Mediastinum and thoracic duct
Mediastinum, oesophagus, thoracic duct, vagus and sympathetic trunk
The mediastinum is the central compartment of the chest, divided by the sternal angle into superior and inferior parts. Exam questions ask what lies where, the narrowings of the oesophagus, the course of the thoracic duct and the paths of the vagus and phrenic nerves.
Divisions
- Superior mediastinum: above the plane through the sternal angle and T4/5 disc.
- Inferior mediastinum: anterior (in front of the pericardium), middle (pericardium, heart, roots of great vessels, phrenic nerves) and posterior (behind the pericardium).
| Region | Main contents |
|---|---|
| Superior | Thymus, brachiocephalic veins, SVC (upper), aortic arch and its branches, trachea, oesophagus, thoracic duct, vagus and phrenic nerves, left recurrent laryngeal nerve |
| Anterior | Thymus remnant, fat, lymph nodes |
| Middle | Heart, pericardium, ascending aorta, pulmonary trunk, phrenic nerves, main bronchi |
| Posterior | Descending aorta, oesophagus, thoracic duct, azygos and hemiazygos veins, vagus nerves, sympathetic trunks and splanchnic nerves |
Anterior mediastinal masses ("4 Ts"): thymoma, teratoma (germ cell), thyroid (retrosternal goitre), terrible lymphoma. Posterior masses: neurogenic tumours.
Oesophagus
- About 25 cm, from C6 (cricopharyngeus) to the cardia at T11; passes through the diaphragm at T10.
- Narrowings (distance from incisors): cricopharyngeus 15 cm, aortic arch 22.5 cm, left main bronchus 27.5 cm, diaphragm 40 cm. Foreign bodies, strictures and carcinomas are common at these points.
- Muscle: upper third skeletal, middle mixed, lower third smooth.
- Blood supply: inferior thyroid (upper), aortic branches (middle), left gastric (lower).
- Venous drainage of the lower end into both left gastric (portal) and azygos (systemic): portosystemic anastomosis, so varices form in portal hypertension.
- Lymph: deep cervical (upper), mediastinal (middle), coeliac (lower).
- The left atrium lies directly in front; an enlarged atrium (mitral stenosis) can indent it and cause dysphagia.
Thoracic duct
- Begins at the cisterna chyli (L1–L2), passes through the aortic hiatus at T12 with the aorta.
- Ascends on the right of the oesophagus behind it, crosses to the left at T5 (sternal angle level), and drains into the left venous angle (junction of left internal jugular and subclavian veins).
- Drains all lymph except the right upper quadrant of the body (right lymphatic duct).
- Injury (oesophagectomy, trauma) causes chylothorax (milky pleural fluid, high triglyceride); right-sided below T5, left-sided above.
Vagus nerves
- Right vagus crosses the subclavian artery (gives the right recurrent laryngeal, which hooks round the subclavian), runs on the trachea and behind the right bronchus.
- Left vagus crosses the aortic arch; the left recurrent laryngeal hooks under the arch behind the ligamentum arteriosum and ascends in the tracheo-oesophageal groove. It is at risk from aortic aneurysm, bronchial carcinoma and mitral stenosis (Ortner syndrome).
- Both vagi pass behind the lung roots and form the oesophageal plexus; they leave as anterior (mainly left) and posterior (mainly right) trunks.
- Phrenic nerves pass in front of the lung roots.
Azygos system
- Azygos vein begins near L1/L2 (ascending lumbar and subcostal), ascends on the right of the vertebrae, arches over the right lung root and joins the SVC at T4.
- Hemiazygos and accessory hemiazygos on the left cross to the azygos at about T8–T9.
- Provides a collateral route when the IVC or SVC is obstructed.
Sympathetic trunk and splanchnic nerves
- Lies on the heads of the ribs in the thorax.
- Greater splanchnic (T5–T9), lesser (T10–T11), least (T12): preganglionic fibres to coeliac, aorticorenal and renal ganglia; carry visceral pain from foregut organs.
- Thoracoscopic sympathectomy (T2–T3) treats palmar hyperhidrosis; damage to the stellate ganglion causes Horner syndrome.
How FCPS asks it
Recalls test: sternal angle plane; contents of each mediastinal division; anterior mediastinal masses; oesophageal narrowings and distances; oesophageal muscle and blood supply; thoracic duct course, crossing level and termination; chylothorax; left recurrent laryngeal and the arch; vagus behind and phrenic in front of the lung root; azygos into SVC; splanchnic nerve roots.
Exam trap The thoracic duct crosses from right to left at about T5.
Exam trap Phrenic nerves pass in front of the lung roots; vagi pass behind.
Exam trap The left recurrent laryngeal nerve hooks under the aortic arch.
Test yourself on this topic
5 of the 52 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A 45-year-old man has sudden severe chest pain after forceful vomiting, with surgical emphysema in the neck and a left hydropneumothorax on X-ray. Where is the tear most likely to be?
Answer: Left posterolateral wall of the lower oesophagus
This is spontaneous (Boerhaave) rupture, caused by a sudden rise in intraluminal pressure against a closed cricopharyngeus during vomiting. The tear is usually longitudinal and in the left posterolateral wall of the lower oesophagus, a few centimetres above the gastro-oesophageal junction. Here the wall is thinner, with less supporting tissue, and it lies against the left pleura, so gastric contents leak into the left chest. Instrumental perforations, by contrast, are commonest near cricopharyngeus.
Boerhaave rupture
- Vomiting against a closed cricopharyngeus
- Left posterolateral, lower oesophagus
- Left hydropneumothorax
- Instrumental tears: upper oesophagus
Remember Vomit, pain, surgical emphysema: lower left oesophagus.
Exam trap Early water-soluble contrast swallow or CT is needed; delay sharply raises mortality.
Why the other options are wrong
- Right lateral wall of the mid-oesophagus Mid-oesophageal tears are uncommon in Boerhaave syndrome.
- Anterior wall at the level of the aortic arch Not the typical site of a pressure rupture.
- Posterior pharynx at Killian's dehiscence Typical site of perforation during endoscopy, not vomiting.
- Lesser curve of the stomach below the cardia A Mallory–Weiss tear is mucosal and at the junction, without leak.
Bronchoscopy in a 58-year-old smoker with a lung mass shows that the carina, normally a sharp ridge, is broad and blunted. Enlargement of which lymph nodes most likely explains this?
Answer: Subcarinal lymph nodes
The carina is the ridge of cartilage at the bifurcation of the trachea, at about the level of the sternal angle (T4–T5). The inferior tracheobronchial nodes lie in the angle below it, so when they enlarge with metastatic cancer they push the two main bronchi apart. This widens the subcarinal angle and blunts the carina at bronchoscopy. A massively enlarged left atrium can also widen the carinal angle.
Blunted carina
- Carina at T4–T5
- Subcarinal nodes lie below it
- Enlarged nodes widen the angle
- Also: big left atrium
Remember A blunt carina means something is pushing up from below.
Exam trap Mediastinal lymph spread changes resectability, so the carina matters for staging.
Why the other options are wrong
- Bronchopulmonary (hilar) nodes Lie at the hilum and compress lobar bronchi rather than the carina.
- Paratracheal nodes Lie beside the trachea and may displace it, not widen the carina.
- Pulmonary (intrapulmonary) nodes Lie within the lung, far from the carina.
- Supraclavicular nodes Lie in the neck, outside the thorax.
A 30-year-old woman has a well-defined, round mass in the posterior mediastinum beside the vertebral bodies on CT, with smooth widening of an adjacent intervertebral foramen. Which of the following is the most likely type of mass?
Answer: Neurogenic tumour
Neurogenic tumours are the commonest masses of the posterior mediastinum, arising from intercostal nerves, nerve roots or the sympathetic chain in the paravertebral gutter. In adults most are benign schwannomas or neurofibromas, while in children ganglioneuroma and neuroblastoma predominate. A tumour growing out through an intervertebral foramen widens it smoothly, giving a 'dumbbell' shape with an extension into the spinal canal. Thymoma, teratoma and goitre lie in the anterior or superior compartments.
Mediastinal compartments
- Anterior: thymoma, teratoma, lymphoma
- Middle: cysts, nodes
- Posterior: neurogenic tumours
- Foraminal widening: dumbbell tumour
Remember Back of the mediastinum: think nerves.
Exam trap A dumbbell tumour needs spinal MRI before removal to avoid cord injury.
Why the other options are wrong
- Thymoma An anterior mediastinal mass.
- Mature teratoma An anterior mediastinal germ cell tumour.
- Bronchogenic cyst Usually lies in the middle mediastinum near the carina.
- Retrosternal goitre Lies in the superior and anterior mediastinum.
During an oesophagectomy, dissection behind the lower oesophagus at the level of T8 injures the thoracic duct. On the third postoperative day the chest drain output becomes milky. Into which space is the chyle most likely leaking?
Answer: Right pleural cavity
The thoracic duct enters the thorax through the aortic hiatus and ascends on the right of the midline, between the descending aorta and the azygos vein, behind the oesophagus. At about T5 it crosses to the left and runs up beside the oesophagus to end at the junction of the left subclavian and internal jugular veins. An injury below T5 therefore usually causes a right chylothorax, and an injury above T5 a left one. The leak often appears once enteral feeding restarts.
Thoracic duct course
- Enters at T12 with the aorta
- Right of midline up to T5
- Crosses to the left at T5
- Low injury: right chylothorax
Remember Below T5 right, above T5 left.
Exam trap Chylous output rises when feeding restarts, so a leak may not show in the first day.
Why the other options are wrong
- Left pleural cavity Expected when the duct is injured above T5.
- Both pleural cavities equally The duct lies on one side or the other at any given level.
- Pericardial cavity The duct does not lie within the pericardium.
- Peritoneal cavity The injury is above the diaphragm, in the posterior mediastinum.
A 62-year-old man has an adenocarcinoma at the lower end of the oesophagus. Which of the following lymph node groups is most likely to be involved first?
Answer: Left gastric and coeliac nodes
Lymph from the oesophagus drains along its arteries, and its submucosa contains a longitudinal lymphatic plexus that allows spread over long distances. The upper third drains to deep cervical nodes, the middle third to posterior mediastinal and tracheobronchial nodes, and the lower third downwards to the left gastric and coeliac nodes. This is why oesophagectomy for lower tumours includes an abdominal lymphadenectomy. Coeliac nodes are reached early by lower third tumours.
Oesophageal lymph drainage
- Upper third: deep cervical
- Middle third: mediastinal
- Lower third: left gastric, coeliac
- Long submucosal spread
Remember Lymph follows the arteries.
Exam trap Skip lesions along the submucosal lymphatics can lie far from the main tumour.
Why the other options are wrong
- Deep cervical nodes Drain the upper third of the oesophagus.
- Paratracheal nodes Receive lymph from the middle third.
- Inguinal nodes Drain the lower limb and perineum.
- Para-aortic nodes at the renal level Drain the gonads and kidneys, not the oesophagus first.
Where this sits in the CPSP syllabus
- Paper 1: Thorax, Mediastinum, oesophagus, thoracic duct, vagus and sympathetic trunk
- Surgery & Allied, Paper 2: Thorax, Mediastinum, oesophagus, thoracic duct, vagus and sympathetic trunk