Areas - Superior Mediastinum

Question 1

What is the mediastinum?

Answer 1

This is the central compartment of the thoracic cavity, located between the two pleural sacs. It contains most of the thoracic organs, and acts as a conduit for structures traversing the thorax on their way into the abdomen.

Anatomically, the mediastinum is divided into two parts by an imaginary line that runs from the sternal angle (angle formed by the junction of the sternal body and manubrium) to the T4 vertebrae:

1) Superior mediastinum - extends upwards, terminating at the superior thoracic aperture.
2) Inferior mediastinum - extends downwards, terminating at the diaphragm. It is further subdivided into the anterior mediastinum, middle mediastinum and posterior mediastinum.

Question 2

Borders?

Answer 2

Superior - thoracic inlet

Inferior - continuous with the inferior mediastinum at the level of the sternal angle.

Anterior - manubrium of the sternum.

Posterior - vertebral bodies of T1-4.

Lateral - pleurae of the lungs.

Question 3

Contents?

Answer 3

Great vessels

1) Arch of the aorta - branches: brachiocephalic, left common carotid and left subclavian.
2) SVC - branches: brachiocephalic, left superior intercostal, supreme intercostal and azygos veins.

Nerves

1) Vagus - branches: right and left vagus nerves.
2) Phrenic
3) Cardiac
4) Sympathetic trunk

Other structures

1) Thymus
2) Trachea
3) Oesophagus
4) Thoracic duct
5) Muscles - infrahyoids - sternohyoid and sternothyroid.

Question 4

Great vessels - branches?

Answer 4

Aorta

• Brachiocephalic artery - supplies right side of head and neck and the right upper limb.
• Left common carotid artery - supplies left side of the head and the neck.
• Left subclavian artery - supplies the left upper limb.

SVC

• Brachiocephalic vein - drains blood from the upper body
• Left superior intercostal vein - collects blood from the left 2nd and 3rd intercostal vein. It drains into the left brachiocephalic vein.
• Supreme intercostal vein - drains the vein from the first intercostal space directly into the brachiocephalic veins.
• Azygos vein - receiving blood from the right posterior intercostal veins. The left intercostal veins drain first into the hemiazygos and accessory hemiazygos veins before joining the azygos vein around T7-T9.

Question 5

Phrenic and Vagus nerves?

Answer 5

Vagus nerve

• Right vagus nerve - runs parallel to the trachea and passes posteriorly to the SVC and the right primary bronchus.
• Left vagus nerve - enters the superior mediastinum between the left common carotid and the left subclavian arteries. It descends anteriorly to the aortic arch, before travelling posterior to the left bronchus.

The left recurrent laryngeal nerve arises from the left vagus nerve as it passes the aortic arch. It loops under the arch, to the left of ligamentum arteriosum, before continuing its journey to the larynx in the tracheal-oesophageal groove.

Phrenic nerve

• from the anterior surface of the anterior scalene muscle, the phrenic nerves (roots C3, C4 and C5) enters the superior mediastinum lateral to the great vessels. They then descend anteriorly into the middle mediastinum, passing anteriorly to the hilum of the lungs.