SVC obstruction can cause SVC syndrome which is the most common condition affecting this vessel. It can be secondary to extrinsic compression or intraluminal thrombosis/stenosis. Collateral pathways, with the azygos vein being the most important collateral vessel, form in response to severe narrowing or obstruction to drain blood into the heart.
Five distinct grades of SVC obstruction have been described:
- grade 0: SVC narrowing without clinical evidence of SVC syndrome
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grade I:
- Ia: mild SVC narrowing without collaterals
- Ib: Moderate SVC narrowing without collaterals
- grade II: severe SVC narrowing above the azygos with the azygos vein serving as partial collateral
- grade III: SVC obstruction below the azygos arch
- grade IV: SVC obstruction at the azygos arch
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