DM1 Patients with Small CTG Expansions are also at Risk of Severe Conduction Abnormalities.

BACKGROUND AND OBJECTIVES A high risk of cardiac arrhythmias was reported in myotonic dystrophy type 1 (DM1). The purpose of the study was to evaluate the risk of severe electrocardiographic abnormalities in DM1 patients with small CTG expansions. METHODS We assessed the ECG done at DM1 diagnosis for 127 patients with ≤200 CTG repeats and for 82 of them who had ≥1 ECG over a period of follow-up of 11.7 ± 7.6 years (mean ± SD). Criteria of severe ECG abnormality are at least one of the following features: PR interval ≥240 msec, QRS duration ≥120 msec, second-degree or third-degree atrioventricular block, atrial fibrillation or flutter, insertion of a pacemaker or cardioverter-defibrillator. RESULTS At baseline, ECG was normal for 109 patients out of 127 (85.8%) and only 4 patients (3.1%) presented severe ECG abnormalities. At follow-up, 46 patients out of 82 (56.1%) had a normal ECG and 25 (30.5%) developed severe ECG abnormalities (p < 0.0001) including 6 of them who needed permanent pacemaker insertion. There were also 3 sudden deaths during the follow-up period. Using multivariate Cox regression analysis, age at entry (relative risk RR, 1.05; (95% CI 1.01-1.08; p = 0.012) and muscular weakness (MIRS) at the entry (RR, 2.03; 95% CI 1.28-3.22; p = 0.003) were significant risk factors for the development of severe ECG abnormality. CONCLUSIONS An increased risk of severe ECG abnormalitiesy and cardiac events is observed even in DM1 patients with small CTG expansions and warrants close cardiac follow-up similar to DM1 patients with larger CTG expansions.