Posterior Mediastinum

The posterior mediastinum is the rearmost subdivision of the inferior mediastinum, located posterior to the pericardium and anterior to the vertebral column. It acts as a conduit for major thoracic structures passing between the neck, thorax, and abdomen. The space contains important vascular, nervous, and lymphatic structures and is closely associated with the thoracic vertebrae and esophagus.

Boundaries

The posterior mediastinum is bordered by the following anatomical landmarks:

• Anterior: Fibrous pericardium and diaphragm
• Posterior: Vertebral bodies of T5 to T12 and associated intervertebral discs
• Superior: A transverse plane at the level of the sternal angle (T4–T5 intervertebral disc), separating it from the superior mediastinum
• Inferior: Diaphragm
• Lateral: Mediastinal pleura of the lungs

Contents of the Posterior Mediastinum

The posterior mediastinum contains several long tubular structures and associated lymphatics and nerves. These include:

• Esophagus
• Thoracic aorta
• Azygos and hemiazygos venous systems
• Thoracic duct
• Vagus nerves and esophageal plexus
• Splanchnic nerves (greater, lesser, and least)
• Posterior mediastinal lymph nodes

Esophagus

The esophagus descends through the posterior mediastinum anterior to the vertebral column and posterior to the pericardium and left atrium. It enters the thorax via the superior mediastinum, runs slightly to the right of the midline, and deviates leftward as it approaches the diaphragm, passing through the esophageal hiatus (T10).

It is closely associated with:

• Thoracic aorta (to the left)
• Azygos vein (to the right)
• Left atrium (anteriorly, through pericardium)

Thoracic Aorta

The thoracic (descending) aorta begins at the level of T4, just inferior to the aortic arch, and descends along the left side of the vertebral column. It continues through the posterior mediastinum and exits via the aortic hiatus (T12) of the diaphragm to become the abdominal aorta.

Major branches of the thoracic aorta in this region include:

• Posterior intercostal arteries (T3–T11)
• Bronchial arteries (usually one right and two left)
• Esophageal branches
• Pericardial branches (posterior part of pericardium)
• Superior phrenic arteries

Azygos and Hemiazygos Veins

Azygos Vein

The azygos vein ascends on the right side of the thoracic vertebral bodies, draining blood from the thoracic wall and posterior abdominal wall. It arches anteriorly over the root of the right lung and empties into the superior vena cava.

Hemiazygos Vein

The hemiazygos vein ascends on the left side and typically crosses to the right around T8 to join the azygos vein. It drains the lower left posterior intercostal veins and some esophageal and mediastinal veins.

Accessory Hemiazygos Vein

This vein drains the upper left posterior intercostal veins and crosses the vertebral column around T7 to join the azygos vein or hemiazygos vein.

Thoracic Duct

The thoracic duct is the largest lymphatic vessel in the body. It enters the thorax through the aortic hiatus (T12), lies posterior to the esophagus and between the thoracic aorta (left) and azygos vein (right), and ascends through the posterior mediastinum toward the superior mediastinum. It crosses from right to left near T5–T6 and eventually drains into the junction of the left internal jugular and subclavian veins.

It receives lymph from:

• Lower limbs
• Abdomen and pelvis
• Left side of thorax
• Left upper limb
• Left side of head and neck

Sympathetic Trunks and Splanchnic Nerves

The sympathetic trunks run longitudinally along the vertebral bodies, posterior to the pleura. From these trunks arise the thoracic splanchnic nerves:

• Greater splanchnic nerve: Arises from T5–T9
• Lesser splanchnic nerve: Arises from T10–T11
• Least (lowest) splanchnic nerve: Arises from T12

These nerves carry preganglionic sympathetic fibers to the abdominal prevertebral ganglia via the diaphragm’s crura. They pass through the posterior mediastinum on their way to the abdomen.

Posterior Mediastinal Lymph Nodes

These lymph nodes are scattered along the thoracic aorta, esophagus, and intercostal spaces. They receive lymph from:

• Esophagus
• Posterior pericardium
• Posterior thoracic wall
• Diaphragm (superior surface)

The efferent lymphatics from these nodes often join the thoracic duct.

Topographical Relationships

• Anterior: Pericardium and left atrium
• Posterior: Thoracic vertebral bodies (T5–T12)
• Lateral: Lungs and mediastinal pleura
• Superior: Continuous with the superior mediastinum
• Inferior: Diaphragm

The posterior mediastinum acts as a protected corridor for structures transitioning between thoracic and abdominal cavities. Its spatial relationships make it essential for understanding both cross-sectional and surgical anatomy of the thorax.