What structures are located in the mediastinum?

Structures of the Mediastinum

The mediastinum contains vital structures including the heart, great vessels, trachea, esophagus, thymus, lymph nodes, nerves, and thoracic duct, all situated between the pleural cavities.

Anatomical Divisions of the Mediastinum

The mediastinum is typically divided into four compartments, each containing specific structures:

1. Superior Mediastinum

• Located above the pericardium, extending from the thoracic inlet to the upper level of the pericardium
• Contains:
  • Brachiocephalic vessels
  • Trachea
  • Esophagus (upper portion)
  • Thymus (superior portion)
  • Lymph nodes
  • Thoracic duct
  • Phrenic and vagus nerves
  • Recurrent laryngeal nerves

2. Anterior Mediastinum

• Located posterior to the sternum and anterior to the heart and great vessels
• Contains:
  • Thymus gland
  • Fat
  • Lymph nodes
  • Internal mammary vessels

3. Middle/Visceral Mediastinum

• Contains:
  • Heart and pericardium
  • Ascending aorta and arch
  • Superior and inferior vena cava
  • Pulmonary vessels
  • Main bronchi
  • Lymph nodes
  • Phrenic nerves
  • Esophagus (mid-portion)

4. Posterior/Paravertebral Mediastinum

• Contains:
  • Descending thoracic aorta
  • Azygos and hemiazygos veins
  • Esophagus (lower portion)
  • Thoracic duct
  • Sympathetic chain
  • Vagus nerves
  • Lymph nodes
  • Adipose tissue

Clinical Significance

Understanding mediastinal anatomy is crucial for:

1. Diagnostic imaging interpretation: CT scanning is the most widely used modality for evaluating mediastinal structures 1. MRI provides superior soft tissue contrast for evaluating invasion of masses across tissue planes 1.

2. Staging of lung cancer: Mediastinal lymph node involvement is critical in determining resectability of lung cancer 1. Normal-sized mediastinal lymph nodes have a short-axis diameter <1 cm on transverse CT scan 1.

3. Evaluation of mediastinal masses: Masses in the anterior mediastinum can be neoplasms (thymomas, lymphomas, thymic carcinomas) or non-neoplastic conditions (intrathoracic goiter, thymic cysts) 1. Most primary cancers in the anterior mediastinum are thymomas 1.

Imaging Considerations

• CT scan with contrast is the primary imaging modality for evaluating mediastinal structures, providing excellent anatomical detail 1.

• MRI offers advantages for:

  • Evaluating invasion of masses into adjacent structures
  • Better tissue characterization
  • Assessment of vascular involvement
  • Evaluation of neurogenic tumors 1

• PET scanning provides functional information and is more accurate than CT for evaluating mediastinal lymph nodes in lung cancer 1.

Important Clinical Pitfalls

1. Lymph node assessment: Relying solely on size criteria for determining malignant involvement of mediastinal lymph nodes can be misleading. Normal-sized nodes may harbor micrometastases, while enlarged nodes may be reactive 1.

2. Mediastinal infiltration: When abnormal tissue in the mediastinum has an irregular, amorphous shape rather than discrete lymph nodes, it suggests extensive extranodal spread of tumor 1.

3. Imaging limitations: Chest radiography is insensitive for mediastinal lymph node involvement and further imaging is usually necessary 1.

4. Diagnostic challenges: It is essential to differentiate between thymic malignancies and other conditions (lung metastases, lymphoma, goiter) before treatment, as management differs significantly 1.

Understanding the complex anatomy of the mediastinum is fundamental for accurate diagnosis and treatment planning of thoracic conditions.