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Mediastinal Compartments

  • What is it:
    • The mediastinum is the central compartment of the thoracic cavity that contains vital structures including the heart, great vessels, trachea, esophagus, thymus, lymph nodes, and connective tissue.
    • It is divided into specific compartments to aid in anatomical localization and differential diagnosis of mediastinal pathologies.
  • Parts:
    • The mediastinum is divided into three main compartments based on imaging criteria (often the Felson’s method in chest radiography or CT-based methods like the International Thymic Malignancy Interest Group (ITMIG) classification):
      • Anterior mediastinum: Bounded by the sternum and anterior pericardium.
      • Middle mediastinum: Bounded by the anterior and posterior pericardium and contains the heart, great vessels, and airways.
      • Posterior mediastinum: Bounded by the pericardium and the vertebral bodies.

  • 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.

    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).

    Posterior Mediastinum

    • Size:
      • Narrowest of the compartments; width depends on adjacent vertebral or soft tissue pathology.
    • Shape:
      • Elongated and cylindrical, following the contours of the spine.
    • Position:
      • Lies posterior to the pericardium and anterior to the vertebral column.
      • Extends vertically from the thoracic inlet to the diaphragm.
    • Character:
      • Contains neural and esophageal structures, the descending thoracic aorta, and lymph nodes.
      • Abnormalities include:
        • Masses: Neurogenic tumors, esophageal neoplasms, or hernias (e.g., Bochdalek hernia).
        • Density changes: May show calcifications in neurogenic tumors or gas-fluid levels in esophageal disorders.
    • The mediastinum consists of soft tissue, fat, lymph nodes, and airways.
    • Variations in density on imaging can indicate pathology, such as mass lesions, lymphadenopathy, or vascular anomalies.
  • Blood supply:
    • Branches of the internal thoracic, bronchial, and intercostal arteries supply the mediastinal tissues.
  • Venous drainage:
    • Mediastinal veins drain into the superior vena cava, azygos system, and internal thoracic veins.
  • Lymphatic drainage:
    • Mediastinal lymph nodes are classified into stations (e.g., paratracheal, subcarinal) and drain into the thoracic duct or right lymphatic duct.
  • Nerve supply:
    • Autonomic innervation via the vagus nerve, sympathetic chain, and phrenic nerves.
  • Embryology:
    • Mediastinal structures arise from different embryological origins:
      • Anterior mediastinum: Thymus from the third pharyngeal pouch.
      • Middle mediastinum: Heart and great vessels from the mesoderm.
      • Posterior mediastinum: Esophagus and spinal structures from foregut and notochord development.
  • Applied anatomy:
    • Mediastinal compartmentalization is critical for:
      • Localization of pathology: Helps in narrowing the differential diagnosis (e.g., anterior mediastinal masses like thymomas vs. posterior mediastinal neurogenic tumors).
      • Surgical planning: Ensures precise access routes for resection or biopsy.
    • Imaging Application:
      • CT and MRI: Provide detailed anatomical delineation of mediastinal compartments.
      • PET-CT: Useful for assessing metabolic activity in mediastinal masses or lymphadenopathy.

Key Compartment-Specific Pathologies

  • Anterior mediastinum:
    • Thymomas, teratomas, thyroid masses, lymphoma.
  • Middle mediastinum:
    • Lymphadenopathy (e.g., sarcoidosis, lymphoma, metastases), vascular anomalies, bronchogenic cysts.
  • Posterior mediastinum:
    • Neurogenic tumors, esophageal abnormalities, descending aortic pathologies.

This framework organizes the mediastinal compartments in a radiology-focused manner, emphasizing imaging and clinical relevance.