3.0 Superior Posterior Mediastina (Exam 3)

Superior Mediastinum

• Lies above the sternal angle
• Contains the great vessels and the trachea
• Nerves continue inferiorly, to be discussed in detail in the posterior mediastinum

Contents by Location

• Retrosternal: thymus, left brachiocephalic vein, superior part of the superior vena cava
• Intermediate: aortic arch, brachiocephalic trunk, left common carotid artery, left subclavian artery
• Prevertebral: trachea, esophagus, thoracic duct, left recurrent laryngeal nerve

Cross-Section Through the Superior Mediastinum

• Anterior to posterior relationships are critical

Structures

• 1 - esophagus
• 2 - trachea
• 3 - left subclavian artery
• 4 - left common carotid artery
• 5 - brachiocephalic trunk
• 6 - superior vena cava
• 7 - brachiocephalic veins
• 8 - thymus gland (upper extent in pre-pubertal individuals)
• 9 - right vagus nerve
• 10 - left vagus nerve
• 11 - right phrenic nerve
• 12 - left phrenic nerve
• Right and left recurrent laryngeal nerves arise from the corresponding vagus nerves at different levels
• The right recurrent laryngeal follows a much shorter, oblique course after looping around the right subclavian artery
• Technically, it's in the root of the neck, rather than superior mediastinum
• Vagus runs posterior to root of the lung
• Phrenic nerve runs anterior to root of the lung

Posterior Mediastinum

• The thoracic region located between the posterior pericardial sac wall and the posterior thoracic wall
• Relationships of major tubular thoracic organs in the region of the tracheal bifurcation are important
• The trachea bifurcates at roughly the T4/T5 level (sternal angle)
• An aortic arch aneurism can impinge on the esophagus, deviating or constricting, making swallowing difficult
• A large bolus of food can impinge on the posterior wall of the trachea, requiring the Heimlich maneuver
• An esophageal tumor can impinge on the tracheal lumen, making breathing difficult

Esophagus Narrowing Points

• Pharyngeal junction
• Aortic arch
• Tracheal bifurcation
• Esophageal hiatus of diaphragm
• Relationships of the vagus nerves in the thorax are clinically important
• The right vagus passes anterior to the subclavian artery, giving off the right recurrent laryngeal nerve (motor to the larynx)
• The nerve lies directly on the apex of the lung
• A pancoast tumor in the lung may result in hoarseness
• The left vagus passes anterior to the aortic arch (lateral to the ligamentum arteriosum) near the tracheal bifurcation
• The left recurrent laryngeal nerve passes posteriorly
• An aortic aneurism could compress this nerve, resulting in persistent hoarseness
• Vagal nerves contribute preganglionic parasympathetic fibers to the esophageal, pulmonary and cardiac plexuses

Visceral Branches of the Aorta

• Important in this region
• Paired bronchial arteries: supply the primary and secondary bronchi
• Casual esophageal arteries: arise from the descending aorta, supply the length of the thoracic esophagus

Parietal Branches of the Aorta

• Paired posterior intercostal arteries: supply the intercostal spaces and body wall
• Paired superior phrenic arteries: supply the diaphragm

Contents of the Posterior Mediastinum

• Descending thoracic aorta
• Sympathetic trunks
• Splanchnic nerves
• The "6 birds of the thoracic cage": esopha-goose, azy-goose, hemiazy-goose, and the left/ right va-goose, thoracic "duck"

Anatomy structures

• Sympathetic trunk
• Azygos vein
• Sympathetic (paravertebral) ganglion
• Ramus communicans
• Intercostal vein, artery and nerve
• Greater splanchnic nerve
• Thoracic duct

Lymphatic Drainage of the Torso

• The body's lymphatic drainage is accomplished through a complex system of lymphatic vessels
• Vessels drain tissue fluid and large macromolecules from tissue spaces and the gastrointestinal tract
• Lymph from the right upper limb, right side of head & neck, and right thorax empties via the right lymphatic duct
• It is emptied into the junction of the right internal jugular and right subclavian veins
• The majority of other lymph (remainder) is drained via the thoracic duct into a similar venous junction on the left side
• Thoracic duct begins in the abdomen at the cisterna chyli (anterior to celiac trunk)
• Lymph from the abdomen and lower limbs collects before passing up through the duct in the posterior mediastinum
• In the posterior mediastinum, the thoracic duct lies just posterior to the esophagus and between the aorta and azygous vein

Posterior Thoracic Wall (Intercostal Space)

• Diagram shows an oblique section of the thorax illustrating the thoracic wall layers
• Arterial blood supply is on the right side, innervation on the left
• Venous drainage parallels arterial supply within the intercostal space
• Posterior intercostal arteries arise from the descending aorta
• They extend into the intercostal space and run forward with the spinal nerve's ventral ramus (intercostal nerve) and the intercostal vein
• About 3/4 of the way forward, the posterior intercostal artery anastomoses with the anterior intercostal artery branch, derived from the internal thoracic artery

Entrance into Intercostal Space

• The ventral ramus of the spinal nerve (intercostal nerve) passes through the space between the innermost and internal intercostal muscles
• The posterior intercostal artery and vein exit via the same pathway
• It takes a few centimeters for the intercostal neurovascular bundle to arrange itself in the typical order: vein, artery, nerve (top to bottom)

Intercostal Space

• Consists of three muscle layers and the neurovascular bundle (between the ribs)
• A costal groove on the rib's inferior aspect semi-protects the neurovascular bundle (vein, artery, nerve)
• The intercostal nerve is the least protected

Venous Drainage of the Posterior Thoracic Wall

• Thoracic/abdominal wall venous drainage is indirect and segmental
• The azygous and hemiazygous receive venous drainage from posterior intercostal, lumbar segmental, and superior intercostal veins, as well as bronchial, esophageal, and mediastinal veins
• Anterior aspects of the thoracic wall are drained by anterior intercostal veins to the internal thoracic veins
• The veins lie on either side of the internal thoracic artery in a venae commitantes arrangement below R3

Azygous System

• The azygous system drains each side separately, but the two halves join in a broken H pattern
• The left-side system is formed by the hemiazygous vein and accessory hemiazygous vein
• The right-side, the azygous vein, drains the thoracic and posterior abdominal walls
• Hemiazygous drains to the azygous vein at between T8 and T10
• Accessory hemiazygous drains to the right side at about T7
• The superior intercostal vein drains the upper 3 or 4 interspaces
• On the left: drains to brachiocephalic vein
• On the right: drains into the azygous vein's arch

Questions about the anatomical structures, vessels, nerves and boundaries of the superior mediastinum. Includes its location, what vessels branch of the aortic arch and the consequence of nerve damage. Also includes surgical considerations.