Discussion
The anomalous retroaortic left brachiocephalic vein is a rare congenital systemic venous anomaly. It has both clinical and surgical fascination. That we were able to identify only 250 cases with this disease since 1980 underscores its rarity, but highlights the need for a high index of suspicion of its existence.37,39,40-42,64-66
Normally, the left brachiocephalic vein enters the superior caval vein above the insertion of azygous vein and the connection is between left and right anterior cardinal components. When taking a retroaortic course, the vein enters the right superior caval vein below and behind the aortic arch, and below the entrance of azygous vein.18 The anomalous vein, therefore, is embryologically distinct from the normal left brachiocephalic vein, as well as having a different spatial pathway and anatomical relations. The question of terminology, therefore, could be debated at length. Because the ultimate connections at either end are identical, it seems reasonable to refer to the structure as an anomalous brachiocephalic vein.
Although of interest anatomically, the retroaortic position is physiologically inconsequential. With echocardiography, it is most easily seen using suprasternal long-axis projections (Figures 2A and 2B). Radiologically, the anomaly can be documented when catheters are placed in both brachiocephalic vein and aorta or with left superior venography. Detection is obviously possible by computed tomographic-angiography, or magnetic resonance imaging (Figures 1A-1F, 3A-3C). On non-contrast-enhanced computed tomographic scans, the subaortic portion of anomalous left brachiocephalic veins mimics enlarged lymphnodes.72,73
Regardless of its pathogenesis, recognition of this anomaly is important in various clinical settings. The preoperative recognition during clinical investigation raises the possibility of associated congenital cardiac malformations, especially right ventricular outflow tract obstruction, tetralogy of Fallot, with pulmonary stenosis or pulmonary atresia, right aortic arch, ventricular septal defect and totally anomalous pulmonary venous connection. Conversely, when the diagnosis of tetralogy of Fallot with pulmonary stenosis or pulmonary atresia is made, the anomalous position of left brachiocephalic vein should be looked for, especially in patients with a right aortic arch.
To the radiologist, and echocardiographer, the descending portion of an anomalous brachiocephalic vein must be differentiated from a persistent left superior caval vein or an ascending vertical vein in supracardiac totally anomalous pulmonary venous connection, and a left partially anomalous pulmonary venous connection.3,17,18,23
Its middle portion needs to be differentiated from the bifurcation of the pulmonary trunk.3,18,23 The retroaortic crossing segment of the anomalous vein may be misinterpreted on unenhanced computed tomographic scan as enlarged lymphnodes, an elevated right pulmonary artery in patients with hypoplastic or atretic central pulmonary arteries, or an early branching upper lobe pulmonary artery on cross-sectional echocardiography.4,11,17-19,21,26
Carefully tracing this vascular channel through sequential images is the key to differentiation. Computed tomography, along with magnetic resonance imaging, are 100% sensitive in diagnosing the anomaly.72,73
During surgery, knowledge of the low entry site of the brachiocephalic vein might prevent is inadvertent obstruction when direct cannulation of the superior caval vein is necessary.12 Separate cannulation of the left superior caval vein is not necessary. Because of the anomalous entry of the retroaortic brachiocephalic vein at the superior cavoatrial junction, the use of direct superior caval venous cannulation must be done with extreme precaution to avoid cannula-induced venous obstruction, or damage to arterial supply of the sinus node.2
In a case with tricuspid atresia and hypoplastic pulmonary arteries, the vein was used to augment the pulmonary arteries during cavopulmonary anastomosis.74 The other situations where this anomaly might affect surgical technique is during superior cavopulmonary anastomosis for staged Fontan procedure. Its presence may also complicate exposure of the pulmonary arteries.10,18Its presence may also complicate the exposure and ligation of the persistently patent arterial duct, coarctation of aorta and distal aortic arch aneurysm.2,5,17,18,50 While performing a systemic-to-pulmonary arterial anastomosis, its presence might complicate exposure of the pulmonary arteries.2 It appears to be associated with a greater degree of obstruction in right ventricular outflow tract in the setting of tetralogy of Fallot. Its presence, therefore, should alert the surgeon to the possible requirement for a more extensive transannular parch to accomplish a satisfactory repair.