July 7, 2008

Sudden Death Risk 3-5 Times Above Average in Type 1 MMD

Adults with type 1 myotonic dystrophy (MMD1, or DM1) and either a severely abnormal electrocardiogram (ECG) or a diagnosis of so-called atrial tachyarrhythmia (an abnormally fast, irregular heart rate originating in the upper chamber, or atrium, of the heart) are at significantly higher than average risk of sudden death, according to a study in the June 19 issue of the New England Journal of Medicine. The study was supported by MDA, the National Institutes of Health and the medical electronics manufacturer Medtronic.

A severely abnormal ECG was correlated with a risk of sudden death that was 3.3 times above average. A clinical diagnosis of atrial tachyarrhythmia resulted in a five-fold increase in the risk of sudden death compared to the average risk.

The investigators defined a severe ECG abnormality as either a heart rhythm not originating from the normal place in the heart; a significant lengthening of the normal time it takes for signals to travel through the heart; or a partial blockage of signals from the upper to the lower part of the heart.

All these abnormalities reflect malfunctions of the heart’s conduction system, which regulates heart rate and rhythm via electricity-like signals. Conduction-system abnormalities, which are common in MMD1, can cause sudden death if they progress to complete blockage of cardiac signals and stoppage of the heartbeat; or if they lead to a fast, nonfunctional rhythm pattern in the lower heart chambers (ventricular tachyarrhythmia) that prevents adequate pumping action.

Cardiologist William Groh, who received MDA support at Indiana University’s Krannert Institute of Cardiology in Indianapolis, with colleagues there and at several other institutions, studied 406 adults with MMD1 seen at any of 23 MDA clinics beginning in April 1997.

During an average follow-up period of 5.7 years, there were 81 deaths among study participants, 27 of which were classified as sudden. In addition, there were 32 deaths from respiratory failure due to respiratory muscle weakness, five nonsudden cardiac deaths and 17 deaths from other causes.

The investigators note that 10 percent of study participants had cardiac pacemakers by the last follow-up visit but that these did not appear to decrease the rate of sudden death or death from any cause.

A small number of participants had implantable cardioverter-defibrillators, which can bring an abnormally slow heartbeat up to a normal rate and also slow a dangerously fast heart rhythm, by delivering an electric shock. Sudden death occurred in two patients with these devices.

The investigators say their findings suggest cardioverter-defibrillators may be helpful in preventing sudden death in MMD1 but that their data didn’t allow firm conclusions to be drawn.

“In conclusion,” the authors write, “our study shows that adult patients with myotonic dystrophy type 1 are at high risk for sudden death.” They noted that the presence of a severe ECG abnormality or a clinical diagnosis of atrial tachyarrhythmia each predicted sudden death, whether or not the other condition existed.