During this complex transformation of the IVC, numerous variations and anomalies to its adult form may occur. The most frequent is azygos continuation of the IVC, which has also been termed absence of the hepatic segment of the IVC with azygos continuation . Failure to form the right subcardinal-hepatic anastomosis (with resulting atrophy of the right subcardinal vein) determines that the anastomosis between the subcardinal and right supracardinal veins collects the blood from the lower body, diverting it to the cranial end of the right supracardinal vein (azygos vein). This passes posterior to the diaphragmatic crura to enter the thorax and joins the superior vena cava at the normal location in the right paratracheal space [7,8]. The hepatic segment is ordinarily not truly absent; rather, it drains directly into the right atrium.
Under normal conditions, the only major vessel that can be observed behind the heart is the descending aorta, which is positioned on the left side of the spine and on the same side as the cardiac apex. The finding of two vessels running behind the heart in the four-chamber view is usually pathological and may be caused by interrupted IVC with azygos continuation (excellent marker of left isomerism) or by total anomalous pulmonary venous connection (typical of right isomerism) . In the first case, the aorta and azygos vein are located in close proximity on the same side of the spine (“double vessel” sign), whereas in the second, the pulmonary venous confluence is situated immediately behind the atrium and a wide gap is apparent between the posterior wall of the atrium and the descending aorta.
Interruption of the IVC with azygos continuation is diagnosed by imaging two vessels (“double vessel” sign) of similar size in a paraspinous location posterior to the heart . This sign is easily detected in transverse views of the thorax and abdomen, and can be confirmed on longitudinal views. Compared to the normal relationship, which shows only the aorta posterior to the heart, interruption of the IVC results in collateral flow through the azygos vein, which becomes enlarged and readily visible. This should be distinguished from the normal azygos vein, which occasionally can be visualized in the third trimester as a small 1 to 2 mm structure.
The “double vessel” sign has been described in a high percentage (80 to 96%) of left isomerism [9,10,11], but it can also be found in few cases of right isomerism  and as a benign vascular malformation with situs solitus and without congenital heart disease [1,2,3,9,13]. In combination with cardiac anomalies or situs abnormalities, interruption of the IVC with azygos continuation should suggest a specific diagnosis of heterotaxy, especially left isomerism. In addition, a strong association between large omphaloceles and interruption of the IVC with azygos continuation has also been documented prenatally .
In any case of diagnosis of an interruption of the IVC, a thorough fetal search for associated anomalies is indicated, with a special emphasis on possible cardiac abnormalities and heterotaxy. It is convenient to inform parents of the good prognosis if it is an isolated finding, although it is important to inform them of possible complications associated with invasive procedures and of the increased risk of deep-vein thrombosis.
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