Concealed left atrial membrane: pitfalls in the diagnosis of cor triatriatum and supravalve mitral ring.

Cor triatriatum and supravalve mitral ring are forms of congenital left ventricular inflow obstruction produced by membranes within the left atrium. Typically, these defects occur as isolated anomalies with manifestations of pulmonary venous obstruction. Four children are presented whose left atrial membrane was associated with other significant cardiac defects, including, in one patient each, simple coarctation of the aorta, sinus venosus atrial septal defect, tricuspid atresia and complex coarctation of the aorta syndrome. The patient with the latter defect had undergone previous pulmonary arterial banding. None of these patients demonstrated significant pulmonary venous obstruction at cardiac catheterization. All patients had a normal value for either pulmonary arterial diastolic or pulmonary arterial wedge pressure. Three mechanisms explained the lack of pulmonary venous obstruction: (1) a large cross-sectional area of membrane openings, (2) an atrial septal defect that was confined to the pulmonary venous chamber and decompressed it by allowing blood to escape into the right atrium, and (3) decreased pulmonary blood flow. The diagnosis was facilitated by two dimensional echocardiography. Accurate diagnosis of left atrial membrane in the setting of other cardiac defects is of practical significance because pulmonary venous obstruction may occur after surgery for the associated defects.