Anatomy of the superior mediastinum

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Introduction0:00–1:31

The mediastinum is the central compartment of the thoracic cavity and lies between the two pulmonary cavities. It’s covered on each side by the mediastinal pleura and contains all thoracic viscera and structures minus the lungs.
The mediastinum is divided into four parts, of which one is superior and three are inferior. The superior part is bounded superiorly by the thoracic aperture and extends inferiorly to the transverse thoracic plane, which is a horizontal plane that extends from the sternal angle anteriorly and the junction of T4 and T5 posteriorly.
The transverse thoracic plane separates the superior mediastinum from the inferior mediastinum, and is also an important anatomical landmark representing the bifurcation of the trachea and the beginning and end of the arch of the aorta.
The inferior mediastinum lies between the transverse thoracic plane superiorly, and the diaphragm inferiorly. It’s further subdivided by the pericardium into anterior, middle and posterior parts, with the pericardium and its contents being in the middle part.
So in this video, let’s focus on the superior mediastinum. From anterior to posterior, it contains the following structures: the thymus, the great vessels, the inferior continuation of the trachea, the inferior continuation of the esophagus, and the thoracic duct and lymphatics trunks.

Thymus1:31–2:04

Now let’s take a closer look at some of these structures one by one. So, the thymus is located in the inferior part of the neck and in the anterior part of the superior mediastinum.
It is a glandular structure and lies posterior to the manubrium and extends into the anterior mediastinum. The thymus is a primary lymphoid organ that plays a huge role in helping our immune cells develop before puberty.
After puberty, however, it undergoes gradual involution and is mostly replaced by fat. Next up, there are the great vessels.

Great vessels2:04–2:23

First we have the veins, which include the brachiocephalic veins and the superior vena cava or SVC, as well as the arteries, which include the arch of the aorta and its major branches, the brachiocephalic trunk, left common carotid artery and left subclavian artery.
Let’s take a look at this picture and identify the left and right brachiocephalic veins, which form posterior to the sternoclavicular joints by the joining of the internal jugular and subclavian veins.

Brachiocephalic veins and superior vena cava2:23–3:19

Then the brachiocephalic veins join to form the SVC at the level of the inferior border of the 1st right costal cartilage, with the left brachiocephalic vein being longer than the right one, since it has to travel farther to drain to the right side of the heart.
Now, the SVC then passes inferiorly and ends at the level of the 3rd costal cartilage at which point it enters the right atrium.
The SVC lies on the right side of the superior mediastinum, anterolateral to the trachea and posterolateral to the ascending aorta.
The SVC is anterior to the root of the right lung, and the azygous vein drains into its posterior surface. Next up, there’s the aortic arch, which continues the ascending aorta and begins at the level of the sternal angle, posterior to the 2nd right sternocostal joint.

Aortic arch3:19–4:03

As you can see in this picture, it arches superiorly, posteriorly and to the left and then inferiorly. The arch goes up anterior to the right pulmonary artery and the bifurcation of the trachea.
It reaches its apex as it passes over the root of the left lung, at the left side of the trachea and esophagus. The arch descends posterior to the left root of the lung beside T4 and ends by becoming the thoracic or descending aorta posterior to the 2nd left sternocostal joint.
Now for a really quick break - can you recall the plane that the beginning and end of the arch of the aorta is divided by?

Quiz4:03–4:17

That's right! We’re talking about the transverse thoracic plane!Now, the branches of the aortic arch are the brachiocephalic trunk, left common carotid artery and left subclavian artery.

Aortic arch branches4:17–5:35

The brachiocephalic trunk is the first and largest branch of the aorta. It arises posterior to the manubrium, at which point is anterior to the trachea and posterior to the left brachiocephalic vein.
Then, it ascends superiorly and laterally to reach the side of the trachea and is found posterior to the right sternocostal joint, where it divides into the right common carotid and right subclavian artery.
The left common carotid artery is the second branch of the aortic arch. It originates posterior to the manubrium, but slightly posterior and to the left of the brachiocephalic trunk.
It ascends anterior to the left subclavian artery and enters the neck posterior to the left sternocostal joint. The left subclavian artery is the third branch of the aortic arch and has its origin in the posterior part of the arch, posterior to the left common carotid artery.
It ascends through the superior mediastinum lateral to the left common carotid artery and It enters the root of the neck by passing posterior to the left sternocostal joint.

Ligamentum arteriosum5:35–6:09

One last important structure in the superior mediastinum to mention is the ligamentum arteriosum. The ligamentum arteriosum is a fibrous band, which is an embryological remnant of the ductus arteriosus from birth, which acted as a shunt for blood to travel from the pulmonary trunk to the aortic arch in order to bypass the fetal lungs, which weren’t functional in utero.
The ligamentum arteriosum remains and passes from the superior portion of the proximal left pulmonary artery to the inferior surface of the aortic arch.

Quiz6:09–6:22

Before moving forward, let’s see if you remember what the superior mediastinum contains, from anterior to posterior. Alright, so we’re gonna assume that you got that exactly right, so we can now move on to the nerves.

Vagus nerves6:22–8:38

As you can surely remember, there is a pair of vagus nerves which carry sensory, motor, and autonomic information to the mediastinum.
They exit the cranium and go down through the neck, posterior and lateral to the common carotid arteries. They then enter the superior mediastinum posterior to the left and right sternocostal joint and brachiocephalic vein.
he right vagus nerve enters the thorax anterior to the right subclavian artery at which point it gives rise to the right recurrent laryngeal nerve.
This nerve goes around the right subclavian artery and ascends between the trachea and esophagus to supply the larynx. Back to the right vagus nerve, it runs posteroinferiorly through the superior mediastinum on the right side of the trachea, then passes posterior to the right brachiocephalic vein, SVC and root of the right lung.
At this point it divides into many branches which contribute to the right pulmonary plexus. The right vagus nerve typically leaves this plexus as a single nerve and passes to the esophagus, where it gives many branches to contribute to the esophageal plexus.
The right vagus nerve also gives rise to nerves that contribute to the cardiac plexus. The left vagus nerve descends in the neck posterior to the left common carotid artery, then enters the mediastinum between the left common carotid artery and left subclavian artery.
When the left vagus nerve reaches the inferior border of the arch of the aorta, it curves medially and gives rise to the left recurrent laryngeal nerve.
This nerve passes inferior to the aortic arch, lateral to the ligamentum arteriosum which we talked about earlier, and climbs to the larynx in the groove between the trachea and the esophagus.
The left vagus nerve passes posterior to the root of the left lung, where it gives many branches to contribute to the left pulmonary plexus.
Then the left vagus nerve leaves this plexus as a single trunk and goes to the esophagus, where it contributes to the esophageal plexus.
Both vagus nerves eventually pierce the diaphragm at the T10 level along with the esophagus to provide further innervation to the abdomen.
Next up, there’s the paired phrenic nerves, which supply the diaphragm with both motor and sensory fibers and they also supply the pericardium and mediastinal pleura with just sensory fibers.

Phrenic nerves8:38–10:37

Quite the overachievers, I’d say! Each phrenic nerve reaches the superior mediastinum between the subclavian artery and the origin of the brachiocephalic vein.
Unlike the vagus nerves, the phrenic nerves pass anterior to the root of the lungs. The right phrenic nerve passes along the right side of the right brachiocephalic vein, SVC and the pericardium over the right atrium.
It also passes anterior to the root of the right lung and descends on the right side of the inferior vena cava to the diaphragm.
The left phrenic nerve descends between the left subclavian and left common carotid arteries. Then crosses the aortic arch anterior to the left vagus nerve.
Now, the left phrenic nerve descends anterior to the root of the left lung and runs along the fibrous pericardium, superficial to the left atrium and ventricle of the heart and it pierces the diaphragm at the level of T8 to the left of the pericardium, providing innervation to it.
The phrenic nerve is a mixed sensory and motor nerve, with contributions from spinal levels C3, C4, and C5. If the phrenic nerve becomes irritated where it innervates the diaphragm or parietal pleura, it can lead to something called referred pain to the dermatomes supplied by the same spinal region the phrenic nerve originates from.
So in this case, phrenic nerve irritation will cause a referred pain to the dermatomes of C3, C4, and C5, and this will manifest as root of the neck pain, which is C3 and C4, or shoulder tip pain, which is C5.
This is why diaphragmatic or lung irritation can cause people to have shoulder tip pain, and this is a classic example to describe referred pain, similar to how heart attacks can cause shooting pain in the neck or arm!
Okay, now, let’s switch gears and look at the trachea, which descends anterior to the esophagus and enters the superior mediastinum.

Trachea10:37–11:03

Looking at it in the median plane, it’s positioned a bit to the right. The posterior surface of the trachea which borders the esophagus is flat, and the trachea ends and divides into the right and left main bronchi at the level of the sternal angle marked by T4 and T5 junction.
And finally, the esophagus is a fibromuscular tube that extends from the pharynx to the stomach. It enters the superior mediastinum between the trachea and vertebral column, lying anterior to the bodies of T1 through T4.

Esophagus11:03–11:33

Typically, the esophagus is flattened anteroposteriorly. To the left side of the esophagus, there’s the thoracic duct, which is also medial or deep to the aortic arch.
Inferior to the aortic arch, the esophagus passes in the diaphragm through the esophageal hiatus. All right, as a quick recap, the mediastinum is divided into the superior mediastinum and an inferior mediastinum, where the inferior mediastinum is further subdivided into the anterior, middle and posterior mediastinum.

Review11:33–12:08

The superior mediastinum lies between the thoracic aperture superiorly and the transverse thoracic plane inferiorly. From anterior to posterior, the superior mediastinum contains the thymus, the great vessels, the inferior continuation of the trachea, the inferior continuation of the esophagus and