Superior vena cava syndrome pathophysiology

Superior vena cava syndrome arises from the obstruction of venous blood drainage of the superior vena cava, which is normally involved in the major blood flow return from head, neck, upper extremities, and upper thorax to the heart. Superior vena cava syndrome is a complication of a partial or complete obstruction due to malignant causes (60%) or benign causes (20%). This syndrome consists on the invasion of the venous wall associated with intravascular thrombosis, enlarged nodes, enlarged ascending aorta, or by extrinsic pressure of a tumor mass against the thin-walled superior vena cava which leads to the development of superior vena cava syndrome. Superior vena cava syndrome is associated with a number of conditions that include malignant tumors, tuberculosis, histoplasmosis, and syphilis.

Knowledge of the anatomy of the superior vena cava and its relationship to the surrounding lymph nodes is essential to understanding the development of the syndrome.^[1]

• The superior vena cava is the major blood vessel for the drainage of the upper body (head, neck, upper extremities, and upper thorax).
• Extrinsic compression of the superior vena cava by a mediastinal tumor is possible because it has a low intravascular pressure. In addition, the superior vena cava is also surrounded by rigid structures, so it is relatively easy to compress.
• Moreover, the superior vena cava is thin-walled, and the blood flowing therein is under low pressure. Therefore, when the nodes or ascending aorta enlarge, the superior vena cava is compressed, blood flow slows, and complete occlusion may occur.^[2]
• The superior vena cava is formed by the junction of the left and right brachiocephalic veins in the mid third of the mediastinum. It extends caudally for 6 to 8 cm, coursing anterior to the right mainstem bronchus and terminating in the superior right atrium, and extends anteriorly to the right mainstem bronchus.
• The superior vena cava is joined posteriorly by the azygos vein as it loops over the right mainstem bronchus and lies posterior to and to the right of the ascending aorta. The mediastinal parietal pleura is lateral to the superior vena cava, creating a confined space, and the superior vena cava is adjacent to the right paratracheal, azygous, right hilar, and subcarinal lymph node groups.^[3]
• In long standing cases with 60% or more stenosis, collateral channels are formed to restore venous return. Various collaterals are formed depending up on the site of the obstruction:

• Pre-azygos: in this conditions mainly the right superior intercostal veins serves as the collateral pathway to drain into the azygos vein.
• Azygos: when the azygos vein is also obstructed the collateral circulation establishes between superior vena cava and inferior vena cava via minor communicating channels i.e. internal mammary veins, superior and inferior epigastric veins to iliac veins and finally into the inferior vena cava.
• Post-azygos: in this case the blood from the superior vena cava is distributed into the azygos and hemiazygos and then into the inferior vena cava tubutaries i.e. ascending lumbar and lumbar veins.

• Superior vena cava syndrome is associated with a number of conditions that include:

• Malignancy^[4]
  • Lymph node metastasis
  • Non–small cell lung cancer
  • Small cell lung cancer
  • Lymphoma
  • Metastatic lesions (most commonly from breast and testicular cancers)
• Infections
• Iatrogenic intravascular devices
• Thyroid goiter
• Thrombosis
• Pericardial constriction
• Idiopathic sclerosing mediastinitis
• Aortic aneurysm

On gross pathology, there are no characteristic findings of superior vena syndrome.

On microscopic pathology, there are no characteristic findings of superior vena syndrome.

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