recorded during invasive testing and the DNA mutation size or severity of peripheral muscle involvement.

Cardiac tachyarrhythmias have rarely been studied in young patients with myotonic dystrophy type 1 (DM1). The authors observed major cardiac rhythm disturbances in 11 patients aged 10 to 18 years. Tachyarrhythmic events were more frequent than impulse conduction disorders. Wide variations in CTG expansion were observed among the population. Since physical exercise was a prominent arrhythmogenic factor, systematic exercise tests with EKG monitoring may be indicated in young patients with DM1.