What are the contents of the superior mediastinum?

Contents of the Superior Mediastinum

The superior mediastinum contains vital structures including the thymus gland, great vessels (aortic arch, superior vena cava, brachiocephalic vessels), trachea, esophagus, thoracic duct, vagus and phrenic nerves, and lymph nodes. 1

Anatomical Boundaries

• The superior mediastinum extends from the thoracic inlet superiorly to the upper level of the pericardium inferiorly 2
• Anteriorly bounded by the manubrium of the sternum 3
• Posteriorly bounded by the upper thoracic vertebrae (approximately T1-T4) 3
• Laterally bounded by the mediastinal pleura on each side 2

Vascular Structures

• Great vessels:

  • Aortic arch and its branches (brachiocephalic trunk, left common carotid artery, left subclavian artery) 1
  • Superior vena cava and its tributaries (right and left brachiocephalic veins) 3
  • Azygous vein (typically considered T3 in staging classifications) 4
  • Intrapericardial portions of the pulmonary arteries 5

• Important venous junctions:

  • Internal jugular-subclavian venous junction 3
  • Left-right brachiocephalic venous junction 3

Neural Structures

• Nerves:

  • Vagus nerves (right and left) 3
  • Phrenic nerves (right and left) 3
  • Recurrent laryngeal nerves (particularly the left, which loops under the aortic arch) 4
  • Sympathetic chain and cardiac nerves 3

• Clinical significance:

  • Left recurrent laryngeal nerve palsy is more common due to its circuitous route under the aortic arch, causing hoarseness and potential aspiration 4
  • Phrenic nerve dysfunction can cause hemidiaphragm elevation 4

Lymphatic Structures

• Lymph nodes:

  • Paratracheal lymph nodes 4
  • Aortopulmonary window nodes 4
  • Anterior mediastinal nodes 4
  • Tracheobronchial lymph nodes 4

• Lymph node stations:

  • The IASLC/ITMIG staging system identifies anterior region (N1) nodes including anterior mediastinal nodes (prevascular, para-aortic, ascending aorta, superior and inferior phrenic) 4
  • Middle mediastinal nodes include internal mammary, upper and lower paratracheal, subaortic, and subcarinal nodes 4

Other Important Structures

• Respiratory:

  • Trachea and its bifurcation 5
  • Proximal portions of the main bronchi 5

• Digestive:

  • Upper portion of the esophagus 5
  • Thoracic duct 5

• Endocrine/Immune:

  • Thymus gland (particularly in children and young adults) 1, 6

Clinical Significance

• Superior vena cava syndrome is commonly caused by lung cancer invading the mediastinum, presenting with facial/neck swelling, dilated neck veins, and prominent chest venous pattern 4
• Mediastinal invasion assessment is critical for lung cancer staging, with invasion of the mediastinal pleura classified as T3 and invasion of great vessels as T4 4
• Advanced imaging techniques like thin-section CT, MRI, and PET-CT are essential for evaluating mediastinal structures and potential invasion 4
• Complete mediastinal lymph node dissection or systematic sampling is recommended during lung cancer resection for accurate staging 4

Imaging Considerations

• CT with contrast is the primary modality for evaluating the superior mediastinum 1, 7
• MRI provides superior soft tissue contrast for assessing invasion of mediastinal structures 4
• Dynamic CT or MRI during free-breathing can assess movement of masses relative to adjacent structures 4
• PET-CT helps in metabolic assessment of mediastinal masses and lymph nodes 1

Surgical Approaches

• Access to the superior mediastinum can be achieved through various approaches including median sternotomy, anterolateral thoracotomy, and minimally invasive techniques 4
• Mediastinoscopy provides access to lymph nodes in the visceral compartment but cannot access the paravertebral compartment 1
• Complete resection with appropriate lymphadenectomy is crucial for optimal outcomes in mediastinal tumors 4