Background Myotonic dystrophy (MD1) is a hereditary autosomal dominant disease with variable penetrance. Cardiac conduction disturbances are frequent and may be responsible for sudden death, but its progression was heretofore unknown.

Aims The aim of the study was to analyse the natural history of infrahissian conduction time in patients with a normal first electrophysiological test, and to identify the predictive value of the clinical and ECG factors accompanying an alteration of infrahissian conduction.

Methods Among 127 consecutive screened MD patients, 25 were enrolled and underwent a second electrophysiological testing. The second electrophysiological test was carried out on patients showing new symptoms, new atrioventricular conduction disturbances on ECG, or significant modifications of signal-averaged (SA)-ECG, and on asymptomatic patients with a follow-up of at least 60 months since the first electrophysiological test.

Results Among the 25 patients, four had new clinical symptoms, four others developed new atrioventricular conduction abnormalities on ECG and six had significant modifications of the SA-ECG. The mean His-ventricle (HV) interval increased significantly between the two electrophysiological studies (initial HV interval 52.1 ms±1.6 ms, final HV interval 61.4 ms±2.2 ms, p<0.005), with a mean increase of 1.2 ms/year. The five patients with HV interval of 70 ms or greater were implanted with a prophylactic dual-chamber pacemaker. Modifications of resting ECG and SA-ECG were strongly associated with HV interval prolongation.

Conclusion In patients with a normal initial electrophysiological study, modifications on the resting ECG and/or SA-ECG, on annual check-up, were associated with an alteration of infrahissian conduction.