How Much Do You Know About Posterior Mediastinum? Quiz

The posterior mediastinum is bounded superiorly by the plane through the sternal angle and T4/5, inferiorly by the diaphragm, anteriorly by the middle mediastinum, and posteriorly by the spinal cord. This area contains the descending thoracic aorta, the azygos system, the esophagus, the thoracic duct, and lymph nodes. Of the answer choices, the esophagus is the only one in the posterior mediastinum. The great vessels and bronchi at the roots of the lung are in the middle mediastinum.

The thoracic duct lies in the posterior mediastinum between the aorta on its left and the azygos vein on its right. These three structures are all found posterior to the esophagus. The greater thoracic splanchnic nerve lies anterior to the vertebral bodies, behind the azygos, thoracic duct, and aorta. The left recurrent laryngeal nerve is found in the middle mediastinum, looping under the aortic arch before it ascends to the larynx. The right pulmonary artery is also in the middle mediastinum--it leaves the heart and enters the root of the right lung. The sympathetic trunk lies on the heads of the ribs on the posterior wall of the thorax

The esophagus is in the posterior mediastinum, along with the descending aorta, thoracic duct, and azygos system.

The thoracic duct is found directly behind the esophagus in the posterior mediastinum, with the aorta to its left and the azygos vein to its right. This relationship between these three vessels is an important one to keep in mind! The other vessels do not share the same relationship with the azygos and aorta.

The left recurrent laryngeal nerve is closely associated with the aortic arch and the ligamentum arteriosum/ductus arteriosus. As the left vagus nerve passes near the aortic arch, it gives offf the left recurrent laryngeal nerve. This nerve then loops under the aortic arch, lateral to the ligamentum arteriosum, and ascends to the larynx in the tracheoesophageal groove. During surgery to close a patent ductus arteriosus, a surgeon must be careful to protect the left recurrent laryngeal nerve, which is lateral to the duct that is being ligated.

The accessory hemiazygos vein is a vein on the left side of the body which drains blood from the left chest wall into the azygos vein. The internal thoracic artery is a branch of the subclavian artery which supplies blood to the anterior chest wall and gives rise to the anterior intercostal arteries. The left phrenic nerve passes through the thorax to innervate the diaphragm. It is a bit lateral to the area where the surgeon is clamping the ductus arteriosus, so it would not be at risk during the procedure. Finally, the thoracic duct is found deep in the posterior mediastinum, lying between the aorta and azygos veins.

A chylothorax is a pleural effusion composed of lymphatic fluid due to disruption of the thoracic duct. Pleurisy refers to the inflammation of the pleura with exudation into the pleural cavity. A pyothorax is an infection that occurs in the pleural space, where pus accumulates within the pleural cavity. A hemothorax involves the accumulation of blood in the pleural space. Finally, lymphedema is a swelling in a body part cause by the obstruction of lymphatic flow or the removal of the lymphatic vessels in a region.

Lymph nodes are found in all of the mediastinal compartments. They are the one structure that can be found in the anterior, middle, and posterior compartments. What else is in each compartment? The anterior compartment contains areolar tissue and sternopericardial ligaments. The middle compartment contains the pericardium,heart, great vessels, and bronchi. The posterior compartment contains the descending thoracic aorta, azygos system, esophagus, and thoracic duct.

The esophagus is closely related to the posterior surface of the pericardial sac. After coming from the heart, the aorta arches over the left pulmonary artery and left bronchus. Eventually, this vessel is posterior to the esophagus. The azygos vein, on the right side of the thorax, arches over the right pulmonary artery and bronchus. It is also posterior to the esophagus. The thoracic duct is posterior to the esophagus as well and does not contact the pericardial sac. Finally, the trachea is superior to the heart.

In the mid thorax, the aorta, thoracic duct, and azygos vein are all posterior to the esophagus. (They are in that order, from left to right.) The superior vena cava and the trachea are not located in the mid thorax--the superior vena cava terminates as it feeds into the right atrium and the trachea ends as it splits into the two mainstem bronchi which enter the lungs.

The left superior intercostal vein drains intercostal spaces 2-4, and then drains into the left brachiocephalic vein. See Netter Plate 231 for a picture of this relationship. The left bronchial vein is a small vein that removes venous blood from the lungs--it drains into the accessory hemiazygos vein. The left pulmonary veins carry oxygenated blood from the lung to the left atrium of the heart. The left subclavian vein is a continuation of the left brachiocephalic vein--this vein drains blood from the arm into the left brachiocephalic vein. The superior vena cava is formed by the junction of the left and right brachiocephalic veins; it delivers blood to the right atrium.

The ascending aorta is located in the middle mediastinum, along with the other great vessels. The aortic arch is located in the lowest part of the superior mediastinum (its lower border lies at the level of the sternal angle). The descending aorta is in the posterior mediastinum. Remember what's in what part of the mediastinum--it's important!

The sympathetic trunk can be found on the posterior wall of the thorax, lying on the heads of the ribs. It contains the cell bodies of postganglionic sympathetic fibers. When a nerve fiber reaches the sympathetic chain, there are three things that can happen. First, the nerve fibers can enter a ganglia in the trunk, synapse at that level, and rejoin the spinal nerve via the grey rami communicantes. Second, the preganglionic nerve fibers can travel up and down the trunk, synapse in a ganglia at another level, and then rejoin a spinal nerve. This is how sympathetic fibers join spinal nerves at the cervical and lumbar levels, which are above and below the lateral horn. Third, some preganglionic fibers do not synapse in the trunk and, instead, form splanchnic nerves. These nerves descend into the abdomen and synapse in other ganglia. You really need to understand all three of these possibilities!

The greater thoracic splanchnic nerve is one of the splanchnic nerves that is carrying preganglionic fibers away from the trunk. It lies medial to the trunk, on the vertebral bodies, and carries fibers to the celiac plexus. The phrenic nerve travels through the anterior thorax to innervate the diaphragm--it's not found near the heads of the ribs. The pulmonary plexus is located along the pulmonary vessels and primary bronchi in the root of the lung--it carries both sympathetic and parasympathetic fibers to the lungs. The vagus nerve is the major nerve carrying parasympathetic fibers in the thorax and to the abdomen. It begins in the anterior portion of the thorax, then enters the posterior mediastinum and forms the esophageal plexus covering the esophagus.

The azygos vein lies immediately behind the right mainstem bronchus. This vein then arches over the right mainstem bronchus to drain blood into the superior vena cava. So, this must be the structure that was damaged during the bronchoscopy.The left brachiocephalic artery doesn't exist, but the brachiocephalic trunk is a branch off the aortic arch which travels far superior to the area of interest. The pericardiacophrenic artery is a branch of the internal thoracic artery which accompanies the phrenic nerve. It is anterior to the right bronchus. The right pulmonary veins are inferior and anterior to the right mainstem bronchus. Finally, the superior vena cava is superior and anterior to the right mainstem bronchus. Take a look at Netter Plate 230 for a better understanding of these relationships

The sympathetic fibers in the greater thoracic splanchnic nerve are preganglionic sympathetic fibers that have left the sympathetic chain and are going to synapse in an abdominal ganglia. These preganglionic sympathetic fibers originate in the lateral horn of the spinal cord grey matter. The celiac ganglia and the superior mesenteric ganglia are the two ganglia where the fibers from the greater thoracic splanchnic nerve can go to synapse. Finally, remember that these fibers did not originate in the chain ganglia--the fibers from there are the postganglionic sympathetic fibers.

The cervicothoracic ganglion is a sympathetic ganglion, formed by the fusion of the inferior cervical sympathetic ganglion and the T1 ganglion of the sympathetic trunk. The postsynaptic sympathetic fibers from this ganglia innervate the vascular smooth muscle and sweat glands of the C8 & T1 cutaneous distribution on chest & upper limb. Since the sweat glands in the right armpit are innervated by fibers coming from the stellate ganglion, this ganglion might need to be removed or cut to alleviate the patient's condition.

The dorsal roots of cervical spinal nerve carry afferent, sensory fibers. These sensory fibers are not involved in innervation to glands. The greater thoracic splanchnic nerve carries preganglionic sympathetic fibers to the abdomen, where they synapse in the celiac ganglion. The lumbar sympathetic trunk is involved with sympathetic innervation in the abdomen--it is far from the area where this patient is experiencing problems. Finally, the vagus nerve carries parasympathetic fibers to the thorax and abdomen; it does not innervate sweat glands.

White rami communicantes carry presynaptic sympathetic fibers to the sympathetic trunk. When a presynaptic nerve fiber reaches the sympathetic chain, there are three things that can happen. First, the nerve fibers can enter a ganglia, synapse at that level, and rejoin the spinal nerve via the grey rami communicantes. Second, the preganglionic nerve fibers can travel up and down the trunk, synapse in a ganglia at another level, and then rejoin a spinal nerve. This is how sympathetic fibers join spinal nerves at the cervical and lumbar levels, which are above and below the lateral horn. Third, some preganglionic fibers do not synapse in the trunk and, instead, form splanchnic nerves. These nerves descend into the abdomen and synapse in other ganglia.

The greater thoracic splanchnic nerve contains preganglionic fibers that are destined to synapse in the celiac plexus. The recurrent laryngeal nerve provides motor and sensory innervation to the upper esophagus and pharynx. Finally, the vagus nerve is a mixed nerve that carries preganglionic parasympathetic fibers. None of these nerves carry postganglionic sympathetic fibers.

The ulnar nerve innervates muscles of the hand and forearm, and provides some sensory innervation to skin of the hand. The ulnar nerve is derived from ventral primary rami, all of which carry postganglionic sympathetic fibers (to innervate vascular smooth muscle, arrector pili muscles, and sweat glands).

Sympathetic fibers innervate sweat glands by synapsing in the sympathetic chain, jumping on the grey rami to rejoin the spinal nerve, and heading for the periphery. The sympathetic nerves to the heart, aorta, lungs, and bronchi are carried in the cardiac and pulmonary plexuses. These fibers are not found in spinal nerves.

The sympathetic nervous system is not responsible for voluntary muscle activity. The neurons which supply voluntary muscles originate from the ventral horn of the spinal cord. One of the main functions of sympathetic nerves is maintaining the tone of blood vessels--if these nerves were damaged, it would be difficult to regulate vascular tone. The sympathetic nervous system also regulates the arrector pili muscles, sweat production, and visceral reflexes

The thoracic splanchnic nerves lie on the anterior surfaces of the vertebral bodies. Remember, the splanchnic nerves lie medial to the sympathetic trunk, which is lying on the heads of the ribs. The vagus nerve lies in the anterior chest and eventually forms the esophageal plexus, covering the esophagus. The phrenic nerve innervates the diaphragm--it is not near the posterior wall of the thorax. Intercostal nerves run in the intercostal groove at the posterior border of the rib--they are not near the vertebral bodies

The internal thoracic vein would provide a collateral route for drainage if the azygos vein was obstructed. In the case of an obstruction, blood could flow from the posterior intercostal veins (which usually drain into the azygos) into the anterior intercostal veins, enter the internal thoracic vein, and drain into the right brachiocephalic vein. This would allow the blood to bypass the blockage. The right brachiocephalic vein would be receiving more blood due to this blockage, but it wouldn't be the vessel that would enlarge--the internal thoracic vein would become distended. The superior epigastric vein is an inferior extension of the internal thoracic vein--it is too inferior to assist with collateral circulation.

Efferent fibers to the skin of the forehead might have their cell bodies located in the lateral horn of the T1 level. Because these fibers are at the superior edge of the thoracolumbar outflow (located from T1 to L2), they might go up the chain, synapse at a higher ganglion, and provide sympathetic innervation to the head and face. Afferent fibers would not have their cell bodies located in the lateral horn--afferent sensory fibers have cell bodies in dorsal root ganglia. Efferent fibers to the sweat glands of the lumbar region would be sympathetic fibers, but these cell bodies would be located at the T12, L1, or L2 levels--not at T1. T1 is too high for the lumbar region! Finally, parasympathetic fibers to the heart come from the vagus nerve.