Anatomy clinical correlates: Mediastinum

We all know how important the heart is, but everyone forgets about the mediastinum, which is the area between the two lungs where the heart actually sits!

The mediastinum doesn't just house the heart, but many important structures in the thoracic cavity from the superior thoracic aperture down to the diaphragm.

So let's take a closer look at the clinical conditions that can affect the mediastinum and the structures within it.

First up, there is widening of the mediastinum, which can be seen on a chest x-ray.

Since the mediastinum contains so many structures, each of them can contribute to pathological widening.

It can be observed after a trauma that causes laceration or dissection of the great-vessels, typically the aorta, which can cause hemorrhaging into the mediastinum.

Other times, malignant tumors such as lymphomas can produce massive enlargement of mediastinal lymph nodes and widening of the mediastinum.

Another cause of mediastinal widening is heart hypertrophy due to congestive heart failure.

Now, next up, there’s the esophagus.

The esophagus may have up to four normal anatomic constrictions as it descends, however there are three sites of constriction that can occur specifically in the posterior mediastinum caused by three structures it meets on its way down: two thoracic constrictions with the first being where the esophagus is crossed by the arch of the aorta, and second where it is crossed by the left main bronchus, and one diaphragmatic constriction where it passes through the esophageal hiatus of the diaphragm.

The fourth site of constriction occurs in the cervical region before the esophagus enters the mediastinum, where constriction may be caused by the cricopharyngeus muscle.

At these locations, there’s a slower passage of substances and is where foreign objects that are swallowed are most likely to lodge.

These narrowings can be seen in chest radiographs of a person who underwent a barium swallow study.

These areas are also at risk of stricture after ingesting caustic liquids such as cleaning products.

Don’t try this at home!!

Now, even though we try to prevent foreign objects going into our esophagus, one thing we are okay with is the use of transesophageal echocardiography, or TEE for short.

TEE is a device which uses ultrasound within the esophagus to show images of the cardiac structures, particularly the left atrium which makes up the majority of the posterior heart and directly anterior to the esophagus.

In addition to the left atrium, TEE can visualize the atrial septum and mitral valve.

TEE allows for assessment of conditions such as atrial enlargement due to mitral stenosis or regurgitation, which is important as severe enough left atrial enlargement can cause external compression on the esophagus leading to dysphagia..

Furthermore, the descending aorta lies posterior to the esophagus, so TEE can visualize aortic abnormalities such as dissection or aneurysm.

Ok, now, a little bit higher up, there are the recurrent laryngeal nerves, which supply all intrinsic muscles of the larynx, except the cricothyroid which is supplied by the external laryngeal nerve, a branch of the superior laryngeal nerve.

You might be thinking, what does this have to do with the thorax?

Well, procedures in certain thoracic regions, like a mediastinotomy, or disease in the superior mediastinum such as esophageal cancers and mediastinal lymph node enlargement can injure these nerves and affect the voice, leading to hoarseness or even loss of voice called aphonia.

Furthermore, as the left recurrent laryngeal nerve wraps around the arch of the aorta, any dilation of the arch of the aorta can stretch and damage this nerve.

Damage to both recurrent laryngeal nerves at the same time would lead to bilateral paralysis of the vocal cords, and would require intubation.

Speaking of the aorta, let’s have a look at some variations of the aortic arch.

Sometimes there may be a right arch of the aorta which courses to the right of the trachea instead of its usual course to the left of the trachea.

In rare cases, a double arch of the aorta can occur which forms a ring around the esophagus and trachea, which can compress these structures and potentially result in difficulty with breathing and swallowing.

The aorta can also be subject to something called coarctation of the aorta, which is when the aortic arch or thoracic aorta has an abnormal narrowing or stenosis of the aortic lumen.

This causes an obstruction of blood flow distal to the stenosis and to the inferior part of the body.