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Mediastinum

Contents of the mediastinum seen from the lateral views.

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If there’s one thing we can say with certainty about our bodies is that they’re both organized and efficient in their layout. Almost every structure in our body, big or small, is compartmentalized in one way or another to ensure that everything stays where it needs to be. One way which our body achieves this is by the presence of defined spaces or cavities. For instance, our cranial cavity keeps our brain safely tucked in our head and our abdominal cavity packs in over seven meters of gut to absorb anything worth taking from that burg you had for lunch.

Our thoracic cavity is also an interesting space with its bony cage protecting your heart and lungs, but between our lungs is a special yet somewhat anatomically complex region whose name alone often brings horror to anatomy students all over. But don’t fear, that’s why we’re here. I assure you, it’s safe to come out because today, we’re going to master the anatomy of the mediastinum.

So let’s get down to business and see what the fuss is all about.

So we’ve established that the mediastinum is located roughly on the midline of our body between the pleural cavities of the thorax. In our tutorial today, we’re going to be looking at the anatomy of this space with these two illustrations with both right and left lateral views of the thorax. So, let’s take a moment to define some of the anatomical boundaries of the mediastinum as a whole.

Anteriorly, the mediastinum is limited by the anterior thoracic wall which is comprised of the sternum as well as the costal cartilages of the first to fifth ribs which you can see here highlighted in green. The posterior boundary of the mediastinum is marked by the vertebral bodies of the superior thoracic vertebrae. Knowing the anterior and the posterior borders of the mediastinum, we can now define the superior most border of the mediastinum as the superior thoracic aperture delineated by the manubrium of the sternum following around the superior border of the first rib to the T1 vertebral body.

Of course, laterally, since the mediastinum is located between the pleural sacs, the parietal pleura of each lung provides the lateral border for each side of the mediastinum. And, finally, the inferior boundary of the mediastinum is formed by the respiratory diaphragm which you can see just over here.

So, to give our borders a little bit more context, let’s take an alternative look at the mediastinum by looking at the diaphragm from a superior perspective. And here you can – so here you can get a better idea of the space occupied by the mediastinum.

Okay, now, let’s get into the nitty-gritty of what the mediastinum is all about.

So as typical anatomists, we like to divide things up into smaller divisions to make it easier for us to explore and, of course, the mediastinum is no different. So, the mediastinum as a whole can be subdivided into two major divisions. And, firstly, we have the superior mediastinum which is found superior to an imaginary line known as the thoracic plane intersecting the sternal angle anteriorly and the T4 to T5 intervertebral discs posteriorly. And, of course, if we have a superior, there must also be an inferior mediastinum, and as you can see, this is the space inferior to the thoracic plane we just mentioned – and guess what? – we can divide this one up even more.

So, the inferior mediastinum is actually further subdivided into three smaller subsections mainly in relation to the pericardial sac around the heart. And anterior to the pericardial sac is the anterior mediastinum which is more of a potential space limited by the anterior thoracic cage. The pericardial sac, its contents, and adjacent parts of the great vessels are located in the middle mediastinum which means that everything else posterior to or behind the pericardial sac is unsurprisingly considered to reside in the posterior mediastinum.

So, in this tutorial, we’re going to explore each region of the mediastinum one by one, examining the organs and the major neurovasculature located within each division. So let’s get to it and begin first with the superior mediastinum.

So just to remind ourselves of where the superior mediastinum begins and ends, remember that it is the space superior to the thoracic plane running from the sternal angle and the T4 to T5 intervertebral disc. And we’re going to work anterior to posterior and quickly identify the major structures found here.

First up is this somewhat fatty glandular-looking organ here which is the thymus, which is one of our primary lymphoid organs. And this means that it’s where our body produces our lymphocytes which work hard to kill off those pesky bacteria and viruses attacking our bodies.

When we’re younger, the thymus is a flat, bilobar gland, however, after puberty, it undergoes a gradual involution – meaning that it shrivels up – and is largely replaced by fat tissue instead.

Our next structure of interest in the superior mediastinum is this one here, which you might be able to identify by its characteristic cartilaginous ring seen here. And that’s right. It’s indeed the trachea, commonly known as the windpipe. And the trachea, of course, descends from the larynx into the neck deviating just a little to the right of the midline. It passes inferiorly to around the level of the T5 vertebra where it bifurcates into the right and left main bronchi which enter the hilum of the lung.

Just posterior to the trachea is the esophagus, which is this fibromuscular tube which carries our swallowed food from the pharynx along the length of the superior and posterior mediastina before continuing into the abdomen via the diaphragm.

Posterior to the thymus are several major arteries and veins, and we’re going to begin with this one here highlighted in green for you which is the superior vena cava. The superior vena cava is formed by the union of the left and right brachiocephalic veins and carries deoxygenated blood received from the head, the neck, the upper limbs, and the thoracic wall. And we can tell from this illustration that the superior vena cava is situated in the right side of the superior mediastinum and empties its contents directly into the right atrium of the heart which is over here.

The superior vena cava also receives the contents of the azygos vein, which runs along the right-hand side of the vertebral bodies before arching over the right main bronchus. And as it courses through the mediastinum, the azygos vein receives several of the right posterior intercostal veins which drain the right intercostal spaces.

So we’re going to turn our focus over to the left view of the mediastinum now so we can get a better view of some of the other vasculature of the superior mediastinum. So again to begin of course with this structure here which is the aortic arch which is a continuation of the ascending aorta beginning at about the level of the second sternocostal joint. And as it courses through the superior mediastinum, it arches posteriorly and to the left before then turning inferiorly as the descending thoracic aorta just to the left of the T4 vertebral body.

The aortic arch gives off three large branches which are the brachiocephalic trunk which is not visible in this illustration, the left common carotid artery which supplies much of the left side of the head and neck, and the left subclavian artery which provides arterial supply of the upper limb as well as the upper spinal cord, the brainstem, the cerebellum, and the posterior cerebrum.

So before we move on from the superior mediastinum, let’s just check out some of the major nerves that are present here. So beginning first with the left vagus nerve which enters the superior mediastinum between the left common carotid and the subclavian arteries. So when the left vagus nerve reaches the aortic arch, it turns posteriorly before coursing medially at the inferior border of the aortic arch where it gives off the left recurrent laryngeal nerve which goes on to ascend back up into the neck running in a groove formed by the trachea and the esophagus.

The left vagus nerve then descends posterior to the root of the left lung giving several branches to the left pulmonary plexus before continuing on to join its right counterpart to form the esophageal plexus.

The phrenic nerve provides both sensory and motor innervation to the respiratory diaphragm as well as sensory innervation to the pericardium of the heart. As you can see in the illustration, the phrenic nerve descends into the superior mediastinum between the subclavian artery and the brachiocephalic vein. The phrenic nerve then passes anterior to the root of the lungs which will help you distinguish it from the vagus nerve in your dissection class.

And with that, we’ve touched on the main structures of the superior mediastinum. So let’s drop down into the inferior mediastinum now and get a hold on the anatomy there.

So you’ll remember from the beginning of our tutorial that the inferior mediastinum is subdivided into three parts which are the anterior, middle, and posterior mediastina. In reality, the anterior mediastinum is extremely limited in volume and is defined as the space between both the pleural sacs anterior to the pericardial sac. And if we look at the anterior mediastinum from a superior view, we can get an even better understanding of its proportions.

So given the fact that it’s almost only a potential space, it will come as no surprise to hear that there’s very little anatomy for us to examine here. In fact, the only structures that you’ll find here are some remnants of the thymus as well as some anterior mediastinal lymph nodes such as the prepericardial lymph nodes.

Moving posteriorly, we’re now entering the middle mediastinum which is the largest portion of the inferior mediastinum. And it’s dominated by the pericardial sac which means that the middle mediastinum is where our heart takes residence in addition to the great vessels such as the inferior part of the superior vena cava, the inferior vena cava, the pulmonary veins, the pulmonary trunk and arteries, and of course the ascending aorta.

Other major vasculature in this region includes the pericardiophrenic arteries as well as its venous counterpart, the pericardiophrenic veins. The main bronchi are also considered to be part of the middle mediastinum.

And that brings us back to the posterior mediastinum which is interposed between the pericardial sac and the T5 to T12 vertebral bodies. And although the posterior mediastinum is smaller than the middle mediastinum, it contains quite a lot of anatomical structures which we’re going to find a little bit more about right now.

So, we’re going to begin with what is arguably the most important structure of the posterior mediastinum which is the descending thoracic aorta. So continuing on from the aortic arch, the descending thoracic aorta runs along the left side of the lower thoracic vertebral bodies before passing through the aortic hiatus of the respiratory diaphragm, and once it’s there, it becomes the abdominal aorta.

As it descends through the posterior mediastinum, the descending thoracic aorta gives off paired lateral branches which supply the intercostal spaces between the third through to the twelfth ribs. And these are known as the posterior intercostal arteries.

Along its anterior aspect, the descending thoracic aorta gives off between two and five unpaired esophageal branches as well as a number of bronchial and pericardial branches. It also gives off a pair of superior phrenic arteries which supply the diaphragm. Running laterally to the descending thoracic aorta are the hemiazygos and accessory hemiazygos veins which receive the contents drainage by the left posterior intercostal veins. And in this illustration, you can also see this vein here which is the left superior intercostal vein which drains the uppermost intercostal spaces.

And finally before we complete our study of the posterior mediastinum, let’s just take a moment to look at the major nerves found in this region.

So many of the nerves which we saw in the superior mediastinum naturally continue their course down into the posterior mediastinum, for example, the vagus and the phrenic nerves as well as their respective branches. And though technically not part of the posterior mediastinum, I want to take a quick look at this collection of nervous structures situated here along the posterior thoracic wall.

So running inferior to each rib alongside the posterior intercostal arteries and veins are the intercostal nerves which are the anterior rami of the thoracic spinal nerves. All of the intercostal nerves supply innervation to the parietal pleura and structures of the thoracic wall.

You also see this chain of nerves and ganglia descending here along the posterolateral border of the vertebral bodies and this is known as the sympathetic trunk and the sympathetic ganglia which, as its name suggests, is one of the major pathways of the sympathetic division of the autonomic nervous system. You can also see several branches coming off the sympathetic trunk which also descend inferiorly into the abdomen and these form what’s known as the greater thoracic splanchnic nerve which provides sympathetic innervation to several parts of the gastrointestinal tract as well as to other abdominal organs.

And with that we’ve reached the end of our tour of the mediastinum. So now that we’ve explored the anatomy of the mediastinum, let’s finish up our tutorial today with some clinical correlates of the mediastinum.

Mediastinitis is an extremely serious condition which involves inflammation of the mediastinal tissues due to chronic or acute infection, and it can be a life-threatening condition and like many infections, needs immediate treatment. Acute mediastinitis may occur as a result of infection originating from one of the structures within the mediastinum or it may migrate from another region of the body altogether.

For example, infections originating from within the mediastinum could arise due to rupture or perforation of the esophageal wall which can occur after foreign body ingestion, cancer or due to iatrogenic causes, which means conditions resulting from medical procedures such as intubation, gastrointestinal endoscopy or cardiothoracic surgery. An infection can also spread from the lungs or even from the bones of the thoracic wall.

Infections originating from outside of the mediastinum most often involve the spread of retropharyngeal and odontogenic infections.

Clinical features of acute mediastinitis are similar to those seen with other major infections such as chills, fever, tachycardia, tachypnea, and general restlessness. Of course, in cases of mediastinitis, the patient will also experience chest pain which may be referred to the neck or to the interscapular region of the back. Chronic mediastinitis is caused by fibrosis of the soft tissue of the mediastina resulting from immune-related diseases such as histoplasmosis and tuberculosis, and this can cause constriction or obstruction of airways or vessels of the mediastinum – for example, superior vena cava syndrome which causes swelling of the upper limbs, the head and the neck region as well as breathing distress and lightheadedness.

And with that, we’ve reached the end of our tutorial on the mediastinum. Before I leave you, let me quickly summarize what we’ve learned today.

So, it began first by defining the main regions of the mediastinum which were the superior mediastinum located between the thoracic inlet and the thoracic plane and the inferior mediastinum which is found between the thoracic plane and the respiratory diaphragm. We then subdivided the inferior mediastinum into three smaller divisions which were the anterior mediastinum, a small restricted space anterior to the pericardial sac; the middle mediastinum, which contains the pericardial sac and much of the great vessels; and finally, the posterior mediastinum, which comprises the space posterior to the pericardial sac.

In the superior mediastinum, we identified several major organs such as the thymus, the trachea, and the esophagus. We then looked at some of the major vessels in this region which included the superior vena cava, the arch of the azygos vein, the aortic arch, the pericardiophrenic artery and vein, the vagus nerve, and the phrenic nerve.

Moving on to the anterior mediastinum, we discovered that this was more of a potential space only and it contained very few structures; for example, the remnants of the thymus, as well as the anterior mediastinal lymph nodes. The middle mediastinum, of course, gave us lots of anatomical structures to consider, none more important than the heart and the pericardium, of course. And we managed to identify several of the great vessels including the ascending aorta, the pulmonary trunk and arteries as well as their venous counterparts, the pulmonary veins, and we also made note of the phrenic nerve descending towards the diaphragm.

Last but not least, we reached the posterior mediastinum, and here we looked at the descending thoracic aorta as well as a number of its branches including the posterior intercostal arteries. We also saw the continuation of the esophagus as it descended towards the abdomen and lateral to the vertebral bodies we looked at the sympathetic trunk and ganglia as well as the greatest splanchnic nerve which provides sympathetic innervation to parts of the gastrointestinal tract and to other abdominal organs.

And that wraps up our tutorial for today. I hope you’ve enjoyed exploring the mediastinum. It’s not so complicated after all. So we’re looking for a few to having you join us for another Kenhub tutorial soon, but until then, happy studying!

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