Mediastinum: Divisions, Contents, and Clinical Anatomy

Overview

The mediastinum is the central thoracic compartment situated between the two pleural cavities (mediastinal pleura). It extends from the thoracic inlet superiorly to the diaphragm inferiorly, and from the sternum anteriorly to the thoracic vertebral column posteriorly. It contains the heart, great vessels, trachea, esophagus, thoracic duct, and numerous nerves — virtually all thoracic structures except the lungs.

Divisions of the Mediastinum

The mediastinum is divided by an imaginary plane passing through the sternal angle (of Louis) anteriorly and the lower border of T4 posteriorly:

Superior Mediastinum

• Above the plane of T4/sternal angle
• Contents:
  • Thymus (large in children, replaced by fat in adults)
  • Great vessels: aortic arch, brachiocephalic trunk, left common carotid artery, left subclavian artery, brachiocephalic veins (left and right), superior vena cava (upper)
  • Trachea and esophagus
  • Thoracic duct
  • Vagus nerves (both sides)
  • Left recurrent laryngeal nerve (loops under aortic arch)
  • Right recurrent laryngeal nerve (loops under right subclavian artery)
  • Phrenic nerves
  • Sympathetic trunks

Inferior Mediastinum

Divided into three compartments:

Anterior Mediastinum

• Between sternum and pericardium
• Contents: thymic remnant/fat, lymph nodes, internal thoracic vessels
• Masses: thymoma (most common in adults), teratoma/germ cell tumors, thyroid/parathyroid extension, lymphoma ("4T's": Thymoma, Teratoma, Thyroid, Terrible lymphoma)

Middle Mediastinum

• Contains the pericardium and its contents
• Contents: heart, ascending aorta, pulmonary trunk, superior and inferior venae cavae (lower SVC), pulmonary veins, main bronchi, phrenic nerves, tracheobronchial lymph nodes, azygos vein (arch)
• Masses: pericardial cyst, bronchogenic cyst, lymphoma, enlarged lymph nodes (sarcoid, metastases)

Posterior Mediastinum

• Between pericardium and T5–T12 vertebral bodies
• Contents: descending thoracic aorta, esophagus, thoracic duct, azygos and hemiazygos veins, splanchnic nerves, sympathetic trunks, posterior intercostal vessels
• Masses: neurogenic tumors (most common — schwannoma, neurofibroma), lymphoma, esophageal tumors, descending aneurysm

Key Anatomical Relationships

• Sternal angle (angle of Louis): marks T4 level; junction of manubrium and body of sternum; articulation of 2nd costal cartilage — key landmark for counting ribs and identifying superior/inferior mediastinum
• Tracheal bifurcation (carina): at T4–T5, marked by the sternal angle; carina angle is normally ~70°; widened angle suggests left atrial enlargement
• Esophagus relationships: anterior to vertebral column, posterior to trachea; left main bronchus crosses it anteriorly; left atrium is close anteriorly — critical in esophageal surgery

Mediastinal Widening

Normal mediastinal width <8 cm (anterior-posterior) or <6 cm in the upper mediastinum on CXR:

• Aortic dissection / aneurysm
• Lymphadenopathy
• Hematoma (traumatic)
• Thymoma
• Germ cell tumor

Blood Supply and Venous Drainage

Supplied by branches of the aorta, internal thoracic arteries, and intercostal arteries.
Drained by: azygos system, internal thoracic veins → brachiocephalic veins → SVC.

Lymphatics

The thoracic duct arises from the cisterna chyli (L1–L2), passes through the aortic hiatus, ascends in the posterior mediastinum right of midline, crosses to the left at T4–T5, and drains into the junction of the left subclavian and internal jugular veins.
The right lymphatic duct drains the right upper quadrant into the right venous angle.

Clinical Relevance

• Superior vena cava syndrome: mediastinal mass compressing the SVC → facial edema, arm swelling, distended neck veins; emergency if severe
• Mediastinitis: infection of mediastinal spaces; following esophageal perforation or sternotomy; life-threatening
• Cardiac tamponade: pericardial fluid in the middle mediastinum compressing the heart
• Esophageal perforation: Boerhaave syndrome; mediastinal air on imaging; rapidly fatal without treatment

Study Points

• Draw and label the four compartments of the mediastinum with their key contents.
• List the "4 T's" of anterior mediastinal masses.
• Explain the significance of the sternal angle as an anatomical landmark.
• Describe the course of the left and right recurrent laryngeal nerves and explain their surgical significance.
• Trace the thoracic duct from cisterna chyli to its termination.