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AN23.1-6 | Mediastinum — Part 1
CLINICAL SCENARIO
A 55-year-old farmer from Villupuram presents to the surgical outpatient department with progressively worsening difficulty swallowing solid food for three months, now also liquids. He has lost 8 kg. Chest X-ray shows a widened mediastinum. CT reveals a mass in the posterior mediastinum compressing the oesophagus.
Where exactly is this mass? What are the normal structures here? Why does mediastinal widening matter?
By the end of this module you will be able to answer these questions — and confidently navigate the mediastinal map in your clinical years.
WHY THIS MATTERS
The mediastinum is the central highway of the thorax — every vital structure connecting the head to the abdomen passes through it. Indian clinicians encounter mediastinal pathology frequently:
- Tuberculosis — mediastinal lymphadenopathy is the most common cause of widened mediastinum in India
- Oesophageal carcinoma — a leading GI cancer, often presenting late with dysphagia
- Superior vena cava syndrome — dramatic presentation from any anterior mediastinal mass (lymphoma, thymoma)
- Aortic aneurysm — increasingly seen with hypertension and atherosclerosis
- Sympathetic chain injury during thoracic surgery causes Horner's syndrome
The mediastinum is also central to Physiology — the heart, great vessels, and autonomic pathways you study in PY are all mediastinal structures.
RECALL
Before we begin, recall your thoracic cage and pleural anatomy:
- The thorax is bounded above by the thoracic inlet (T1, first rib, manubrium) and below by the diaphragm
- The pleural cavities occupy the lateral compartments; the space between them is the mediastinum
- The sternal angle (angle of Louis) — junction of manubrium and body of sternum — is at the level of T4/T5 and marks the plane separating superior from inferior mediastinum
Divisions of the Mediastinum (AN23.1)
The mediastinum is divided by the sternal angle plane (T4/T5) into:
Superior mediastinum — above the sternal angle plane
Inferior mediastinum — below, further divided into:
- Anterior mediastinum — between sternum and pericardium (contains: thymus/fat, internal thoracic vessels, lymph nodes)
- Middle mediastinum — contains the pericardium and heart
- Posterior mediastinum — between pericardium and vertebral column
Why the sternal angle matters: It is the highest reliable bony landmark palpable in the thorax. Clinically: the second costal cartilage articulates here → used to count ribs (identify intercostal spaces). On CXR: upper limit of the aortic knuckle is just above this.
Anatomical boundaries of the mediastinum:
| Boundary | Structure |
|---|---|
| Superior | Thoracic inlet (T1, first rib, manubrium) |
| Inferior | Diaphragm |
| Anterior | Sternum and costal cartilages |
| Posterior | Thoracic vertebrae T1–T12 |
| Lateral | Mediastinal pleura (each side) |
Superior Mediastinum — Contents (AN23.2)
The superior mediastinum (above T4/T5) is a crowded space. Mnemonic — from anterior to posterior: "The Big Artery Trunks Very Slowly":Thymus, Brachiocephalic veins (L+R), Arch of aorta, Trachea, Vagus nerves, Sympathetic chain + oesophagus (posterior-most).
Key structures, anterior → posterior:
- Thymus — in children: large lymphoid organ; in adults: replaced by fat. Site of T-lymphocyte maturation. Thymoma can cause myasthenia gravis.
- Brachiocephalic veins — right brachiocephalic vein is short (2.5 cm) and nearly vertical; left is long (6 cm) and horizontal. Both join to form the superior vena cava (SVC) behind the right first costal cartilage.
- Arch of aorta — curves from front (at sternal angle) to back (T4). Three branches: brachiocephalic trunk (right) → left common carotid artery → left subclavian artery.
- Trachea — descends slightly to the right of midline; bifurcates at the carina (T4/T5) into left and right main bronchi. The right main bronchus is wider, shorter, more vertical → foreign bodies and aspirated liquids preferentially enter the right lung.
- Oesophagus — posterior; begins at C6, enters thorax at T1, passes through the diaphragm at T10.
- Vagus nerves (X) — pass on either side of trachea. In the superior mediastinum the left recurrent laryngeal nerve loops under the aortic arch.
- Phrenic nerves — run on the lateral aspect of the pericardium ("phrenic = lateral to vagus"). Mnemonic: "P before V" — Phrenic is anterior (lateral) to Vagus.
- Thoracic duct — enters thorax through aortic hiatus (T12), ascends on the right in the posterior mediastinum, crosses to the left at T5, ascends in superior mediastinum to drain into the left brachiocephalic vein.
SELF-CHECK
A. Right subclavian artery
B. Aortic arch
C. Brachiocephalic trunk
D. Pulmonary trunk
Reveal Answer
Answer: .
Posterior Mediastinum (AN23.3)
The posterior mediastinum lies between the pericardium anteriorly and T5–T12 posteriorly.
Contents (anterior to posterior):
- Oesophagus — central structure; deviates left at T4–T7 then returns to midline at T10. Surrounded by the oesophageal plexus of vagal fibres.
- Descending thoracic aorta — begins at T4 (left of vertebral column), descends midline, passes through the aortic hiatus at T12. Gives off posterior intercostal arteries (9 pairs), bronchial arteries, and oesophageal branches.
3. Azygos venous system:
- Azygos vein (right side) — drains posterior thoracic wall; arches over right main bronchus at T4 → enters SVC. Acts as collateral pathway if IVC obstructed.
- Hemiazygos vein (left lower) — joins azygos at T8–T9
- Accessory hemiazygos vein (left upper) — joins azygos at T7
- Thoracic duct — most posterior; ascends between aorta and azygos vein (see AN23.4)
- Sympathetic chains — run lateral to vertebral bodies, lateral to the azygos/hemiazygos system
Clinical: Rupture of the thoracic aorta (trauma) or oesophagus (Boerhaave syndrome) both present as posterior mediastinal emergencies. Widened mediastinum on CXR after trauma suggests aortic injury.