Inferior Mediastinum

The inferior mediastinum is the lower part of the mediastinal cavity, lying beneath the superior mediastinum and extending to the diaphragm. It serves as a central passageway and support zone for vital thoracic structures, including the heart, large vessels, esophagus, lymphatic system, and major nerves. Due to the density and diversity of its contents, it is clinically and anatomically subdivided into three parts: anterior, middle, and posterior.

Location and Boundaries

The inferior mediastinum is defined by its position relative to the transverse thoracic plane—a horizontal plane from the sternal angle to the T4/T5 vertebrae:

• Superior boundary: Transverse thoracic plane (sternal angle to T4/T5 intervertebral disc)
• Inferior boundary: Diaphragm
• Anterior boundary: Body of the sternum and costal cartilages
• Posterior boundary: Bodies of thoracic vertebrae T5 to T12
• Lateral boundaries: Mediastinal pleura of the lungs

It is positioned directly below the superior mediastinum and divided into three compartments: anterior, middle, and posterior, each with unique contents and clinical significance.

Divisions of the Inferior Mediastinum

1. Anterior Mediastinum

The anterior mediastinum is the smallest subdivision, lying between the sternum and the pericardium. It is a narrow space, more prominent in children than adults, and contains primarily connective and lymphatic tissue.

Boundaries of the Anterior Mediastinum

• Anterior: Sternum and transverse thoracic muscles
• Posterior: Pericardium
• Superior: Continuous with the superior mediastinum
• Inferior: Diaphragm
• Lateral: Mediastinal pleura

Contents of the Anterior Mediastinum

• Loose connective tissue and fat
• Inferior remnants of the thymus gland (particularly in children)
• Anterior mediastinal lymph nodes
• Small branches of the internal thoracic (mammary) arteries
• Sternopericardial ligaments

2. Middle Mediastinum

The middle mediastinum is the central and most prominent division of the inferior mediastinum. It primarily contains the heart enclosed within the pericardium, along with associated great vessels, nerves, and lymphatics. This region is of particular importance due to its cardiovascular contents and surrounding anatomical relationships.

Boundaries of the Middle Mediastinum

• Anterior: Anterior pericardium
• Posterior: Posterior pericardium
• Superior: Transverse thoracic plane (continuous with the superior mediastinum)
• Inferior: Diaphragm
• Lateral: Mediastinal pleura of lungs

Contents of the Middle Mediastinum

• Heart enclosed in the pericardial sac
• Ascending aorta
• Pulmonary trunk and right and left pulmonary arteries
• Superior vena cava (terminal portion)
• Inferior vena cava (enters the right atrium through the diaphragm)
• Right and left pulmonary veins
• Main bronchi (just inferior to the carina)
• Phrenic nerves (travel between pleura and pericardium)
• Pericardiacophrenic vessels
• Tracheobronchial lymph nodes

The phrenic nerves pass anterior to the root of the lung and supply motor innervation to the diaphragm, making their relationship to the pericardium and lungs of functional importance.

3. Posterior Mediastinum

The posterior mediastinum lies behind the pericardium and in front of the vertebral bodies of T5 to T12. It forms a longitudinal corridor that allows passage of major thoracic and abdominal structures. This area is deep and bordered by critical neurovascular and digestive elements.

Boundaries of the Posterior Mediastinum

• Anterior: Posterior surface of the pericardium
• Posterior: Bodies of thoracic vertebrae T5 to T12
• Superior: Transverse thoracic plane (T4/T5 level)
• Inferior: Diaphragm
• Lateral: Mediastinal pleura

Contents of the Posterior Mediastinum

• Descending thoracic aorta and its branches (bronchial, esophageal, posterior intercostal arteries)
• Esophagus
• Thoracic duct (lymphatic drainage into venous angle)
• Azygos vein (right side)
• Hemiazygos and accessory hemiazygos veins (left side)
• Posterior mediastinal lymph nodes
• Sympathetic trunks and thoracic splanchnic nerves (greater, lesser, and least)

The thoracic duct ascends on the right side of the vertebral column, crosses over to the left at the level of T4–T6, and enters the venous system at the junction of the left subclavian and internal jugular veins. The azygos system of veins provides collateral circulation between the superior and inferior vena cava, which becomes significant in cases of obstruction.

Functional Overview

The inferior mediastinum, as a whole, supports the following key physiological functions:

• Cardiac function: Through housing the heart and pericardium, the middle mediastinum enables systemic and pulmonary circulation.
• Vascular distribution: The ascending aorta and pulmonary arteries emerge here; venous return from the upper and lower body also converges in this region.
• Airway bifurcation: The trachea divides into left and right bronchi just above this area; the main bronchi lie in the middle mediastinum.
• Esophageal transit: The esophagus travels from the pharynx to the stomach, passing through the posterior mediastinum.
• Autonomic control: Sympathetic chains and splanchnic nerves descending through the posterior mediastinum innervate thoracic and abdominal organs.
• Lymphatic drainage: The thoracic duct and associated lymph nodes regulate thoracic and abdominal lymphatic return.

The anatomical complexity and proximity of vital organs in the inferior mediastinum make it a subject of critical focus in thoracic imaging, surgery, and pathology. Understanding its subdivisions and relationships forms the backbone of thoracic anatomy.