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CLINICAL INSIGHTS JULY/AUGUST 2018 63 Vascular anomalies other than those arising from the portal system are rare. Other imaging modalities, such as, computed tomographic angiography are usually required for definitive diagnosis. Caudal Vena Cava Thrombosis Caudal vena cava abnormalities are most commonly associated with neoplasia of the right adrenal gland, but can occur secondary to invasion by left adrenal, hepatic, lymph node, or renal neoplasia or from extension of femoral or iliac venous thrombosis. It is difficult to differentiate between thromboses that are blood clots and those that represent true tumor thrombi. Both blood clots and tumor thrombi appear as echogenic structures within the lumen of the vessel. Color Doppler imaging shows a filling defect within the vessel ( FIGURE 6 ). Complete occlusion results in absence of flow beyond the lesion. Partial occlusion may result in turbulence adjacent to the lesion or aliasing. However, if the tumor is vascular, a flow pattern may be visible within the thrombus, particularly when using power Doppler. This finding would be consistent with a tumor thrombus. Portal Vein Thrombosis Thrombosis of the portal vein occurs with numerous diseases associated with the development of coagulopathies. 2,13 It is recognized ultrasonographically as intraluminal structures of moderate to high echogenicity and the absence of color Doppler signals within the lumen 14 ( FIGURE 7 ). Thrombosis can be focal or can extend into all branches of the portal venous system, resulting in secondary portal venous hypertension, acquired extrahepatic portosystemic shunts (PSSs), and ascites. Aortoiliac Thrombosis Aortic and iliac arterial thromboembolism is most commonly seen in cats with primary cardiomyopathy; in dogs, it may be secondary to neoplasia, cardiac disease, or hypercoagulable states. The ultrasonographic features are similar to those of a venous thrombus. Hepatic Venous Congestion Congestion usually occurs secondary to increased resistance to blood flow of the caudal vena cava to the right atrium. Causes of hepatic venous congestion include obstruction by a right atrial mass, pericardial effusion, invasion of the caudal vena cava by a tumor, and right-sided congestive heart failure.15 Distention of the hepatic veins and caudal vena cava is best visualized adjacent to the diaphragm at the level of the right cranial dorsal abdomen (10th or 11th intercostal space approach). Concurrent findings include a diffusely hypoechoic hepatic parenchyma, hepatomegaly, and peritoneal effusion. Portosystemic Shunts Congenital Portosystemic Shunts Congenital PSSs are single, large, anomalous connections between the portal and systemic veins that allow portal blood from the gastrointestinal tract to bypass the liver. Congenital PSSs are more frequently FIGURE 6. (A) Long-axis and (B) short-axis views of the caudal vena cava at the level of the right adrenal gland in a dog with a pheochromocytoma invading the adjacent vena cava. In A, note the formed echogenic structure filling much of the caudal vena caval lumen, causing little flow in the far field of the caudal vena cava when investigated with color Doppler. In B, the formed echogenic thrombus fills most of the caudal vena cava (semilunar anechoic structure) and part of the adjacent phrenicoabdominal vein. A hypoechoic right adrenal gland mass is also present. A B

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