Isolated leftward prolapse or deviation of the primary atrial septum is a rare CHD that can mimic abnormal pulmonary venous return at first sight. We present a case of a newborn infant, referred for surgical correction of totally anomalous pulmonary venous return into the right atrium, with the peri-operative finding of a leftward deviation of the superior margin of the primary atrial septum. The distinction with a dividing atrial shelf could not be confirmed with certainty. Fifty-three similar cases from the literature are incorporated. A detailed review of the current account on atrial septation is studied. The embryological and clinical features of a dividing partition of the left atrium are discussed.
