• What is it:
  • The mediastinum is the central compartment of the thoracic cavity, located between the pleural cavities.
  • It contains vital structures such as the heart, great vessels, trachea, esophagus, thymus, lymph nodes, and nerves, surrounded by connective tissue.
• Parts:
  The mediastinum is divided into three main compartments for clinical and radiological purposes:
  1. Anterior mediastinum: Located between the sternum and pericardium.
  2. Middle mediastinum: Contains the heart, great vessels, and surrounding structures.
  3. Posterior mediastinum: Positioned between the pericardium and vertebral column.

• Anterior Mediastinum

  • Size:
    • Extends vertically from the thoracic inlet to the diaphragm.
    • Width varies but is generally narrow in healthy individuals; enlargement suggests pathology (e.g., masses or effusion).
  • Shape:
    • Triangular in cross-section, with the base along the diaphragm and apex at the thoracic inlet.
  • Position:
    • Bounded anteriorly by the sternum.
    • Posteriorly by the pericardium and great vessels.
    • Lies medial to the pleural spaces.
  • Character:
    • Contains fat, the thymus gland (in children and young adults), lymph nodes, and small vessels.
    • Abnormalities include:
      • Masses: Thymomas, teratomas, lymphomas, or ectopic thyroid tissue.
      • Density changes: Increased density (on CT) due to fat stranding, calcifications, or soft tissue mass.

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  Middle Mediastinum

  • Size:
    • Larger compared to the anterior compartment, spanning the heart and great vessels.
    • Width can increase in cases of lymphadenopathy or cardiomegaly.
  • Shape:
    • Oval or rectangular in shape, depending on the imaging plane.
    • Well-defined borders in healthy individuals.
  • Position:
    • Bounded anteriorly by the pericardium.
    • Posteriorly by the posterior pericardium and trachea.
    • Centrally located, containing the heart, pericardium, great vessels, trachea, and esophagus.
  • Character:
    • Contains vascular structures (aorta, pulmonary arteries and veins, superior vena cava), airways (trachea and main bronchi), and lymph nodes.
    • Abnormalities include:
      • Masses: Bronchogenic cysts, lymphadenopathy, or vascular anomalies.
      • Airway changes: Tracheal deviation or compression from adjacent masses.
      • Calcifications: Often seen in lymph nodes due to old infections (e.g., tuberculosis).
• • The mediastinum consists of various tissues, including fat, connective tissue, lymph nodes, and organs.
  • Each compartment has unique structures and pathologies associated with it.
• Blood supply:
  • Mediastinal structures receive blood from branches of the internal thoracic, bronchial, intercostal, and subclavian arteries.
• Venous drainage:
  • Mediastinal veins drain into the superior vena cava, azygos vein, hemiazygos vein, and internal thoracic veins.
• Lymphatic drainage:
  • Mediastinal lymph nodes are distributed across various stations (e.g., paratracheal, subcarinal).
  • These nodes drain into the thoracic duct or the right lymphatic duct.
• Nerve supply:
  • Mediastinal structures are innervated by the vagus nerves, sympathetic chains, and phrenic nerves.
• Embryology:
  • Mediastinal structures develop from distinct embryological origins:
    • Heart and great vessels: Derived from mesoderm.
    • Thymus: Derived from the third pharyngeal pouch.
    • Trachea and esophagus: Derived from the foregut.
• Applied anatomy:
  • Imaging Application:
    • Plain chest radiography: Useful for identifying mediastinal widening or gross masses but lacks compartmental specificity.
    • CT (preferred): Provides detailed compartmentalization and characterization of structures and masses.
    • MRI: Used for soft tissue characterization and vascular involvement.
    • PET-CT: Assesses metabolic activity of mediastinal masses or lymphadenopathy.
  • Surgical Application: The mediastinum’s compartments guide thoracic surgeons for resection of masses or lymph node sampling.

Key Points and Pearls

• Anterior mediastinum: Most common site for thymomas, germ cell tumors, and thyroid masses.
• Middle mediastinum: Dominated by lymph nodes, vascular abnormalities, and bronchogenic cysts.
• Posterior mediastinum: Often involved in neurogenic tumors, esophageal disorders, or descending aortic pathology.
• Clear delineation of compartments on imaging helps narrow the differential diagnosis and guide clinical management.

The mediastinum is densely populated with important structures.  Often the first signs of disease are recognized on the chest X-ray as a result of subtle changes in the shape of the mediastinum. It is therefore imperative that radiologists are extremely familiar with the normal shape of the mediastinum. Review of the shape may start with the left side at the apex – Think of yourself skiing down a mogul trail and consider the bumpy ride that may challenge you.   There is a left sided slope and the right sided slope – both are black diamonds.   Get your skis and helmets on– these are tough slopes

Applied Anatomy 

The following image represents one of the many shape changes that occurs in the mediastinum that will tip the radiologist off to a disease process in the chest.

 

Enlarged Mediastinum Caused by Adenopathy

Normal vs Abnormal

CT of the Above Patient