Oesophagus and azygos system
Oesophagus, thoracic duct, thymus, azygos system and mediastinal nerves
These are the soft, often invisible structures of the mediastinum, shown on barium swallow, CT and the chest film mainly by what happens when they are narrowed, enlarged, torn or compressed. CPSP asks their levels and relations, the oesophageal constrictions and barium patterns, the course of the thoracic duct and azygos veins, the thymus at different ages, and the phrenic, vagus and recurrent laryngeal nerves.
Oesophagus: course and relations
- About 25 cm long, from C6 (cricopharyngeus) to the gastro-oesophageal junction at T11, left of the midline, passing through the diaphragm at T10.
| Constriction | Distance from incisors | Level |
|---|---|---|
| Cricopharyngeus (commonest site for a swallowed coin in a child) | About 15 cm | C6 |
| Aortic arch (pulsating wall at endoscopy) | About 22 cm | T4 |
| Left main bronchus (left wall indentation) | About 27 cm | T5–T6 |
| Diaphragm | About 40 cm | T10 |
- Right side: azygos vein and right pleura; the aortic arch lies to the left of the upper oesophagus, so the thoracic oesophagus is mobilised through a right thoracotomy. In front: trachea, left main bronchus, then the left atrium. Behind and to the left lower down: descending aorta. Behind: thoracic duct.
- Wall: non-keratinised stratified squamous epithelium; upper third skeletal muscle, middle mixed, lower third smooth. There is no serosa, so cancer invades the trachea and aorta early and anastomoses leak.
| Part | Arterial supply | Venous drainage |
|---|---|---|
| Cervical | Inferior thyroid | Inferior thyroid veins |
| Thoracic | Oesophageal branches of the aorta (and bronchial) | Azygos system |
| Abdominal | Left gastric (and inferior phrenic) | Left gastric to portal vein: portosystemic anastomosis, varices |
- Downhill varices in the upper third with facial swelling suggest SVC obstruction.
- Squamous carcinoma (tobacco) is commonest in the middle third; adenocarcinoma in the lower third.
Barium swallow patterns
| Appearance | Diagnosis |
|---|---|
| Dilated oesophagus, smooth short beak at the junction, no gastric air bubble, wide right mediastinum with an air–fluid level on the chest film | Achalasia |
| Long, asymmetric narrowing extending below the diaphragm in an older man with weight loss | Carcinoma of the cardia (pseudoachalasia) |
| Simultaneous non-propulsive contractions (corkscrew) | Diffuse oesophageal spasm |
| Thin smooth shelf at the squamocolumnar junction | Schatzki ring |
| Multiple fixed thin rings, food bolus in an atopic young man | Eosinophilic oesophagitis |
| Smooth intramural defect, obtuse angles, intact mucosa | Leiomyoma |
| Serpiginous filling defects | Varices |
| Pouch above cricopharyngeus | Pharyngeal pouch through Killian dehiscence |
- Oesophageal atresia: commonest type has a distal tracheo-oesophageal fistula: nasogastric tube coils in the upper pouch and the stomach contains gas. An H-type fistula without atresia is shown by a prone oesophagogram as the tube is pulled back. A duplication cyst has two smooth muscle layers and a gut lining.
Thoracic duct
- Begins at the cisterna chyli (about L1–L2), enters through the aortic hiatus at T12, ascends behind the oesophagus between the descending aorta and the azygos vein, crosses to the left at about T5, and ends at the left jugulosubclavian junction.
- Injury below the crossing gives a right chylothorax; above it, a left one. Chyle appears milky once feeding restarts (after oesophagectomy or thoracoabdominal aortic surgery).
Azygos system
- The azygos vein rises on the right of the vertebral bodies, arches over the right lung root at T4 and enters the SVC at the level of the sternal angle. On an erect PA film it is seen at the right tracheobronchial angle; enlargement (compared with old films) suggests raised central venous pressure (fluid overload), or azygos continuation of an interrupted IVC.
- The hemiazygos crosses to the azygos at about T8, behind the aorta. The left superior intercostal vein curving round the arch makes a small "aortic nipple" on the knuckle.
- SVC obstruction at or below the azygos entry forces blood down through chest and abdominal wall veins to the IVC, so superficial veins fill from above. Above the azygos, the azygos vein can still carry blood to the heart.
Thymus
- From third pouch endoderm (with the inferior parathyroids), with Hassall corpuscles in the medulla. Supply: internal thoracic and inferior thyroid arteries.
- In infants it is large (sail shape); after puberty it is replaced by fat but keeps a triangular shape.
| Finding | Meaning |
|---|---|
| Smooth enlargement keeping its shape after chemotherapy | Rebound hyperplasia |
| Normal-looking gland in myasthenia gravis | Lymphoid follicular hyperplasia with germinal centres |
| Anterior mass in an adult; pleural nodules after resection | Thymoma; spreads by pleural seeding |
| Huge soft fatty mass draping round the heart | Thymolipoma |
| Enhancing mass with ectopic ACTH | Thymic carcinoid |
Mediastinal nerves
| Nerve | Course | Clinical and imaging link |
|---|---|---|
| Phrenic (C3–C5) | On scalenus anterior, in front of the lung root, on the pericardium with the pericardiacophrenic vessels | Raised hemidiaphragm after interscalene block or surgery; an accessory phrenic (C5) may keep partial function |
| Vagus | Behind the lung root; forms the oesophageal plexus; left vagus becomes the anterior trunk | Truncal vagotomy |
| Left recurrent laryngeal | Hooks under the arch at the ligamentum arteriosum | Hoarseness from arch aneurysm, PDA ligation, aortopulmonary window nodes, giant left atrium |
| Right recurrent laryngeal | Hooks round the right subclavian artery | Apical lung tumour; non-recurrent with an aberrant right subclavian |
| Sympathetic trunk | On the rib heads | Horner syndrome with an apical tumour |
Remember Oesophageal narrowings at 15, 22, 27 and 40 cm: cricopharyngeus, arch, left bronchus, diaphragm.
Exam trap The thoracic duct crosses from right to left at about T5, so a low duct injury gives a right-sided chylothorax.
Exam trap The phrenic nerve passes in front of the lung root and the vagus behind it.
Test yourself on this topic
5 of the 42 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A 38-year-old man has mild dysphagia. Endoscopy shows a smooth bulge in the mid oesophagus covered by intact mucosa. The mass is enucleated from the muscularis propria and shows interlacing bundles of bland spindle cells that are desmin positive and CD117 negative. What is the lesion?
Answer: Leiomyoma
Leiomyoma is the commonest benign tumour of the oesophagus. It arises from smooth muscle in the muscularis propria, so it grows within the wall beneath intact mucosa and can be shelled out. Histology shows interlacing fascicles of bland smooth muscle cells, positive for desmin and smooth muscle actin and negative for CD117 (KIT) and DOG1, which separates it from a gastrointestinal stromal tumour. Carcinomas arise from the mucosa and ulcerate it.
Oesophageal leiomyoma
- Commonest benign oesophageal tumour
- Arises from muscularis propria
- Mucosa intact, can be enucleated
- Desmin positive, CD117 negative
Remember Smooth intramural lump with intact mucosa: leiomyoma.
Exam trap In the stomach, an intramural spindle cell tumour is more often a GIST; check CD117.
Why the other options are wrong
- Squamous cell carcinoma Arises from mucosa with keratinising malignant cells.
- Fibrovascular polyp A long intraluminal mass on a stalk of fibrous tissue and fat.
- Adenocarcinoma Malignant glands from Barrett mucosa in the lower oesophagus.
- Squamous papilloma A small mucosal wart-like squamous lesion.
A 60-year-old man has occasional episodes of a large bite of steak sticking low in his chest. Endoscopy shows a thin, smooth, symmetrical mucosal ring at the squamocolumnar junction, at the top of a small sliding hiatus hernia. What is this lesion?
Answer: Schatzki ring
A Schatzki (lower oesophageal mucosal, or B) ring is a thin ring of mucosa and submucosa at the squamocolumnar junction, usually at the top of a small sliding hiatus hernia. It is seen only when the oesophagus is well distended. If narrower than about 13 mm it causes episodic dysphagia to solid boluses ('steakhouse syndrome'). A web is a thin membrane in the upper oesophagus, a peptic stricture is longer and tapered, and carcinoma is irregular with shouldered edges.
Schatzki ring
- Thin ring at squamocolumnar junction
- On top of a sliding hiatus hernia
- Episodic solid bolus dysphagia
- Seen only with full distension
Remember Steak sticks, thin low ring: Schatzki.
Exam trap An under-distended oesophagus can hide the ring on barium, so a normal swallow may be falsely reassuring.
Why the other options are wrong
- Oesophageal web Usually in the upper oesophagus, often with iron deficiency.
- Peptic stricture Longer tapered narrowing, often with oesophagitis.
- Achalasia cardia Smooth bird's beak with a dilated body.
- Oesophageal carcinoma Irregular, shouldered narrowing with progressive dysphagia.
Six months after chemotherapy for testicular cancer, a 20-year-old man's thymus has enlarged diffusely while keeping its normal shape. Biopsy shows normal thymic tissue with cortex, medulla and Hassall corpuscles. What is the explanation?
Answer: Rebound thymic hyperplasia
Stress, illness and chemotherapy cause the thymus to shrink through glucocorticoid-mediated lymphocyte loss. When the stress ends, the gland may regrow, sometimes to more than its original size, a process called rebound thymic hyperplasia. It usually appears a few months after chemotherapy, is smooth and keeps the normal shape and histology of the thymus. Recurrent tumour tends to be nodular and shows malignant cells.
Rebound thymic hyperplasia
- Thymus shrinks with stress
- Regrows after chemotherapy
- Smooth, keeps normal shape
- Months after treatment
Remember Thymus back after chemo: often rebound, not relapse.
Exam trap Mild FDG uptake in rebound thymus can mimic recurrent lymphoma on PET.
Why the other options are wrong
- Thymic metastasis from testis Would show malignant germ cells, not normal thymus.
- Newly arising thymoma Neoplastic epithelial cells, not normal architecture.
- Recurrent mediastinal lymphoma Malignant lymphoid cells replacing the architecture.
- Thymic carcinoid tumour Neuroendocrine tumour cells, sometimes with Cushing syndrome.
In the lower posterior mediastinum, the thoracic duct ascends on the front of the vertebral bodies behind the oesophagus. Between which two structures does it lie at this level?
Answer: Descending aorta and azygos vein
The thoracic duct enters the thorax through the aortic hiatus and ascends in the posterior mediastinum on the front of the vertebral bodies, behind the oesophagus, between the descending aorta on its left and the azygos vein on its right. At about T5 it crosses to the left, behind the oesophagus, and ascends along the left side of the oesophagus into the neck to end at the junction of the left internal jugular and subclavian veins. Knowing this course guides cannulation and embolisation for chylous leaks.
Thoracic duct course
- Enters via aortic hiatus
- Lower chest: between aorta and azygos
- Crosses to the left at about T5
- Ends at the left venous angle
Remember Low in the chest the duct runs between the aorta and azygos.
Exam trap Injury to the duct below T5 gives a right chylothorax; injury above T5 gives a left one.
Why the other options are wrong
- Oesophagus and trachea These lie in front of the duct, not on either side of it.
- Azygos vein and SVC The SVC lies far anterior in the middle mediastinum.
- Descending aorta and left atrium The left atrium lies in front of the oesophagus.
- Hemiazygos vein and left subclavian artery These lie on the left, higher up and further apart.
A 5-month-old has coughed and choked with feeds since birth and has had three pneumonias. A nasogastric tube passes easily to the stomach. Bronchoscopy shows a small fistula running obliquely upward from the oesophagus into the back of the trachea at the level of T1. What is the anomaly?
Answer: H-type tracheo-oesophageal fistula
An H-type fistula connects a patent oesophagus to the trachea without atresia, so the tube passes and the diagnosis is often delayed. Swallowed milk enters the airway, causing choking with feeds and recurrent aspiration pneumonia, and air enters the gut. The fistula runs obliquely upward from oesophagus to trachea, usually at the lower neck or upper thorax (C7–T2), and is best shown by a prone pull-back oesophagogram or bronchoscopy. Atresia types present at birth because the tube coils in the upper pouch.
H-type fistula
- No atresia; tube passes
- Choking with feeds, pneumonias
- Runs obliquely upward to trachea
- At about C7–T2
Remember Choking feeds with a patent oesophagus: H-type fistula.
Exam trap A routine contrast swallow often misses the fistula; a prone pull-back study or bronchoscopy is needed.
Why the other options are wrong
- Oesophageal atresia with distal fistula The tube would coil in the blind upper pouch.
- Pure oesophageal atresia The tube would coil and the abdomen would be gasless.
- Laryngotracheal cleft Lies at the larynx, higher, and causes aspiration via a cleft.
- Congenital oesophageal stenosis Narrowing without communication with the airway.
Where this sits in the CPSP syllabus
- Diagnostic Radiology, Paper 2: Thorax, Oesophagus, thoracic duct, thymus, azygos system and mediastinal nerves