Anatomy of the Mediastinum
Three classification schemes, compartment contents, and surgical approaches to the mediastinum.
The mediastinum is the median partition of the thoracic cavity, covered by the mediastinal part of the parietal pleura and containing all thoracic viscera and structures except the lungs.
Boundaries
Classification schemes
Three classification systems are in common use. Knowing all three matters — examiners use them interchangeably and radiologists use Felson's, surgeons use Gray's, and oncologists now use ITMIG.
Gray's four-compartment model
The classical anatomical model divides the mediastinum into a superior compartment and three inferior compartments.
- Superior mediastinum: thoracic inlet to the thoracic plane (line from sternal angle of Louis to T4/5 disc)
- Inferior mediastinum: thoracic plane to diaphragm — divided into anterior, middle, and posterior
| Compartment | Key contents |
|---|---|
| Superior | Aortic arch and branches · SVC and innominate vein · vagus, phrenic, left RLN · thoracic duct · thymus · trachea · oesophagus · lymph nodes |
| Anterior inferior | Thymus (remnant) · lymph nodes · fat |
| Middle inferior | Heart and pericardium · great vessels · phrenic nerve and pericardiophrenic vessels |
| Posterior inferior | Descending aorta · azygos vein · thoracic duct · oesophagus · sympathetic ganglia · vagus nerve · lymph nodes |
Felson's three-compartment model
Used by radiologists. The superior and anterior compartments of Gray's model are combined into a single anterosuperior compartment. Three zones are identified on the lateral chest radiograph:
- Anterior (anterosuperior): anterior to a line along the posterior border of the heart and trachea — thymus, lymph nodes, fat, intrathoracic goitre, germ cell tumours
- Middle: between the anterior border and a line 1 cm posterior to the anterior margin of the vertebral bodies — heart, pericardium, trachea, oesophagus, thoracic duct
- Posterior: posterior to that line — neurogenic tumours, descending aorta, sympathetic chain, vertebral column
The location of a mediastinal mass on lateral CXR predicts the most likely diagnosis: anterior masses = thymic/GCT/lymphoma/goitre; middle masses = foregut cysts/lymphoma/vascular; posterior masses = neurogenic tumours. Not absolute, but a powerful first filter.
ITMIG classification
The International Thymic Malignancy Interest Group (ITMIG) classification uses cross-sectional imaging (CT) rather than plain radiograph geometry. It is now accepted as the standard for mediastinal tumour staging and research.
| Compartment | Boundaries | Key contents |
|---|---|---|
| Prevascular (anterior) | Anterior — sternum Posterior — anterior aspect of pericardium | Thymus · fat · lymph nodes · left brachiocephalic vein |
| Visceral (middle) | Anterior — posterior pericardium Posterior — 1 cm anterior to vertebral body margin (visceral–paravertebral line) | Heart and great vessels · trachea and carina · oesophagus · thoracic duct · lymph nodes |
| Paravertebral (posterior) | Anterior — visceral–paravertebral line Posterolateral — posterior chest wall | Thoracic spine · paravertebral soft tissues · sympathetic chain |
All three compartments are bounded superiorly by the thoracic inlet and inferiorly by the diaphragm.
Surgical access to the mediastinum
Standard approach for anterior and superior mediastinal pathology — thymectomy, anterior mediastinal tumours, cardiac surgery.
For posterior mediastinal tumours — neurogenic, oesophageal, descending aorta.
Staging of superior and middle mediastinal nodes (stations 2, 4, 7). Standard approach for lymph node biopsy and mediastinal staging.
Left 2nd interspace parasternal approach. Accesses aortopulmonary window nodes (stations 5, 6) — left-sided tumours.
Posterior mediastinal tumours, sympathectomy, pleural biopsy, lung biopsy. Increasingly used for thymectomy.
Thymectomy, posterior mediastinal tumour resection — growing evidence base; excellent visualisation in confined space.
All clinical content should be verified against current guidelines before clinical application. This resource is intended for revision and educational purposes only.
Standard textbooks
- Shields TW, LoCicero J, Reed CE, Feins RH. General Thoracic Surgery. 7th ed. Lippincott Williams & Wilkins.
- Sellke FW, del Nido PJ, Swanson SJ. Sabiston & Spencer Surgery of the Chest. 9th ed. Elsevier.
- Pearson FG, et al. Thoracic Surgery. 3rd ed. Churchill Livingstone.