Mediastinum

Competency

  • AN24.1: Mention the blood supply, lymphatic drainage and nerve supply of pleura, extent of pleura and describe the pleural recesses and their applied anatomy.

Introduction

  • The mediastinum is the central compartment of the thoracic cavity located between the right and left pleural sacs.
  • The term mediastinum is derived from Latin and means “middle septum.”
  • It contains the major vessels, viscera, lymphatics, and nerves of the thorax.

Boundaries of Mediastinum

  • The mediastinum extends from the thoracic inlet superiorly to the diaphragm inferiorly.
  • It is bounded laterally by the mediastinal pleurae on both sides.
  • Anteriorly, it is bounded by the sternum.
  • Posteriorly, it is bounded by the vertebral column.
  • Superiorly, it is bounded by the thoracic inlet.
  • Inferiorly, it is bounded by the diaphragm.
  • On each side, it is bounded by the mediastinal pleura.

Contents of Mediastinum

  1. Thymus, heart enclosed within the pericardial sac, trachea, and esophagus.
  2. Thoracic aorta, pulmonary trunk, superior vena cava, and inferior vena cava.
  3. Lymph nodes and the thoracic duct.
  4. Vagus nerves, sympathetic trunks, and phrenic nerves.

Divisions of Mediastinum

  • For descriptive purposes, the mediastinum is divided into superior and inferior parts by an imaginary transverse plane.
  • This plane passes through the sternal angle anteriorly and the lower border of the body of the fourth thoracic vertebra posteriorly.
  • The inferior mediastinum is further divided by the pericardium into anterior, middle, and posterior mediastina.
  • The anterior mediastinum lies in front of the pericardium.
  • The middle mediastinum contains the pericardium and its contents.
  • The posterior mediastinum lies behind the pericardium.

Superior Mediastinum

  • The superior mediastinum is the upper part of the mediastinum.
  • It extends from the thoracic inlet superiorly to the imaginary transverse thoracic plane inferiorly.

Boundaries

  • Anteriorly, the superior mediastinum is bounded by the manubrium sterni.
  • Posteriorly, it is bounded by the bodies of the upper four thoracic vertebrae (T1–T4).
  • Superiorly, it is bounded by the oblique plane of the thoracic inlet.
  • Inferiorly, it is bounded by an imaginary transverse thoracic plane that passes through the sternal angle anteriorly and the lower border of the body of the T4 vertebra posteriorly.
  • Laterally, it is bounded on both sides by the mediastinal pleura.

Contents

  1. Viscera – trachea, esophagus, and thymus
  2. Arch of the aorta, brachiocephalic artery, left common carotid artery, and left subclavian artery
  3. Right and left brachiocephalic veins, upper half of the superior vena cava, and left superior intercostal vein
  4. Right and left phrenic nerves, right and left vagus nerves, left recurrent laryngeal nerve, right and left sympathetic trunks, and cardiac nerves.
  5. Lymph nodes and the thoracic duct.
  6. Sternohyoid, sternothyroid, and lower ends of the longus colli muscles.
  7. The thymus undergoes involution after puberty and gradually decreases in size.

Clinical Integration

  • Mediastinum – as a potential dead space: The major veins of the superior mediastinum are situated mainly on the right, whereas the major arteries are situated mainly on the left. The superior mediastinum contains only a small amount of loose connective tissue. This loose connective tissue allows the veins to expand when venous return increases. Tumors and fluid collections may extend into the space around the major veins on the right side. This region surrounding the veins therefore acts as a potential dead space.
  • Mediastinitis: Mediastinitis is inflammation of the mediastinum, which may be associated with pus formation.
  • Extension of infection from neck to the mediastinum: The prevertebral fascia extends from the neck into the superior mediastinum up to approximately the level of the fourth thoracic vertebra. Therefore, infection originating in the neck may spread downward along this fascial plane to the level of T4. The pretracheal fascia extends inferiorly and blends with the fibrous structures around the arch of the aorta. Infection in the fascial space between the pretracheal and prevertebral fasciae may spread from the neck into the superior and posterior mediastinum.

Anterior Mediastinum

The anterior mediastinum is a narrow part of the inferior mediastinum located in front of the pericardium.

Boundaries

  • Anteriorly, it is bounded by the body of the sternum.
  • Posteriorly, it is bounded by the pericardium enclosing the heart.
  • Superiorly, it is limited by the transverse thoracic plane passing through the sternal angle anteriorly and the lower border of the T4 vertebra posteriorly.
  • Inferiorly, it is bounded by the diaphragm.
  • Laterally, it is bounded on both sides by the mediastinal pleura.

Contents

  1. Superior and inferior sternopericardial ligaments
  2. Loose areolar tissue
  3. Mediastinal branches of the internal thoracic arteries
  4. Lower part of the thymus is present up to puberty
  5. A few lymph nodes.

Clinical Integration

  • Anterior mediastinitis: The anterior mediastinum communicates superiorly with the pretracheal space of the neck through the superior mediastinum. Therefore, infection from the neck may spread into the anterior mediastinum and cause anterior mediastinitis.

Middle Mediastinum

The middle mediastinum is the central part of the inferior mediastinum and contains the pericardium and its contents.

Boundaries

  1. Anteriorly, it is related to the anterior mediastinum.
  2. Posteriorly, it is related to the posterior mediastinum, including the esophagus and descending thoracic aorta.
  3. Superiorly, it is continuous with the superior mediastinum.
  4. Inferiorly, it is bounded by the diaphragm.
  5. Laterally, it is bounded on both sides by the mediastinal pleura.

Contents

  1. Pericardium.
  2. Heart.
  3. Aorta, pulmonary trunk, and right and left pulmonary arteries.
  4. Lower half of the superior vena cava, terminal part of the azygos vein, and right and left pulmonary veins.
  5. Phrenic nerves and deep cardiac plexus
  6. Lymph nodes
  7. Trachea and principal bronchi.

Posterior Mediastinum

  • Posterior mediastinum is a part of inferior mediastinum that lies behind the pericardium.

Boundaries

Anterior: Pericardium, bifurcation of trachea, pulmonary vessels

Posterior: Bodies of lower eight thoracic vertebrae and intervertebral discs

Superior: Transverse thoracic plane passing through the angle of sternum and lower border body of T4 vertebra

Inferior: Diaphragm

On each side (lateral): Mediastinal pleura

Contents

  1. Esophagus
  2. Arteries: Descending thoracic aorta
  3. Veins: Azygos, hemiazygos, and accessory hemiazygos veins
  4. Nerves: Vagus nerves, sympathetic chains
  5. Lymphatics: Posterior mediastinal lymph nodes, thoracic duct.

Clinical Integration

Widening of the Mediastinum

  • Widening of the mediastinum may be detected on a chest radiograph. It causes include hemorrhage from injured blood vessels, enlarged mediastinal lymph nodes, cardiac enlargement, and other mediastinal masses or lesions.

Mediastinal Shift

  • A mediastinal shift occurs when the mediastinum is displaced from its normal central position. In lung collapse (atelectasis), the mediastinum shifts toward the affected side. In a large pleural effusion, the mediastinum may shift toward the opposite side.
  • Normally, the trachea can be palpated in the midline at the suprasternal notch. When the mediastinum shifts, the trachea is usually displaced in the same direction as the mediastinal shift.

Mediastinoscopy

  • Mediastinoscopy is a procedure in which an endoscope is used to visualize mediastinal structures. It also allows biopsy of mediastinal tissues, particularly lymph nodes.

Mediastinal Syndrome

Mediastinal syndrome refers to a group of signs and symptoms caused by compression of mediastinal structures by a tumor, fluid collection, or other mass as follows:

  1. Compression of the superior vena cava causes engorgement of veins in the upper half of the body.
  2. Compression of the trachea may cause dyspnea and cough.
  3. Compression of the esophagus may cause dysphagia.
  4. Compression of the left recurrent laryngeal nerve may produce hoarseness of voice.
  5. Compression of the phrenic nerve may result in paralysis of the diaphragm.
  6. Compression of an intercostal nerve may cause intercostal neuralgia, with pain along the area supplied by the affected nerve.
  7. Common causes of mediastinal syndrome include bronchogenic carcinoma, mediastinal lymphadenopathy in Hodgkin lymphoma, and aortic aneurysm.

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