Accessory hemiazygos vein

The accessory hemiazygos vein descends on the left side of the vertebral column, and varies inversely in size with the highest left intercostal vein.

The hemiazygos vein is a component of the azygos venous system that consists of three main veins: the azygos vein (on the right), the hemiazygos vein (lower left, previously termed as inferior hemiazygos vein), and the accessory hemiazygos vein (upper left, previously termed as superior hemiazygos vein). These veins develop from the caudal cardinal veins during embryonic development. The accessory hemiazygos vein runs along the left side of the vertebral column in the upper posterior mediastinum, parallel to the descending thoracic aorta.

It receives veins from the three or four intercostal spaces between the highest left intercostal vein and highest tributary of the hemiazygos; the left bronchial vein sometimes opens into it. It either crosses the body of the eighth thoracic vertebra to join the azygos vein or ends in the hemiazygos. When this vein is small, or altogether wanting, the left highest intercostal vein may extend as low as the fifth or sixth intercostal space.

In obstruction of the superior vena cava, the azygos and hemiazygos veins are one of the principal means by which the venous circulation is carried on, connecting as they do the superior and inferior venæ cavæ, and communicating with the common iliac veins by the ascending lumbar veins and with many of the tributaries of the inferior vena cava.