The mediastinum is an essential component of the thoracic cavity, located in between the two pleural sacs. It serves as the conduit for structures passing through the thorax en route to the abdomen, while also containing a majority of the organs within the thoracic region. The mediastinum can be divided into two distinct parts by an imaginary line passing from the sternal angle, formed at the convergence of the sternal body and manubrium, to the T4 vertebrae.
The superior mediastinum comprises the upper section of the mediastinum, extending from the thoracic inlet up to the superior thoracic aperture. The inferior mediastinum is continuous with the superior mediastinum along the level of the sternal angle, extending downwards to the diaphragm. The superior mediastinum can be further subdivided into anterior, middle, and posterior mediastinum.
In this article, we shall look into the anatomy of the superior mediastinum, focusing on its borders, contents, and clinical correlations.
The superior mediastinum is encompassed by four major thoracic structures:
The superior mediastinum contains an array of neural, vascular, and respiratory structures, many of which are responsible for transferring essential elements from the adjacent regions of the neck and abdomen (via the inferior mediastinum).
The great systemic blood vessels of the heart are located within the superior mediastinum, and their primary branches arise before passing through the superior thoracic aperture of the thorax (Fig. 2).
The three major branches of the aortic arch, namely the brachiocephalic artery, left common carotid artery, and the left subclavian artery, emerge from the superior mediastinum.
The superior vena cava is the first major vessel located within the superior mediastinum, lined with tributary pathways. This includes the brachiocephalic veins, which drain blood from the upper body, the left superior intercostal vein collecting blood from the left 2nd and 3rd intercostal veins, and draining into the left brachiocephalic vein, the supreme intercostal vein, draining directly into the brachiocephalic veins from the first intercostal space, and the azygos vein, which receives blood from the right posterior intercostal veins and the left intercostal veins which drain into the hemiazygos and accessory hemiazygos veins first before joining to the azygos vein around T7-T9.
The vagus nerve has two distinct pathways in the superior mediastinum. The right vagus nerve runs parallel to the trachea and passes posteriorly to the superior vena cava and the right primary bronchus, while the left vagus nerve enters the superior mediastinum between the left common carotid and the left subclavian arteries, descending anteriorly to the aortic arch, and posterior to the left bronchus. The left recurrent laryngeal nerve also arises from the left vagus nerve as it passes the aortic arch, looping underneath the arch to the left of ligamentum arteriosum, before continuing to the larynx in the tracheal-oesophageal groove.
From the anterior surface of the anterior scalene muscle, the phrenic nerves (roots C3, C4 and C5) enter the superior mediastinum, on the lateral aspect of the great vessels. They proceed to descend anteriorly into the middle mediastinum, passing anteriorly to the hilum of the lungs.
The superior mediastinum is also home to cardiac nerves, which originate from the superior, middle and inferior cardiac ganglion, forming the superficial and deep cardiac plexuses in the superior mediastinum. The superior plexus is situated between the aortic arch and the right pulmonary artery, while the deep plexus lies on the surface of the trachea at the point of bifurcation. Additionally, the sympathetic trunk runs bilaterally to the vertebral bodies along the entire length of the vertebral column.
In conclusion, the anatomy of the superior mediastinum is quite intricate and host to several essential structures, ranging from respiratory and vascular elements to neural pathways. Its borders, contents, and clinical correlations, as well as its implications for other components of the thoracic cavity, must be thoroughly understood in order to provide accurate diagnoses and treatment for any issues related to thoracic diseases.
The mediastinum is a central structure of the thoracic cavity, and is further subdivided into the anterior, middle, and posterior mediastinum. In this article, we shall look at the superior mediastinum and its anatomy in greater detail. This includes the borders, contents, great vessels, vagus nerve, and other pertinent factors.
The superior mediastinum is bordered by the following thoracic structures:
The superior mediastinum contains neural, vascular and respiratory structures passing from the adjacent regions of the neck and abdomen. The great systemic blood vessels of the heart (i.e. the aorta and superior vena cava) lie within the superior mediastinum and their main branches arise before passing through the superior thoracic aperture.
The three major branches of the aortic arch arise within the superior mediastinum:
The following tributaries of the superior vena cava are located within the superior mediastinum:
The vagus nerve has two distinctive paths within the superior mediastinum:
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. The phrenic nerves enter the superior mediastinum lateral to the great vessels before descending anteriorly into the middle mediastinum.
The cardiac nerves originate from the superior, middle and inferior cardiac ganglion and form the superficial and deep cardiac plexuses in the superior mediastinum. The superior plexus sits between the aortic arch and right pulmonary artery, while the deep plexus lies on the surface of the trachea at the point of bifurcation.
The sympathetic trunk runs bilaterally to the vertebral bodies along the entire length of the vertebral column. The thymus gland is the most anterior structure within the superior mediastinum. It sits flush against the posterior surface of the sternum and extends into the anterior mediastinum. The trachea bifurcates into the primary bronchi posterior to the ascending aorta at the level of the sternal angle, while the oesophagus ascends towards the pharynx, which it joins at the level of C6.
The thoracic duct passes to the left of the oesophagus on its path to the junction of the left internal jugular and subclavian veins. The sternohyoid and sternothyroid muscles originate from the posterior surface of the manubrium, and the inferior aspect of the longus colli muscle originates within the superior mediastinum.
The superior mediastinum is both an important structure for blood flow and a site of passage for several structures travelling between the neck, thorax, and abdomen. It is important to visualize and understand the anatomy of the superior mediastinum for proper diagnosis and treatment of medical conditions affecting this region.
