Thoracic Mediastinum

Mediastinum

A centrally located, interpleural (between pleural sacs) mass of tissue containing all thoracic viscera except the lungs and pleurae as well as most other thoracic structures.

Posterior Thoracic Wall

including the thoracic vertebral bodies, sympathetic trunks and intercostal spaces - is also extrapleural (outside the pleural sacs) but is not considered part of the mediastinum.

Boundaries of Thoracic Mediastinum

Superiorly - thoracic inlet
Inferiorly - thoracoabdominal diaphragm
Extends from the sternum anteriorly to the thoracic vertebral bodies posteriorly.
Laterally - bounded by the mediastinal pleura of the right and left pleural sacs.

Superior Mediastinum Boundaries

between thoracic inlet and the transverse thoracic plane which extends from the sternal angle to the IV discs between T4/T5

Contents of Superior Mediastinum

Thymus (infants and adolescents), great veins, aortic arch with its branches, vagus and phrenic nerves, trachea, esophagus, thoracic duct

Inferior Mediastinum Boundaries

Extends from the transverse thoracic plane to the thoracoabdominal diaphragm

Anterior Mediastinum

Between the sternal body and pericardial sac.

Contents of Anterior Mediastinum

Thymus, connective tissue, fat, lymph nodes

Middle Mediastinum

Coincident with the pericardial sac and its contents

Contents of Middle Mediastinum

pericardium, heart, roots of great vessels, phrenic nerves

Posterior Mediastinum

Between the pericardial sac and the vertebral column.

Contents of Posterior Mediastinum

Esophagus, esophageal plexus and vagal trunks, thoracic duct, descending aorta and its branches, azygos venous system

Mediastinum as an Anatomical Conduit

The mediastinum is an anatomical conduit for structures traveling between the head, neck and upper extremity and the thorax. Several longitudinal structures pass from the superior mediastinum into the posterior mediastinum and on into the abdomen.

Viscera of Mediastinum

Mediastinal viscera are mobile and their positions relative to the transverse thoracic plane vary. Viscera lie approximately one vertebral level higher in the supine position and lie lower during inspiration than during expiration.

Thymus

Located in the superior mediastinum - primary lymphoid organ with two asymmetrical lobes - source of T-Lymphocytes - large in children and adolescents but largely replaced by fatty tissues in adults.

Great Veins

Located in Superior Mediastinum - drain head, neck and upper extremities. Include the brachiocephalic veins and superior vena cava

Brachiocephalic Veins

formed by the union of the internal jugular and subclavian veins - right vein is shorter and descends vertically; the left vein is longer and courses anterior to the aortic arch and its branches.

Superior Vena Cava

Formed by the union of right and left brachiocephalic veins and terminates in the medial mediastinum

Aortic Arch

in the superior mediastinum - direct continuation of the ascending aorta - courses posteriorly and to the left giving three branches to supply the head, neck and upper extremities - brachiocephalic trunk (unpaired) - divides to form right common carotid a

Ligamentum Arteriosum

The remnant of the fetal ductus arteriosus - connects the aortic arch and left pulmonary artery.

Coarctation of Aorta

Most often occurs just dital to the attachment of the ligamentum arteriosum. In cases of postductal coarctation, collateral circulation can develop via the subclavian, internal thoracic, anterior intercostal and posterior intercostal arteries, thus mainta

Trachea

membranous tube extending from the larynx to the level of T4-T5 IV discs. The trachea terminates by bifurcating into the left and right primary bronchi. The point of bifurcation is marked internally by the keel-shaped, cartilaginous carina.

Tracheal Cartilages

Maintain the patency of the trachea, c-shaped cartilage "rings", deficient posteriorly to permit the passage of food through the adjacent esophagus.

Structures Continuous Between Superior and Posterior Mediastinum

Esophagus, thoracic duct

Esophagus

Musculomembranous tube connecting the pharynx with the stomach. In the superior mediastinum, it lies immediately posteriorly to the trachea and anterior to the vertebral bodies. In the posterior mediastinum, it lieds immediately posterior to the pericardi

Dysphagia

Difficulty swallowing can be caused by enlargement of the mediastinal lymph nodes or hypertrophy of the left atrium. Prior to the advent of modern imaging technologies, barium swallows were used to assess the left atrium radiographically.

Thoracic Duct

Thin walled vessel that carries lymph from structures inferior to the thoracoabdominal diaphragm. It enters the thorax via the aortic hiatus and drains into the venous system at the junction of the left subclavian and left internal jugular veins. In the p

Posterior Mediastinum

Descending aorta, azygos system

Descending Aorta

Continuation of the aortic arch, descends on the left side of the vertebral column. Branches - posterior intercostal arteries, bronchial arteries, esophageal arteries, superior phrenic arteries

Posterior Intercostal Arteries

supply intercostal spaces and abdominal wall - because the aorta lies to the left, right posterior intercostal arteries are longer and must cross anterior to the vertebral bodies.
Note: the 1st and 2nd posterior intercostal arteries are NOT branches of th

Bronchial Artery

to the bronchial tree and lungs

Esophageal Artery

to the esophagus

Superior Phrenic Artery

to the superior surface of the diaphragm

Aortic Aneurysms and Dissections

Aortic aneurysms are abnormal dilations of the aorta, occur most frequently on the descending aorta. Aortic dissection occurs when a tear in the aortic intima permits formation of a subintimal hematoma, usually on the ascending aorta or aortic arch. In ea

Azygos System

Venous network draining the intercostal spaces and thoracic wall.

Azygos Vein

Vertical venous channel that ascends on the right of the vertebral column. At the T4 vertebral level, the arch of the azygos vein passes anteriorly superior to the root of the right lung to drain into the superior vena cava. Inferiorly, the azygos vein co

Azygos Vein Receives

right superior intercostal vein - formed by union of posterior intercostal veins 2-4
Right posterior intercostal veins - 5-12
Note: the first 1st posterior intercostal vein typically drains directly into the right brachiocephalic vein.
Esophageal and bron

Hemiazygos and Accessory Hemiazygos Vein

Vertical venous channels lying to the left of the vertebral column.

Hemiazygos Vein

receives left posterior intercostal veins 9-12. Crosses anterior to the vertebral bodies to drain into the azygos vein. Communicates inferiorly with the ascending lumbar vein and left renal vein.

Accessory Hemiazygos Vein

Receives left posterior intercostal veins 5-8 before draining into the hemiazygos vein. The accessory hemiazygos vein may also communicate directly with the azygos vein.

Left Superior Intercostal Vein

And left 1st posterior intercostal vein drain directly to the left brachiocephalic vein.

Lymphatic system

Valved vessels that return interstitial fluid and large molecules (plasma proteins, fats) to the general circulation. Lymph nodes filter cellular debris, bacteria and circulating antibodies and play a majore role in immune response. The lymphatic system p

Jugular Lymph Trunks

major lymphatic vessel - (right/left) - drain the head and neck

Subclavian Lymph Trunks

Major lymphatic vessel - right/left - drain the upper extremities

Bronchomediastinal Lymph Trunks

Major lymphatic vessel - right/left - drain the lung and bronchial tree, mediastinum, anterior thoracic wall (paratracheal, mediastinal, and parasternal nodes)

Thoracic Duct

Drains lymph from all structures inferior to the thoracoabdominal diaphragm including the intestines, whose fat-rich lymph (chyle) has a milky appearance.

Thoracic Duct Receives From

Posterior intercostal lymph vessels (right/left): from posterior thoracic wall
Left bronchomediastinal lymph trunk
Left subclavian lymph trunk
Left jugular lymph trunk

Right Lymphatic Duct

Formed by the union of of the right bronchomediastinal, subclavian and jugular trunks. Drains into the venous system at the junction of the right subclavian and right internal jugular veins.

Chylothorax

Damage to the thoracic duct may allow lymph to accumulate in the mediastinum or pleural cavity. Permanent ligation of the thoracic duct has no harmful effects; lymph returns to the venous system by alternate pathways.

Phrenic Nerve

Right and left - C3, 4, 5 Ventral rami - provide motor and sensory innervation to the thoracoabdominal diaphragm; sensory innervation to the pericardium and mediastinal pleura.

Phrenic Nerve in Superior Mediastinum

The left phrenic nerve courses inferiorly posterior to the left brachiocephalic vein and anterior to the aortic arch.
Right Phrenic Nerve - lies lateral to the right brachiocephalic vein and superior vena cava.

Phrenic Nerve in Middle Mediastinum

Lies anterior to the roots of the lungs and between the pericardium and mediastinal pleura.

Periocardiophrenic Artery

Branch of the internal thoracic artery and its corresponding vein; accompanies the phrenic nerve.

Vagus Nerve

CNX - presynaptic parasympathetic fibers to thoracic and abdominal viscera; motor and sensory innervation to the larynx.

Left Vagus Nerve

Superior Mediastinum - courses anterior and lateral to the arch of the aorta

Left Recurrent Laryngeal Nerve

motor and sensory innervation of the larynx - hooks around the aortic arch near the ligamentum arteriosum and ascends lateral to the trachea.

Esophageal Plexus

In the posterior mediastinum, the left vagus nerve descends posterior to the root of the lung and lateral to the descending aorta and esophagus before joining the plexus.

Compression of Left Recurrent Laryngeal Nerve

Persistent hoarseness due to the compression of this nerve is a warning sign of several life-threatening conditions including aortic aneurysm, metastatic carcinoma of the lung, and mediastinal lymphoma.

Right Recurrent Laryngeal Nerve

Branch of right vagus nerve - motor and sensory innervation to larynx - arises in the root of the neck and hooks around the right subclavian artery.

Right Vagus Nerve

Superior Mediastinum - descends posterior to the great veins and medial to the arch of the azygos vein.
Posterior Mediastinum - lies posterior to the root of the lung and lateral to the esophagus before joining the esophageal plexus.

Cardiac and Pulmonary Branches

of the right vagus nerve contribute to cardiac and pulmonary autonomic plexuses as well as the esophageal plexus and vagal trunks.

Esophageal Plexus and Vagal Trunks

Fibers of the right and left vagus nerves unite to from the esophageal plexus. These fibers subsequently assemble to form anterior and posterior vagal trunks, which accompany the esophagus through the esophageal hiatus and innervate structures in the abdo

Anterior Vagal Trunk

composed primarily but not exclusively of fibers from the left vagus nerve

Posterior Vagal Trunk

composed primarily but not exclusively of fibers from the right vagus nerve

Thoracic Sympathetic Trunks

right and left - formed by the thoracic sympathetic chain (paravertebral) ganglia - trunks lie lateral to vertebral bodies and anterior to the neck of the ribs

Stellate Ganglion

1st thoracic sympathetic ganglion often fuses with inferior cervical ganglion

Postsynaptic Sympathetic Neurons

Chain ganglia contain the cell bodies

Rami Communicantes

Connect the sympathetic chain ganglia with thoracic ventral rami (intercostal nerves)

White Rami Communicantes

carry presynaptic sympathetic fibers from the intercostal nerves (ventral rami of thoracic spinal nerves) to the chain ganglia - lie lateral and slightly inferior to gray rami

Gray Rami Communicantes

Carry postsynaptic sympathetic fibers from the chain ganglia back to their respective intercostal nerves

Thoracic Branches of Sympathetic Chain

Cardiac branches, thoracic splanchnic nerves

Cardiac Branches

Postsynaptic sympathetic fibers exit the T1-T4 ganglia and course medially to join the cardiac and pulmonary plexuses.

Thoracic Splanchnic Nerves

Presynaptic fibers that course medially and pierce the diaphragm before synapsing on prevertebral (preaortic) ganglia in the abdomen.

Greater Splanchnic Nerves

Roots from the T5-T9 ganglia

Lesser Splanchnic Nerves

Roots of T10-T11 ganglia

Least Splanchnic Nerves

From the T12 ganglion

Visceral Afferent Fibers

Supplying thoracic structures follow both sympathetic and parasympathetic pathways.

Sympathetic Associated Visceral Afferents

convey pain sensations from the heart and other thoracic and abdominal viscera

Parasympathetic-Associated Visceral Afferents

convey sensations from bronchial tree (cough reflex), unconscious sensations related to homeostasis, and sensations of nausea.