14 The Mediastinum

THE MEDIASTINUM 

Learning Objectives

By the end of the course students will be able to:

1. Identify the contents of the posterior, superior and middle mediastinum
2. Appreciate the significance of the pericardium, its blood supply and innervation
3. Identify and describe the location of the lungs, heart and great vessels in the thoracic cavity
4. Describe the course of major structures passing between the neck and thorax and those which pass through the diaphragm between the thorax and the abdomen.

Reference: Moore, Clinically Oriented Anatomy, chapter 1

Particularly relevant Blue Boxes in Moore:

●Levels of the Viscera Relative to the Mediastinal Divisions, p. 132

●Widening of the Mediastinum, p. 133

●Pericarditis, Pericardial Effusion,Cardiac Tamponade, p. 133

●Pericardiocentesis, p. 134

To access the Netter Presenter Database click here

Grant’s Dissector, 15th Edition, pp 82 – 87

To access Gray’s Photographic Dissector section on the Mediastinum click here

To access the Primal Pictures software click here 

Check out the Primal Pictures model of the Mediastinum 

The MEDIASTINUM (Netter 227, 228; Moore 127-134)

phrenic nerves (discussed in last chapter).

●posterior — descending aorta, azygos and hemiazygos veins, esophagus, vagi nerves,  thoracic duct, posterior mediastinal lymph nodes, thoracic splanchnic nerves.

  (4 birds – “Vagoose, Azygoose, Esophagoose and Thoracic Duck”)

POSTERIOR MEDIASTINUM (Netter 227­, 228, 229)

Overview: In the lab, the posterior mediastinum is exposed by removing the pericardial sac and the heart. When this is done, we see the structures that lie immediately behind the pericardium. The relationships of the structures in the posterior mediastinum should also be studied on cross sections through the thorax.

After the heart has been removed, you will see the cut borders of the major vessels entering and leaving the heart. (see Netter 212) The oblique pericardial sinus is clearly seen within the four pulmonary veins and inferior vena cava at this point.

After the pericardium has been removed the esophagus can seen along with its plexus derived from the left and right vagus nerves as well as branches from the sympathetic chain (Netter 229).

As the esophagus passes through the diaphragm to enter the abdomen, the left vagus emerges from the plexus as the anterior vagal trunk and the right vagus becomes the posterior vagal trunk. (Use the mnemonic to remember: LARP – Left Anterior, Right – Posterior)

Netter 229 also shows how the trachea splits into the right and left primary bronchi at the level of the junction between the superior and inferior mediastinae (at the level of the sternal angle – T4/T5 junction)

The thoracic aorta starts at the T4-T5 junction and extends the full length of the posterior mediastinum passing through the aortic hiatus of the diaphragm into the abdomen.

The thoracic aorta gives rise to the posterior intercostal arteries which pass posterior and lateral to join the posterior intercostal veins and intercostal nerves to form the neurovascular bundle that travels along the inferior border of a rib or the upper aspect of an intercostal space. (Netter 233); It also gives rise to bronchial branches that enter the root of the lung to supply the bronchi and lung tissue.

Behind and between the esophagus and thoracic aorta, you will find the thoracic duct, a fragile lymphatic duct that begins in the abdomen as the cisterna chyli. It passes through the aortic opening of the diaphragm and ascends through the posterior mediastinum then arches laterally over the apex of the left pleura and between the left carotid sheath and the vertebral artery and then usually empties into the junction of the left internal jugular nad subclavian veins.

Alongside and behind the thoracic duct, you will find the veins that drain the walls of the thorax, the azygos and hemiazygos veins (Netter 189). Each of these veins begin in the abdomen as the ascending lumbar veins. The ascending lumbar veins drain parts of the posterior abdominal wall.

On the right side, the azygous vein begins at the junction of the ascending lumbar vein and the subcostal vein and passes deep to the right crus of the diaphragm to enter the posterior mediastinum. It continues upward along the right side of the bodies of the thoracic vertebrae and to the right of the descending aorta receiving posterior intercostal veins along the way. At about T8, it forms an arch that passes over the root of the right lung and then enters the posterior aspect of the superior vena cava. The arch receives the left superior intercostal vein that drains the upper 2 or 3 posterior intercostal spaces.

Located on the left side, the hemiazygous veins are quite variable in their makeup but the classic description is that the upper intercostal spaces are drained by the superior hemiazygos vein and the lower the inferior hemiazygos vein. These two veins may join as one hemiazygos vein that passes behind the thoracic duct to empty into the azygos vein. The two veins may just as frequently pass into the azygos separately, forming two hemiazygos veins. Or, there may be multiple veins crossing into the azygos vein, whereby a true hemiazygos vein doesn’t exist at all.

If you could reflect or remove the azygos system of veins, the next structures in the posterior mediastinum would be the splanchnic nerves, specifically the greater splanchnic nerves (Netter 236). These nerves are derived from the sympathetic chains, thoracic ganglion T5 to T9. There are also lesser splanchnic nerves that are derived from ganglion T10 and T11. Some people may even describe a least splanchnic that is derived from ganglion T12.

THORACIC AORTA (Netter 203)

The thoracic aorta arises from the left ventricle, passes superiorly to the right, forms aortic arch which passes POSTERIORLY and to the left. It descends along the left side of the esophagus.

In the region of the arch, typically, the three major branches – 1) right brachiocephalic (which forms right subclavian to superior extremity) and right common carotid artery to head/neck, 2) left common carotid artery to head/neck, and 3) left subclavian artery to superior extremity.

Occasionally, another vessel may appear to arise from the arch, thus a total of four vessels Should this vessel be present, it most likely would be the left vertebral artery and would be found arising between the left common carotid and the left subclavian arteries.

Summary of branches of the thoracic aorta:

Descending aorta

• from the left side of the body of T5, descends on the left of the vertebral column posterior to the root of the left lung (Netter 228).
• is in the midline at T8
• passes through the aortic hiatus at T11/12 (Netter 230).
• The greater splanchnic nerve from the sympathetic trunk joins the descending aorta and enters the abdomen with it (Netter 234).

The visceral branches of the descending aorta are (Netter 204, 234):

• 1-3 bronchial arteries
• 1-3 esophageal arteries   ● branches to pericardium and diaphragm.

The parietal (thoracic) branches are:

• the right and left posterior intercostal arteries from the 3rd intercostal space to the subcostal arteries below rib 12.
• The right posterior intercostal arteries from T3-T8 cross the vertebral column anteriorly. They anastomose with the anterior intercostal arteries of the internal thoracic artery at the midclavicular line.

• Clinical Considerations: An aneurysm of the aortic arch is a sac formed by dilation of the aortic arch that compresses the left recurrent laryngeal nerve, leading to coughing, hoarsness and paralysis of the ipsilateral vocal cord. It may cause dysphagia (difficulty swallowing) due to pressure on the esophagus and dyspnea (difficulty breathing) due to pressure on the trachea, root of the lung or phrenic nerve. Coarctation of the aorta usually occurs distal to the point of entrance of the ductus arteriosus into the aorta, in which case an adequate collateral circulation develops before birth. If this condition occurs proximal to the origin of the left subclavian artery, adequate collateral circulation does not develop. Coarctation of the aorta results in enlarged vessels, especially the internal thoracic, intercostal, epigastric and scapular arteries. It also results in elevated blood pressure in the radial artery and decreased pressure in the femoral artery. Coarctation of the aorta causes the femoral pulse to occur after the radial pulse (normally, the femoral pulse occurs slightly before the radial pulse and is under about the same pressure.

VENOUS DRAINAGE OF THE MEDIASTINUM (Netter 189; 234)

Venous drainage passes into several veins which join together to form the azygos vein, which in turn forms an arch over the right root of the lung and then enters the superior vena cava.

On the right side, the azygos vein passes superiorly along the right side of the vertebral bodies. At approximately the level of the 7th/8th intercostals a cross-over occurs from the left side. Passing upward on the left is the hemiazygos vein and passing downward on the left is the accessory hemiazygos vein. These veins cross to join the azygos on the right side.

On the left side, the upper 2-3 intercostal spaces may drain directly into the left innominate (brachiocephalic) vein.

The azygos and hemiazygos venous systems (Netter 189; 234)

• Drain the posterior thoracic wall from the 3rd intercostal space to the subcostal veins.
• The posterior intercostal veins on the left side drain into the hemiazygos veins. They join with the azygos sytem in the mid-thorax by passing anterior to the vertebral column .
• The azygos system ascends on the right side of the vertebral column and arches over the right bronchus to enter the posterior aspect of the superior vena cava at the level of the costal cartilage of rib 3 (Netter 227).
• The right superior intercostal vein joins the azygos after draining the 2nd, 3rd, 4th right intercostal spaces.
• The left superior intercostal vein joins the left brachiocephalic vein by crossing the anterior aspect of the aortic arch.
• The highest posterior intercostal veins drain the 1st intercostal space and join the brachiocephalic veins.
• The anterior intercostal veins drain into the internal thoracic veins

THE ESOPHAGUS

• extends from the posterior aspect of the pharynx at the level of C6 (Netter  230) to the stomach, below the left dome of the diaphragm.
• pierces the diaphragm at the level of the rib 7 costal cartilage at the level of T10.
• is constricted in 4 regions:
  • C6 (upper esophageal sphincter-voluntary),
  • T2/3 (crossing of aortic arch),
  • T4/5 (crossing of left primary bronchus),
  • T10 (diaphragm).
• Obstructions may occur at these levels. These levels are respectively 15 cm, 22 cm, 27 cm and 40 cm from the incisor teeth.
• lies anterior to the vertebral bodies of C7-T8
• swings to the left in the lower thorax, in front of the descending aorta to pass through the left dome at T10.
• is anterior to the thoracic duct, right posterior intercostal arteries (T3-T7), azygos and hemiazygos systems in the midthoracic region.
• The trachea is anterior to the esophagus from C7 to T4 (Netter  230). Then the esophagus lies posterior to the base of the heart (left atrium).

The superior esophageal sphincter is the cricopharyngeus (a voluntary muscle – we’ll see this during the Head and Neck section of the course))

The inferior esophageal cardiac sphincter is under the control of vagal (opener) and sympathetic fibers (closer).

Clinical Note:

Achalasia of the esophagus is a condition of impaired esophageal contractions because of a failure of relaxation of the inferior esophageal sphincter (see later chapter), resulting from degeneration of autonomic nerve plexuses in the esophagus. It causes an obstruction to the passage of food in the termnal esophagus and exhibits symptoms of dysphagia for solids and liquids, weight loss, chest pain, cough and recurrrent bronchitis or pneumonia.

THE RECURRENT LARYNGEAL NERVES

In the cervical region, both recurrent laryngeal nerves lie between the trachea and the esophagus (Netter  77). We will also discuss these in the Head and Neck portions of the course.

The vagus nerves

• inferior to the root of the lung, the left vagus lies anterior to the esophagus (Netter 228) and the right vagus lies posterior.
• Their branches form the anterior and posterior esophageal plexuses (Netter 229) which form single nerves to pierce the diaphragm at the esophageal hiatus.
• Below the diaphragm they are renamed the anterior and posterior gastric nerves.
• Vagal branches induce peristalsis in the esophagus and are secretomotor to mucous glands.

LYMPHATIC DRAINAGE (Netter 205)

Within the thorax, one may locate the thoracic duct lying against the posterior wall, usually situated between the azygos vein and the aorta (Netter 238).  As it ascends, it crosses to the left side of the aorta and enters the neck to empty into the point of union of left subclavian vein and the intern jugular vein to form the left brachiocephalic vein.

Further explanation of the lymphatic drainage system will be presented in the syllabus on the abdomen.   As you dissect the thorax, you will naturally learn the various contents of these imaginary spaces. Because the area has such three dimensional complexity, there is no good way to verbally describe it for you, and the illustrations will have to substitute.

SUMMARY OF THE MEDIASTINUM:

Refer to Netter plate 203 and Note:

●the division of the aorta into an ascending portion, an arch, and a descending portion.

●the differing origins of the left and right subclavian and carotid arteries.

●the very regular supply to the thoracic wall via the posterior intercostal branches.

●the somewhat random bronchial, esophageal, pericardial, and mediastinal branches, which arise as needed to supply structures adjacent to the descending aorta.

●the ligamentum arteriosum,which is a tough cord which ties the pulmonary trunk to the descending aorta. In the fetus, this is an open connection between the two–the ductus arteriosus–which plays an important role in the fetal circulation.

The important nerves of the superior and posterior mediastina are the LEFT and RIGHT VAGI (Netter 206), the RECURRENT LARYNGEAL NERVES, LEFT and RIGHT, given off by the vagi, and the LEFT AND RIGHT PHRENIC NERVES (Netter 227, 228).

The vagi descend from the skull through the neck and into the major body cavities, where they give much of the parasympathetic innervation received by the thoracic and abdominal viscera. The phrenic nerves descend from the cervical plexuses to innervate the muscles of the diaphragm. The recurrent laryngeal nerves innervate muscles in the larynx necessary for phonation

Also note these important nerves, the phrenics and the vagi:

● descend through the neck; the phrenics lateral to the jugular vein, the vagi lateral to the common carotid arteries.

● pass into the thorax deep to the great veins

● The phrenic nerves then pass down the lateral aspects of the pericardium to reach the diaphragm.

● The vagi pass posterior to the heart to form a plexus on the surface of the esophagus, and in this way pass into the abdomen to distribute to the viscera there. The vagi also give off the recurrent laryngeal branches; and notice the difference in the courses of these nerves. This has an embryologic explanation we will see later.

Cross-sections through the mediastinum at the T2, T4, T5, T7 and T8 levels

The Mediastinum quiz click here