Boundaries and divisions
The transverse thoracic plane, from the sternal angle to the disc between T4 and T5, separates the superior mediastinum from the inferior mediastinum. The inferior mediastinum is subdivided by the pericardium.
| Division | Position | Main contents |
|---|---|---|
| Superior | Above the transverse thoracic plane | Thymus, brachiocephalic veins, upper superior vena cava, aortic arch and its branches, trachea, oesophagus, thoracic duct, vagus, phrenic and left recurrent laryngeal nerves |
| Anterior | Between sternum and pericardium | Remnants of thymus, fat, lymph nodes, sternopericardial ligaments |
| Middle | The pericardium and its contents | Heart, ascending aorta, pulmonary trunk, lower superior vena cava, phrenic nerves, main bronchi |
| Posterior | Behind the pericardium, in front of T5–T12 | Oesophagus, descending aorta, azygos and hemiazygos veins, thoracic duct, vagi, splanchnic nerves |
The mediastinum is continuous above with the fascial spaces of the neck, which is how infection from the neck can spread into it.
Contents of the superior mediastinum
The superior mediastinum is best learnt in layers from front to back: thymus, veins, arteries, airway, oesophagus, then vertebrae.
Veins
Each brachiocephalic vein forms behind the sternoclavicular joint from the internal jugular and subclavian veins. The left brachiocephalic vein is longer and crosses obliquely to the right, in front of the arch branches. The two veins join behind the first right costal cartilage to form the superior vena cava, which receives the azygos vein at about T4 and enters the pericardium. The inferior thyroid veins descend on the front of the trachea to end in the brachiocephalic veins.
Arteries
The aortic arch begins and ends at the level of the sternal angle. It runs backwards and to the left over the left main bronchus and left pulmonary artery, in front of and then to the left of the trachea, and continues as the descending aorta. Its three branches from right to left are the brachiocephalic trunk, left common carotid artery and left subclavian artery. The brachiocephalic trunk divides behind the right sternoclavicular joint into the right common carotid and right subclavian arteries.
Airway and oesophagus
The trachea lies behind the arch branches and divides at the sternal angle plane. The oesophagus lies behind it, with the thoracic duct on its left.
Relations of the nerves
On each side the phrenic nerve runs in front of the lung root and the vagus nerve behind it.
- Right phrenic nerve: descends on the right brachiocephalic vein, the superior vena cava and the pericardium over the right atrium.
- Left phrenic nerve: crosses the left side of the aortic arch, lateral to the left vagus, and descends on the pericardium over the left ventricle.
- Right vagus: runs on the side of the trachea, behind the right brachiocephalic vein, and passes behind the root to the oesophagus. Its recurrent laryngeal branch hooks under the right subclavian artery in the neck.
- Left vagus: descends between the left common carotid and left subclavian arteries, crosses the arch and gives the left recurrent laryngeal nerve, which hooks under the arch beside the ligamentum arteriosum.
- Sympathetic trunks: lie on the heads of the ribs at the back, in the paravertebral gutters.
Clinical relevance
Mediastinal disease presents by compressing the veins, airway, oesophagus or nerves that share this narrow space.
- Mediastinal widening on a chest radiograph after deceleration trauma raises suspicion of aortic injury, typically near the ligamentum arteriosum.
- Anterior mediastinal masses are classically the four Ts: thymoma, teratoma, retrosternal thyroid and lymphoma.
- Superior vena cava obstruction, usually by tumour, causes facial swelling and distended neck and chest wall veins.
- Hoarse voice: the left recurrent laryngeal nerve can be compressed by an arch aneurysm or left hilar nodes.
- Mediastinitis can follow oesophageal perforation or spread from deep neck infection.
On the specimen
This station shows the superior mediastinum with the sternum and thymus removed, exposing the veins in front and the arteries behind.
- Veins first: the left brachiocephalic vein crosses obliquely from left to right; follow it to the superior vena cava on the right. The internal jugular veins enter from above, and the left subclavian vein joins from the side.
- Inferior thyroid vein: a thin vein coming down in the midline in front of the trachea.
- Arch and branches: from right to left, the brachiocephalic trunk, left common carotid and left subclavian arteries. The ascending aorta is below the arch in front; the descending aorta continues behind on the left.
- Left pulmonary artery: runs under the arch towards the left lung root.
- Phrenic or vagus nerve: the phrenic lies more anterior and lateral, on the veins and pericardium; the vagus lies deeper, beside the arteries and trachea. On the left, the nerve that gives a branch hooking under the arch is the vagus.
- Sympathetic trunk: the most posterior nerve, on the necks of the ribs.