SVC ostium obstruction with secondary right-to-left shunt
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Suspicion of pulmonary embolism. Past history of Ebstein anomaly and tricuspid valves replacement decades ago, and no pulmonary disease nor anemia. A recent echocardiography showed normal right and left contractile functions, yet an enormous thrombus in the right atrium. No intracardiac shunt was visible on US.
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As expected, artificial tricuspid valves and a huge thrombus in the right atrium were found. However, upon contrast injection in the pulmonary arterial phase, pulmonary artery and aorta showed reversal of the expected enhancement: contrast followed the superior vena cava then flowed through an unsual "bag of worms"-like ductus into the superior left pulmonary vein, thus creating a right-to-left shunt. This so-called ductus was located around the right hilar region and included the azygos vein.
The rest of the cardiac anatomy was normal, with situs solitus of the viscera and atria (S), ventricular D-loop (D), and solitus-related great arteries (S): S, D, S.
No pulmonary embolism was seen.
CT evidence showed this unusual right-to-left shunt causing dyspnea.
This case emphasized an unusual pathway through preexisting bronchial venous plexuses which link the right atrium to the left superior pulmonary vein.
Final diagnosis: systemic-to-pulmonary venous acquired shunt as collateral pathway.
Chronic obstruction of superior vena cava is commonly an acquired condition due to thrombus, mediastinal mass or fibrosis. Thus, bloodstream often goes through the aygos-hemiazygos pathway, or internal mammary. However, in cases of SVC's ostium obstruction or enlarged right atrium, shunting of systemic-to-pulmonary venous pathway, normally prevented by venous valves, may be observed 1.
In a patient with cardiac insufficiency symptoms with preserved contractile function, clinicians have to rule out anemia and possible shunts such as atrial septal defect (ASD), arterio-venous fistula from dialysis or other arterio-venous malformation. The first imaging needed is echocardiography. If non-contributive, consider contrast-enhanced body CT to look for shunts.
Imaging is expected to find the location of a shunt, its primary cause, the anatomy of collateral pathways, as well as its severity in order to plan surgical treatment. The appropriate protocol could be pulmonary arterial phased acquisition as the table moves onward, followed by arterial phase acquisition as the table moves backward. Otherwise, a single acquisition with mixed phased can be performed.
Therapy consists of endovascular or surgical closure of the collateral pathways, and treatment of their primary cause.
Teaching point: In case of cardiac insufficiency with enlarged right atrium or SVC’s ostium obstruction, radiologists may find unusually shunting collateral pathways such as systemic-to-pulmonary venous pathway: bronchial plexus veins -> azygos vein -> left superior pulmonary vein.
- 1. Kapur S, Paik E, Rezaei A, Vu DN. Where there is blood, there is a way: unusual collateral vessels in superior and inferior vena cava obstruction. (2010) Radiographics : a review publication of the Radiological Society of North America, Inc. 30 (1): 67-78. doi:10.1148/rg.301095724 - Pubmed
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