Signal-averaged electrocardiography in myotonic dystrophy

We performed signal-averaged electrocardiography and 24-h ambulatory electrocardiographic monitoring in 53 patients with myotonic dystrophy to determine the incidence and clinical significance of ventricular late potentials. Patients were followed up for a mean period of 31 ± 17 months (range 11–68 months). At entry, none of the patients had bundle branch block on 12-lead electrocardiogram and none had wall motion abnormalities on routine echocardiogram. Also, no patient had history of syncope or clinical evidence of ischemic heart disease or a documented sustained ventricular tachycardia. A group of 47 healthy subjects matched for age and sex also underwent signal-averaged electrocardiography for comparison with the patient group. Late potentials were diagnosed in the presence of at least two of the following measures: duration of the filtered QRS > 114 ms, root-mean-square voltage of the terminal 40 ms of the filtered QRS < 20 μV, and duration of the low-amplitude (< 40 μV) signals of terminal filtered QRS > 38 ms. Late potentials were more frequent in patients than in controls: 18 of the 53 patients (34%) showed late potentials compared with four of the 47 controls (8.5%) (P < 0.01). In 45 patients (85%) no ventricular ectopy (40 cases) or infrequent premature ventricular complexes (five cases) were detected on Holter monitoring. Complex ventricular arrhythmias were traced in the remaining eight patients. These were six of the 18 patients with, and two of the 45 patients without late potentials (33% vs. 6%, respectively; P < 0.01). Only two of the eight patients with complex ventricular arrhythmias were documented to have repeated three-beat runs of ventricular tachycardia; both patients also had late potentials. During the period of observation there were no sudden deaths. Two patients required permanent pacemakers for appearance of serious conduction defects and recurrent syncope. Therefore, ventricular late potentials are a frequent finding in patients with myotonic dystrophy. In our series they were sensitive in predicting complex ventricular arrhythmias being present in 75% of cases and correctly identifying the patients with ventricular tachycardia. However, specificity and positive predictive value were unacceptable due to the high false-positive rate. Also, the absence of cardiac catastrophic events during follow-up calls in question the prognostic value of ventricular late potentials in myotonic dystrophy.