Mediastinum 2 — MRCS Part B Anatomy

Two mediastinal prosections, with the relations of the great vessels and nerves at the thoracic inlet. The examiners' interest is where the compartments are and what goes wrong in each.

Compartments

The mediastinum is everything between the two pleural sacs, from the thoracic inlet to the diaphragm. The plane from the sternal angle to the lower border of T4 divides the superior from the inferior mediastinum; the pericardium divides the inferior into anterior, middle and posterior parts.

• Superior: thymus, the brachiocephalic veins forming the superior vena cava, the aortic arch with its three branches, the trachea and oesophagus, the phrenic and vagus nerves, the left recurrent laryngeal nerve hooking under the arch, and the thoracic duct.
• Anterior: thymic remnant, fat and lymph nodes between the sternum and pericardium.
• Middle: the pericardium and heart, the roots of the great vessels, the tracheal bifurcation and main bronchi, the phrenic nerves.
• Posterior: descending aorta, oesophagus with the vagal plexus, azygos and hemiazygos veins, thoracic duct, sympathetic trunks and splanchnic nerves.

Great vessels

The brachiocephalic veins form behind the sternoclavicular joints; the left is longer and crosses behind the manubrium in front of the arch branches to join the right at the first right costal cartilage, forming the superior vena cava, which enters the right atrium at the third cartilage after receiving the azygos vein. The aortic arch begins and ends at the sternal angle, giving the brachiocephalic trunk, left common carotid and left subclavian arteries from its convexity.

Nerves

The phrenic nerve runs down in front of the lung root, between the mediastinal pleura and the pericardium. The vagus runs behind the lung root: the right vagus on the trachea then the oesophagus, forming the posterior vagal trunk; the left vagus crosses the arch, gives the left recurrent laryngeal nerve, which loops under the arch behind the ligamentum arteriosum, and forms the anterior vagal trunk on the oesophagus. A tumour at the left hilum or an aneurysm of the arch therefore presents with a hoarse voice.

Pathology by compartment

Anterior: thymoma, teratoma, lymphoma and a retrosternal thyroid (the “four Ts”). Middle: enlarged nodes from lymphoma, sarcoidosis or carcinoma, bronchogenic cysts and aortic aneurysm. Posterior: neurogenic tumours, oesophageal lesions and paravertebral abscess. A widened mediastinum on a supine trauma film raises the possibility of aortic injury at the isthmus, just distal to the left subclavian artery, where the mobile arch meets the fixed descending aorta.

Mediastinitis and pneumomediastinum

Mediastinitis is infection of the mediastinal tissues, after sternotomy or from oesophageal perforation, and needs urgent drainage; untreated it is often fatal. Pneumomediastinum is air in the mediastinum, from alveolar rupture with air tracking along the bronchovascular sheaths or from a torn oesophagus or airway. Boerhaave's syndrome is spontaneous oesophageal rupture after vomiting; the clue is surgical emphysema at the neck with severe chest pain, and the air on the film comes from the oesophagus.