Superior Vena Cava Syndrome
Knowledge of the anatomy of the SVC and its relationship to the surrounding lymph nodes is essential to understanding the development of the syndrome. The SVC is formed by the junction of the left and right brachiocephalic veins in the mid third of the mediastinum. The SVC 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 SVC 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 SVC, creating a confined space, and the SVC is adjacent to the right paratracheal, azygous, right hilar, and subcarinal lymph node groups. The vessel itself is thin-walled, and the blood flowing therein is under low pressure. Thus, when the nodes or ascending aorta enlarge, the SVC is compressed, blood flow slows, and complete occlusion may occur.
The severity of the syndrome depends on the rapidity of onset of the obstruction and its location. The more rapid the onset, the more severe the symptoms because the collateral veins do not have time to distend to accommodate an increased blood flow. If the obstruction is above the entry of the azygos vein, the syndrome is less pronounced because the azygous venous system can readily distend to accommodate the shunted blood with less venous pressure developing in the head, arms, and upper thorax. If the obstruction is below the entry of the azygos vein, more florid symptoms and signs are seen because the blood must be returned to the heart via the upper abdominal veins and the inferior vena cava, which requires higher venous pressure.
One study suggested that the general recruitment of venous collaterals over time may lead to remission of the syndrome, although the SVC remains obstructed.
Once SVCS is recognized, prompt clinical attention is important. A diagnosis should be established prior to initiating therapy for the following reasons:
- 75% of patients have symptoms and signs for longer than 1 week before seeking medical attention.
- Cancer patients diagnosed with SVCS do not die of the syndrome itself but rather from the extent of their underlying disease.
- 3% to 5% of the patients diagnosed with SVCS do not have cancer.