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.