Markedly dilated right atrium in an infant: what is the diagnosis?

An 18-month-old boy presented with recurrent episodes of irritability and documented fast heart rate suggestive of supraventricular tachycardia. Cardiovascular examination revealed significant cardiomegaly, normal heart sounds and no murmurs. The differential diagnosis of marked right atrial dilatation and management principles of idiopathic dilatation of the right atrium are described.