A female infant aged 5 months with a diagnosis of Down syndrome and complete atrioventricular septal defect (AVSD) was transferred from a local hospital due to failure to thrive and for optimized management of congenital heart disease. The transthoracic echocardiography suggested the presence of a membranous structure crossing the left atrium (LA). The CT angiogram confirmed the definitive diagnosis of complete AVSD (double arrows) and the rare co-occurrence of cor triatriatum sinister. The LA and its upper/dorsal aspect communicated with the right atrium through a hole in the membrane and the atrial septal defect (red arrow), and with the lower portion of the left atrium through another opening (yellow arrow) (Fig. 1A). The dimensions of both communications were adequate to preclude obstruction of pulmonary venous return. An inverted MIP (maximum intensity projection) was selected to allow enhanced visualization of the septum and membrane. A modified sagittal view (Fig. 1B) showed by a vertically oriented structure across the LA (red dashed circle) that divided it into dorsal/upper (**) and ventral/low (*) compartments. Despite the rare coexistence of these findings, hemodynamic circulation remained balanced, which allowed for complete surgical correction and a favorable clinical outcome with preserved ventricular function and absence of residual shunting.

Figure 1.

(A) CT angiogram axial view revealing the rare coexistence of complete atrioventricular septal defect and cor triatriatum. The blue double arrow indicates the ostium primum defect, while the white double arrow points to the ventricular septal defect. A red arrow highlights the communication of the upper left atrium (LA) with the right atrium (RA) via an atrial septal defect. The yellow arrow indicates the communication between the upper and lower portions of the divided left atrium. Finally, the white ellipse marks the cor triatriatum membrane, (also in axial view).

(B) The red dashed circle in the sagittal view delineates the divided left atrium, with the asterisk (*) indicating the ventral/lower portion and the double asterisk (**) marking the dorsal/upper portion.