Ambulatory electrocardiographic recording in mild or moderate myotonic dystrophy and myotonia congenita (Thomsen's disease).

Ambulatory electrocardiographic recording (AER) was performed on 27 adult patients with mild or moderate myotonic dystrophy (MD) and in 12 adult patients with mild myotonia congenita (MC) to determine whether characteristic arrhythmias and conduction disturbances occur in the early stages of these myotonic disorders. In MD and MC, AER showed at least one finding regarded as normal in 93% and 0% of cases, respectively. The most common abnormalities in MD were first degree atrioventricular (A-V) block (in 70% of cases) and QTc greater than or equal to 0.46 s (in 33%). Abnormal sinus intervals and frequent second degree A-V block were rare, each occurring in 4% of patients. Ventricular tachycardia did not occur. It is concluded that signs of involvement of cardiac conductive tissue in the MD disease process are very common, even in the early stages. Delayed A-V conduction in the most common single finding. Sinus nodal dysfunction, severe disturbances in A-V conduction, and ventricular arrhythmias are not common in mild disease. Repeated AER is indicated, to evaluate whether conduction disturbances in MD are progressing.