The three routes are:
- Superior route: Seen with subclavian or brachiocephalic vein obstruction. Blood flows via through the ipsilateral external jugular vein into horizontal veins that communicate across the midline via the transverse arch of the anterior jugular venous system. Once on the contralateral side, blood flows into the external jugular vein into the subclavian vein, and finally into the superior vena cava.
- Posterior route: Seen in cases of obstruction at the level of the supraazygos superior vena cava. This leaves the azygos vein as a conduit for blood to get into the superior vena cava. Blood from the head and neck flows through paravertebral collaterals into intercostal and paravertebral veins and then the superior intercostal vein, which drains into the azygos vein.
- Anterolateral: Seen in cases of central superior vena cava obstruction (shown above). Blood flows through anterior intercostal, internal mammary (pink arrow), and long thoracic veins, which, flow to the inferior vena cava (green arrow) via pericardiophrenic (yellow arrow), musculophrenic (blue arrow), lumbar, and hepatic veins. The internal mammary vein can also connect to the left portal vein via the paraumbilical vein and result in increased activity or enhancement in segment IV of liver (white arrow) and is the basis of the focal hepatic hot spot sign on 99mTc sulfur colloid "liver and spleen" scans. IN the image above, we also see aortopulmonary window collaterals (red arrow) that drain into the infraazygos superior vena cava(S), in this patient with combined infraazygos superior vena cava and azygos vein obstruction.
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