Prophylactic Pacing in Myotonic Muscular Dystrophy

Abstract & Commentary

By John P. DiMarco, MD, PhD, Professor of Medicine, Division of Cardiology, University of Virginia, Charlottesville.

Source: Wahbi K, et al. Electrophysiological study with prophylactic pacing and survival in adults with myotonic dystrophy and conduction system disease. JAMA 2012;307:1292-1301.

This paper reports registry data from a large, single-center patient cohort with myotonic dystrophy type 1 (DM1). Between January 2000 and December 2009, 914 adult patients were included in the DM1 Heart Registry at the authors' institution. After enrollment, patients were followed at yearly intervals as part of a natural history study. The authors identified patients with either a prolonged PR interval (> 200 msec) or QRS duration (> 100 mesc) or both at registry entry or during follow-up. Outcomes using two nonrandomized strategies were then evaluated. The invasive strategy group included patients who underwent electrophysiologic study with prophylactic cardiac pacing if the HV interval was > 70 msec. The noninvasive group included patients who underwent neither electrophysiologic study nor implantation of a prophylactic pacing device. Decisions on whether to perform the electrophysiologic study were at the discretion of the individual patient's cardiologist. Electrophysiologic study included measurement of normal conduction system intervals and programmed ventricular stimulation. Either a permanent pacemaker or an implantable cardioverter defibrillator (ICD) could be implanted in patients with positive findings. The authors then analyzed the effects of these two strategies on total mortality, sudden death, respiratory death, and death from other causes. Since this was an observational study, the authors used several techniques to limit biases in the group comparisons. Propensity scoring in the entire cohort and a case-control substudy were used for this purpose.

There were 914 patients entered into the DM1 Heart Registry during the period of study. Of these, 373 (41%) did not have a qualifying ECG conduction defect. An additional 55 patients were excluded from analysis due to either prior implantation of a pacing device (46), symptomatic AV block at study entry (5), or they received prophylactic pacing without electrophysiologic study (4). Among the remaining 486 patients, 341 (70%) underwent electrophysiologic study and were included in the invasive strategy group and 145 (30%) were included in the noninvasive strategy group. Patients undergoing electrophysiologic study more commonly had prior syncope, supraventricular arrhythmias and heart failure, lower left ventricular ejection fractions, and longer PR and QRS intervals. At electrophysiologic study, an HV interval greater than 70 msec was documented in 164 patients (48%). Sustained ventricular tachyarrhythmias were induced in 70 patients (21%). Any overlap between these two groups was not described, but was likely to be substantial. After electrophysiologic study, pacemakers were implanted in 150 patients and ICDs were implanted in 14 patients. During follow-up, none of the 14 patients who received ICDs received a shock or antitachycardia pacing for the treatment of a ventricular tachyarrhythmia. Antiarrhythmic drugs were used in 49 patients, but most of these were beta-adrenergic blockers. During a median follow-up of 7.4 years, seven patients (1.4%) were lost to routine follow-up, but their survival was confirmed using the French National Death Registry. There were 80 deaths during follow-up corresponding to an overall 9-year survival of 74.4%. Fifty of 341 patients in the invasive strategy group (14.7%) died compared to 30 of 145 patients in the noninvasive strategy group (20.7%). The crude hazard ratio based on management strategy was 0.74 in favor of the invasive approach. After adjustment for other risk factors including age, gender, history of supraventricular arrhythmia, left ventricular ejection fraction, PR interval, QRS duration, and heart rate, the hazard ratio was 0.61 (95% CI, 0.38-0.98). Similar hazard ratios were obtained when the analysis was adjusted for propensity score quintiles. Finally, case-control matching of 212 patients in the invasive strategy group with 106 patients in the noninvasive strategy group yielded a mortality hazard ratio of 0.55 unadjusted and 0.47 after adjustment for known baseline prognostic factors.

The cause of death could be determined in all 80 patients. In the invasive strategy group, 10 patients died suddenly, including nine pacemaker recipients, one ICD recipient, and one patient who did not receive a device and whose HV interval was less than 70 msec. Device analysis from the time of sudden death showed ventricular fibrillation in three patients, asystole with pacing in three patients, and asystole without pacing stimuli in one patient. In the noninvasive strategy group, there were 16 sudden deaths. ECG data at the time of sudden death could not be obtained in the noninvasive strategy group. In the entire study group, sustained ventricular tachyarrhythmias were diagnosed in only five patients, including four in the invasive strategy group and one in the noninvasive strategy group. The 9-year point survival estimate for sudden death was 4.5% in the invasive strategy group compared to 18% in the noninvasive strategy group. The hazard ratios for sudden death in favor of the invasive strategy ranged between 0.24 and 0.28, depending on the analytical methods used. Respiratory failure was the cause of death in 23 patients in the invasive strategy group and in nine patients in the noninvasive strategy group. Deaths from other causes were observed in 17 patients in the invasive strategy group and in five patients in the noninvasive strategy group.

The authors conclude that among myotonic DM1 patients with surface ECG conduction abnormalities, management with an invasive strategy using electrophysiologic study and prophylactic permanent pacing for patients with positive findings is associated with enhanced survival due to prevention of sudden death.

Commentary

Decisions on the best management for asymptomatic patients with myotonic dystrophy are often difficult. The data presented here suggest that most sudden deaths are related to bradyarrhythmias and that permanent pacing based on the HV interval measured at electrophysiologic study is an effective strategy. This implies that bradycardia is the most common cause for sudden death. However, in another DM1 registry study, Bhatka and his colleagues present data that both bradyarrhythmias and ventricular tachyarrhythmias are common and argue that an ICD should be the device selected.1 In the current report, the authors don't correlate their results during ventricular stimulation with outcomes, so value of the full electrophysiologic study to help in device selection is not addressed.

Reference

1. Bhatka D, et al. Pacemaker and implantable cardioverter-defibrillator use in a US myotonic dystrophy type 1 population. J Cardiovasc Electrophysiol 2011;22:1369-1375.