Progressive cardiac conduction defect and cardiomyopathy in siblings with syncope.

Three siblings evaluated for syncope were found to have left bundle-branch block. Progression to complete heart block occurred in all. Pacemaker implantation eliminated syncopal epidoses in each case. Echocardiographic manifestations of cardiomyopathy were present in each child despite normal roentgenographic heart size. Careful family study identified no other affected members with conduction defects. These children area believed to represent a form of familial cardiomyopathy in which clinical manifestations of cardiac conduction system disease predominate.