The Mediastinum: The Crowded Corridor Between the Lungs

A corridor, not a room

The mediastinum is defined entirely by what surrounds it. The mediastinum is the median partition of the thorax: everything that lies between the two pleural sacs, from the thoracic inlet above to the diaphragm below. Its walls are borrowed from its neighbours. In front is the sternum with the costal cartilages, described in the thoracic cage. Behind are the bodies of the twelve thoracic vertebrae. Above, the superior thoracic aperture — the thoracic inlet — opens the mediastinum directly into the root of the neck, which is why an infection or a haemorrhage can travel between the two without crossing any barrier. Below, the diaphragm closes it, pierced by three openings that let the aorta, the oesophagus and the inferior vena cava continue into the abdomen. On each side stands the mediastinal pleura, the medial wall of each pleural sac, covered in the pleura and the pleural cavity — a membrane, not a wall, and therefore the reason mediastinal disease so readily produces a pleural effusion.

Two properties of this corridor matter clinically before any of its contents are named. First, it is mobile: apart from the vertebrae behind it, the mediastinum is soft tissue slung between two elastic lungs, so a pressure difference between the two pleural cavities will physically push it sideways. Second, it is continuous — with the neck above and, through the diaphragmatic openings, with the abdomen below — and it is filled with loose areolar tissue. Loose tissue offers no resistance to spreading pus, and no resistance to air. That single fact explains both surgical mediastinitis and the crackling subcutaneous emphysema that can appear at a patient's neck after an oesophageal tear.

The plane of the sternal angle

Run a finger down the front of your chest and you will find the single most useful landmark in the body. About five centimetres below the suprasternal notch the sternum changes angle: the manubrium meets the body at a palpable transverse ridge, the sternal angle of Louis. Take a horizontal plane through it — backwards to the disc between the fourth and fifth thoracic vertebrae, T4/T5 — and that plane is a roll-call of events. It passes through the second costal cartilage, the rib you count from. It cuts the trachea exactly at its bifurcation into the two main bronchi. It marks both the beginning and the end of the arch of the aorta, so that the whole arch lies above it. It is where the arch of the azygos vein turns forwards to join the superior vena cava, and roughly where the thoracic duct crosses the midline from right to left. And, for our purposes, it is the line that divides the SUPERIOR mediastinum above from the INFERIOR mediastinum below.

The inferior mediastinum is then subdivided not by a bony plane but by an organ — the fibrous pericardium. Whatever lies in FRONT of the pericardium, between it and the body of the sternum, is the anterior mediastinum. The pericardium itself, together with everything inside it, is the middle mediastinum. Whatever lies BEHIND the pericardium, between it and the thoracic vertebrae, is the posterior mediastinum. So the classical scheme is four compartments: superior, and then anterior, middle and posterior. Radiologists often use a slightly different working version on cross-sectional imaging — a prevascular, a visceral and a paravertebral compartment — but the logic is identical, and the classical names are still the language of the wards and the exam.

The superior mediastinum: everything on its way through

Between the thoracic inlet and the sternal angle lies the busiest short stretch of anatomy in the trunk, and almost nothing in it belongs there — it is all traffic. From front to back the layers are: the thymus (a large organ in the child, a pad of fat and fibrous tissue in the adult); then the great veins — the right and left brachiocephalic veins uniting behind the right first costal cartilage to form the superior vena cava, with the long left brachiocephalic vein crossing the midline in front of the arch; then the arch of the aorta with its three branches in order — the brachiocephalic trunk, the left common carotid artery and the left subclavian artery; then the trachea descending in the midline to the carina; then the oesophagus flat against the vertebral column, with the thoracic duct behind and to its left. Threaded among them run the vagus nerves, the phrenic nerves, the left recurrent laryngeal nerve, the cardiac plexus and the sympathetic trunks, with paratracheal and brachiocephalic lymph nodes packed into every gap.

One relationship in this compartment produces more clinical signs than any other. The left recurrent laryngeal nerve leaves the left vagus at the arch of the aorta, hooks underneath the arch just lateral to the ligamentum arteriosum, and climbs back up in the groove between the trachea and the oesophagus to reach the larynx. Anything that enlarges in that hook — an aortic arch aneurysm, a left hilar tumour, enlarged subaortic lymph nodes, a grossly dilated left atrium — stretches or invades the nerve and paralyses one vocal cord. The patient does not complain of a chest problem; they complain that their voice has gone hoarse. It is one of medicine's purest examples of an anatomical relationship becoming a symptom.

Anterior, middle, posterior: three compartments, three worlds

The anterior mediastinum is the narrowest space in the chest and holds almost nothing. In front of the pericardium and behind the body of the sternum lies a slot that in a healthy adult is barely a potential space. Its contents are modest: the inferior extension of the thymus or its fatty remnant, loose areolar tissue and fat, a few lymph nodes draining the anterior chest wall and the diaphragm, small branches of the internal thoracic vessels, and the sternopericardial ligaments — delicate bands that tether the fibrous pericardium forwards to the manubrium and to the xiphoid process. That emptiness is precisely why an anterior mediastinal mass declares itself so dramatically on a chest X-ray: there is nothing here normally, so anything at all is abnormal, and it has nowhere to expand except backwards onto the heart and the great veins.

The middle mediastinum is the opposite: it is full, and what fills it is the heart. It contains the pericardium — fibrous and serous layers alike, described in the pericardium — and everything the pericardium encloses: the heart itself, the ascending aorta, the pulmonary trunk and its bifurcation into the right and left pulmonary arteries, the lower half of the superior vena cava with the terminal arch of the azygos vein, the short intrathoracic segment of the inferior vena cava, and the four pulmonary veins. Outside the heart but still within this compartment run the phrenic nerves with the pericardiacophrenic arteries and veins, descending on each side of the fibrous pericardium to reach the diaphragm. And at the back of it sits the airway: the bifurcation of the trachea, the right and left main bronchi, and the tracheobronchial lymph nodes that sit in the angle of that bifurcation — nodes whose enlargement is the commonest cause of a middle mediastinal mass.

The posterior mediastinum, behind the pericardium and in front of the lower eight thoracic vertebrae, is not a chamber but a set of conduits heading for the abdomen. The descending thoracic aorta runs down it, giving off posterior intercostal, bronchial, oesophageal and superior phrenic branches. The oesophagus lies in front of the aorta at first and crosses to its front and left lower down, wrapped in the oesophageal plexus formed by the two vagi — the left vagus rotating onto its anterior surface and the right onto its posterior surface as they follow the stomach's embryological turn. Behind and to the right lie the thoracic duct and the azygos vein, with the hemiazygos and accessory hemiazygos veins crossing the midline from the left. Against the heads of the ribs on each side run the sympathetic trunks, and from their lower ganglia the greater (T5–T9), lesser (T10–T11) and least (T12) splanchnic nerves slope forwards and downwards to pierce the crus of the diaphragm and supply the abdominal viscera. Posterior mediastinal lymph nodes fill the spaces between.

Phrenic in front, vagus behind

Two nerves descend through the mediastinum on each side, and they pass the lung root on opposite sides of it. The phrenic nerve (C3, C4, C5 — "C3, 4, 5 keeps the diaphragm alive") descends on the lateral surface of the fibrous pericardium, accompanied by the pericardiacophrenic vessels, and passes ANTERIOR to the root of the lung. The vagus nerve descends alongside the great vessels and passes POSTERIOR to the root of the lung, breaking up into the pulmonary and then the oesophageal plexus. State it as a discriminator and it becomes unforgettable: phrenic in front, vagus behind. It is not a piece of trivia. It tells you why a hilar tumour that grows backwards causes hoarseness or oesophageal symptoms, while one growing forwards paralyses a hemidiaphragm that then rises on the film and moves paradoxically on sniffing. It tells the cardiac surgeon exactly where not to place a clamp on the pericardium. And it explains why irritation of the parietal pleura or pericardium over the diaphragm refers pain to the C4 dermatome at the tip of the shoulder — the full course of both nerves is traced in the nerves of the thorax.