A persistent left superior vena cava is not an uncommon finding, and if entered at catheter most of us would document it with a quick hand injection of contrast and with low dose cinefluorgraphy.
Imagine our surprise when such a run gave this image. There was a pulsatile pouch connected to the LSVC (star*). Moreover its contraction pattern was that of an atrium. It looked like the left atrial appendage, and was in the region where the LAA was to be expected. The child had normal situs but was critically ill with aortic stenosis and a large VSD. Subsequent necropsy confimed that the chamber was indeed the morphological left atrial appendage.
It is nearly impossible to explain this anomaly using the platform of classic cardiac embryology. I have not heard it explained to any satisfaction, nor have I seen another example anywhere.
Not so unusual is the presence of a left brachiocephalic vein (arrowed). It is of good size though smaller than normal. This can cause some confusion if a pacemaker is fitted from the left, with the lead sometimes going down the LSVC and somtimes finding the normal route to the RSVC. Below the left brachiocephalic vein is the left hemiazygos vein.