Learning Objective: Understand the anatomical borders, contents, and clinical significance of the superior mediastinum, including the great vessels, nerves, thymus, trachea, oesophagus, and associated structures.
The mediastinum is the central compartment of the thoracic cavity, located between the two pleural sacs. It contains most thoracic organs and serves as a conduit for structures traversing the thorax to reach the abdomen. Anatomically, it is divided into two parts by an imaginary line from the sternal angle (junction of manubrium and body of sternum) to the T4 vertebra:
| Mediastinum Part | Boundaries |
|---|---|
| Superior mediastinum | Extends upwards to the superior thoracic aperture |
| Inferior mediastinum | Extends downwards to the diaphragm; subdivided into anterior, middle, and posterior mediastinum |
This article focuses on the superior mediastinum, covering its borders, contents, and clinical correlations.
Activity:
Borders of the Superior Mediastinum
| Border | Structure |
|---|---|
| Superior | Thoracic inlet |
| Inferior | Continuous with the inferior mediastinum at the sternal angle |
| Anterior | Manubrium of the sternum |
| Posterior | Vertebral bodies of T1–T4 |
| Lateral | Pleurae of the lungs |
Contents of the Superior Mediastinum
The superior mediastinum contains neural, vascular, and respiratory structures passing between the neck, thorax, and abdomen.
Great Vessels
Arch of Aorta
The aortic arch gives rise to three major branches:
| Branch | Distribution |
|---|---|
| Brachiocephalic artery | Right head, neck, and upper limb |
| Left common carotid artery | Left side of the head and neck |
| Left subclavian artery | Left upper limb |
Superior Vena Cava (SVC)
Tributaries within the superior mediastinum:
| Vein | Function / Drainage |
|---|---|
| Brachiocephalic veins | Drain the upper body |
| Left superior intercostal vein | Drains left 2nd–3rd intercostal veins into the left brachiocephalic |
| Supreme intercostal vein | Drains the 1st intercostal space directly into the brachiocephalic veins |
| Azygos vein | Receives right posterior intercostal veins; left intercostal veins → hemiazygos/accessory hemiazygos → azygos |
Activity:
Nerves
Vagus Nerve (CN X)
- Right vagus: Runs parallel to the trachea, posterior to the SVC and the right primary bronchus.
- Left vagus: Enters between the left common carotid & left subclavian arteries, descends anterior to the aortic arch, and posterior to the left bronchus.
- Left recurrent laryngeal nerve: Loops under the aortic arch near the ligamentum arteriosum, ascending in the tracheal-oesophageal groove to the larynx.
Phrenic Nerve (C3–C5)
- Enters laterally to the great vessels, descends anteriorly to the hilum of the lungs into the middle mediastinum.
Other Nerves
- Cardiac nerves: Form superficial and deep cardiac plexuses; superficial between the aortic arch and right pulmonary artery, deep on the tracheal bifurcation.
- Sympathetic trunk: Bilateral along vertebral bodies.
Other Structures
| Structure | Location & Notes |
|---|---|
| Thymus | Most anterior; posterior to sternum; extends into anterior mediastinum and neck |
| Trachea | Bifurcates posterior to the ascending aorta at the sternal angle |
| Oesophagus | Ascends to the pharynx at C6 |
| Thoracic duct | Passes left of the oesophagus, empties at the junction of the left internal jugular & subclavian veins |
| Muscles | Sternohyoid & sternothyroid (posterior manubrium); inferior aspect of longus colli |
Clinical Correlations
- Compression or injury to great vessels → compromised circulation.
- Damage to vagus or recurrent laryngeal nerve → hoarseness or vocal cord paralysis.
- Mediastinal masses (thymoma, cysts) → respiratory or vascular obstruction.
