Families with myotonic dystrophy with and without cardiac involvement.

Study of 30 patients with myotonic dystrophy and 17 unaffected family members in a total of 18 families by echocardiography and ECG, including noninvasive His-bundle recording, appeared to show two phenotypes of myotonic dystrophy: those with and those without substantial cardiac involvement. Besides the 29% incidence of asymptomatic mitral valve prolapse common to many neuromuscular diseases, 20% of families had progressive involvement of the cardiac conduction system. It is important to recognize these families in order to treat patients with cardiac pacemakers who are rapidly progressive or symptomatic. If one patient with myotonic dystrophy is found to have myotonic heart disease with cardiac block or arrhythmia, it should be expected in other family members with myotonic dystrophy.