Azygos Vein,Hemiazygos Vein, Accessory Hemiazygos Vein,Thoracic Sympathetic Trunk

Topic Overview

Azygos Vein

• The azygos vein drains the thoracic wall and upper lumbar region and provides an important connection between the superior and inferior vena cava, as well as the portal and vertebral venous systems.

• Formation:

  • Formed by the union of right subcostal vein and right ascending lumbar vein.

  • Sometimes includes a contribution from the lumbar azygos vein.

• Course:

  • Enters the thorax through the aortic opening of the diaphragm.

  • Ascends on the right side of the vertebral column and arches over the root of the right lung to join the superior vena cava.

• Relations:

  • Anteriorly: Oesophagus.

  • Posteriorly: Lower eight thoracic vertebrae and right posterior intercostal arteries.

  • Right side: Right lung and pleura, greater splanchnic nerve.

  • Left side: Thoracic duct and aorta below, oesophagus and vagus nerve above.

• Tributaries:

  1. Right superior intercostal vein (from 2nd–4th intercostal spaces).

  2. 5th–11th right posterior intercostal veins.

  3. Hemiazygos and accessory hemiazygos veins.

  4. Right bronchial vein.

  5. Oesophageal, mediastinal, and pericardial veins.

• Clinical Note:

  • Acts as an important collateral channel during obstruction of the inferior vena cava

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Hemiazygos Vein

• Also known as the inferior hemiazygos vein, it mirrors the lower part of the azygos vein on the left side.

• Formation:

  • Union of left ascending lumbar vein, left subcostal vein, and left lumbar azygos vein.

• Course:

  • Enters thorax by piercing the left crus of diaphragm, ascends along the left side of vertebral column behind the aorta, and at the level of the 8th thoracic vertebra, turns to the right behind the oesophagus and thoracic duct to join the azygos vein.

• Tributaries:

  • 9th–11th left posterior intercostal veins.

  • Oesophageal veins.

• Function:

  • Provides venous drainage for the lower left posterior thoracic wall and connects with the azygos system for collateral circulation

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Accessory Hemiazygos Vein

• Also called the superior hemiazygos vein, representing the upper counterpart of the hemiazygos vein.

• Formation and Course:

  • Begins around the 4th or 5th intercostal space, descends along the left side of vertebral column, and at the level of the 8th thoracic vertebra, crosses to the right behind the aorta and thoracic duct to join the azygos vein.

• Tributaries:

  • 5th–8th left posterior intercostal veins.

  • Sometimes left bronchial veins.

• Function:

  • Drains the upper left posterior thoracic wall and provides a connection between the superior intercostal and hemiazygos systems

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Thoracic Sympathetic Trunk

• The thoracic part of the sympathetic trunk is a chain of ganglia lying on each side of the vertebral column.

• Extent:

  • Continuous above with the cervical and below with the lumbar sympathetic trunks.

• Structure:

  • Usually 11 ganglia, corresponding to T1–T12 spinal nerves.

  • The first thoracic ganglion often fuses with the inferior cervical ganglion to form the stellate (cervicothoracic) ganglion.

• Course and Relations:

  • Descends across the neck of the 1st rib, heads of 2nd–10th ribs, and bodies of 11th–12th thoracic vertebrae.

  • Lies anterior to posterior intercostal vessels and nerves; continues below behind the medial arcuate ligament into the abdomen.

• Branches:

  1. Grey rami communicantes – to all thoracic spinal nerves (T1–T12), carrying postganglionic fibres to blood vessels, sweat glands, and arrector pili.

  2. White rami communicantes – from T1–T12 to corresponding ganglia; T1–T5 ascend to cervical ganglia, T10–L2 descend to lumbar or sacral ganglia.

  3. Visceral branches:

     • Cardiac, pulmonary, and oesophageal branches (from upper 5 ganglia).

     • Splanchnic nerves from lower 8 ganglia:

       • Greater splanchnic nerve (T5–T9)

       • Lesser splanchnic nerve (T10–T11)

       • Least splanchnic nerve (T12)

• Clinical Significance:

  • Involved in autonomic control of thoracic viscera.

  • Stellate ganglion block is performed for relief of vasospasm or hyperhidrosis.

  • Damage can cause Horner’s syndrome (ptosis, miosis, anhidrosis