M01.05.004 Posterior Mediastinum

The posterior mediastinum is a key anatomical region located between the two pleural sacs in the thoracic cavity. It houses major organs, vessels, and nerves, serving as a conduit for structures passing through the thorax into the abdomen.

1. Borders of the Posterior Mediastinum

The posterior mediastinum is bordered by the following structures:

2. Contents of the Posterior Mediastinum

The posterior mediastinum contains vital structures including major vessels, the esophagus, and various nerves.

A. Thoracic Aorta

The thoracic aorta begins at the lower edge of the T4 vertebra and descends to the left of the vertebral column, becoming the abdominal aorta at T12.

• Major branches of the thoracic aorta:
  • Posterior intercostal arteries: Nine pairs supplying the intercostal spaces (except the first two).
  • Bronchial arteries: Supply the tracheobronchial tree.
  • Oesophageal arteries: Supply the oesophagus.
  • Superior phrenic arteries: Supply the superior diaphragm.

B. Oesophagus

The esophagus is a muscular tube that connects the pharynx to the stomach, passing through the posterior mediastinum. It deviates leftward as it descends and exits via the oesophageal hiatus of the diaphragm.

• Oesophageal plexus: A network of nerves from the vagus nerves that surround the oesophagus.

C. Thoracic Duct

The thoracic duct is the largest lymphatic vessel, returning lymph from most of the body into the venous system.

• It originates from the cisterna chyli in the abdomen and ascends through the posterior mediastinum.

D. Azygos Venous System

The azygos system drains blood from the body walls and mediastinal viscera, emptying into the superior vena cava.

• Azygos vein: Formed by the right lumbar and subcostal veins.
• Hemiazygos vein: Formed by the left lumbar and subcostal veins.
• Accessory hemiazygos vein: Drains from the fourth to eighth intercostal veins.

E. Sympathetic Trunks

The sympathetic trunks are paired nerve bundles running from the base of the skull to the coccyx. In the thoracic region, they form the thoracic sympathetic trunks.

• These trunks give rise to lower thoracic splanchnic nerves that supply the abdominal viscera.

3. Points to Remember

• The posterior mediastinum is located between the pleural sacs and extends from the thoracic inlet to the diaphragm.
• Major structures include the thoracic aorta, esophagus, thoracic duct, azygos veins, and sympathetic trunks.
• The posterior mediastinum serves as a conduit for structures passing between the thorax and abdomen.
• Clinical conditions such as aortic aneurysms, esophageal cancer, and thoracic duct injuries are commonly associated with the posterior mediastinum.

4. Clinical Correlations

• Aortic Aneurysms: Can develop in the thoracic aorta, potentially leading to compression of nearby structures like the esophagus or trachea.
• Esophageal Disorders: Conditions such as esophageal cancer or strictures can affect the oesophagus as it traverses the posterior mediastinum.
• Lymphatic Drainage: The thoracic duct is critical for lymphatic return, and its obstruction can lead to conditions like chylothorax (lymph accumulation in the pleural cavity).

5. Learning Objective

• Understand the anatomical boundaries and the contents of the posterior mediastinum. Identify the structures within this region and explain their functional roles, along with common clinical conditions related to this area.