3.0 Superior Posterior Mediastina (Exam 3)

Superior Mediastinum

• Superior mediastinum sits above the sternal angle
• The largest features are the great vessels and the trachea
• Nerves here continue inferiorly and will be discussed with the posterior mediastinum

Viscera in Superior Mediastinum

• Prevertebral includes trachea, esophagus, thoracic duct and left recurrent laryngeal n.
• Intermediate includes aortic arch, brachiocephalic trunk, left common carotid a., left subclavian a.
• Retrosternal includes thymus, left brachiocephalic v. and the superior part of the SVC

Cross Section of Superior Mediastinum

• Anterior to posterior relationships are important for identification here
• Relative locations:
  • 1 - esophagus (most posterior)
  • 2 - trachea (anterior to esophagus)
  • 3 - left subclavian a.
  • 4 - left common carotid a.
  • 5 - brachiocephalic trunk
  • 6 - superior vena cava
  • 7 - brachiocephalic vv.
  • 8 - thymus gland (its upper extent in pre-pubertal individuals) (most anterior)
  • 9 - right vagus n.
  • 10 - left vagus n.
  • 11 - right phrenic n.
  • 12 - left phrenic n.

Recurrent Laryngeal Nerves

• Right and left recurrent laryngeal nn. come from the corresponding vagus nn.
• The right recurrent laryngeal follows a shorter, more oblique course after looping around the right subclavian a.
• It is not found within the superior mediastinum, but at the root of the neck

Clinical Relations - Vagus & Phrenic Nerves

• Relationship between the vagus and phrenic nerves and the root of the lung is important
• Phrenic nn. run anterior to root of the lung
• Vagus nn. run posterior to root of the lung

Posterior Mediastinum

• The posterior mediastinum is the thoracic region between posterior wall of the pericardial sac and the posterior thoracic wall
• Relationships of major tubular thoracic organs near the tracheal bifurcation is important
• Trachea bifurcates at T4/T5 (sternal angle)
• Aortic arch can impinge on the esophagus and compress/deviate it, making swallowing difficult
• Bolus of food can impinge on posterior wall of trachea, requiring Heimlich
• Esophageal tumor can impinge on the tracheal lumen
• The esophagus is narrowed at the following regions: pharyngeal junction, aortic arch, tracheal bifurcation, and esophageal hiatus of the diaphragm
• The vagus nn. relationships in the thorax is important clinically

Clinical Relations - Vagus Nerve (Posterior Mediastinum)

• The right vagus passes anterior to subclavian artery, giving off right recurrent laryngeal n. (motor to the larynx), which sits on the apex of the lung
• Pancoast tumor in lung can result in hoarseness
• Left vagus passes anterior to aortic arch (lateral to ligamentum arteriosum) near tracheal bifurcation, giving off the left recurrent laryngeal n. posteriorly
• An aortic aneurism may compress this nerve resulting in hoarseness
• Vagal nn. contribute preganglionic parasympathetic fibers to esophageal, pulmonary and cardiac plexus

Aorta Branches

• Visceral branches that arise are the paired bronchial aa. which supply the primary and secondary bronchi
• Esophageal aa. arise from the descending aorta and supply the length of the thoracic esophagus
• Parietal branches include paired posterior intercostal aa., that supply the intercostal spaces and body wall, and the paired superior phrenic aa. to supply the diaphragm

Contents of Posterior Mediastinum

• Descending thoracic aorta
• Sympathetic trunks
• Splanchnic nn
• "6 birds of the thoracic cage," which are the esopha-goose, azy-goose, hemiazy-goose, left va-goose, right va-goose and the thoracic "duck"

Diagram of Mediastinum

• Shows the appearance of a dissecting approach viewing the heart with the posterior aspect of the pericardial sac removed

Lymphatic Drainage of Thorso

• Accomplished through lymphatic vessels draining tissue fluid from tissues spaces and GI tract
• Right lymphatic duct empties lymph from the right upper limb, right side of head & neck and the right side of the thorax into the junction of the right internal jugular and right subclavian vv.
• The rest of the body's lymph drains 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 here before passing up through the duct in the posterior mediastinum, where it lies posterior to the esophagus and between the aorta and azygous vein

Posterior Thoracic Wall

• Arterial blood supply is on the right side of the diagram and innervation on the left , venous drains parallels the arterial supply
• Posterior intercostal aa. arise from the descending aorta, extend into the intercostal space accompanied by ventral spinal nerve ramus (intercostal nerve) & the intercostal vein
• About 3/4 of the way forward through the intercostal space the posterior intercostal a. anastomoses with the anterior intercostal a. (a branch of the internal thoracic artery)

Entrance into Intercostal Space

• The ventral ramus of the spinal n. (intercostal n.), posterior intercostal artery and posterior intercostal vein enter/exit the intercostal space between the innermost intercostal mm. and the internal intercostal m. posteriorly - at the sides of the vertebral column.
• The intercostal neurovascular bundle order from top to bottom is Vein, Artery, Nerve

Intercostal Space Contents

• Consists of three muscle layers and the neurovascular bundle.
• A costal groove on the rib's inferior aspect semi-protects the neurovascular bundle (Vein, Artery and Nerve) from top to bottom, with the intercostal nerve the least protected

Venous Drainage of Posterior Thoracic Wall

• Venous drainage of thoracic and abdominal walls is indirect and segmental
• Azygous and hemiazyous receive venous drainage from posterior intercostal vv., lumbar segmental vv., superior intercostal vv. as well as bronchial, esophageal and mediastinal vv.
• Anterior aspects of the thoracic wall are drained by anterior intercostal vv. to the internal thoracic vv
• The azygous system drains each side separately but the two halves of the system join in a pattern resembling a broken H
• Left side formed by a hemiazygous v. and an accessory hemiazgyous v
• Right side is the azygous v. which drains the thoracic and posterior abdominal walls
• Hemiazygous typically drains to the azygous v. at between T8 and T10
• Accessory hemiazygous drains to the right side at about T7
• Superior intercostal v. drains the upper 3 or 4 interspaces. On the left it drains to the brachiocephalic v., on the right it drains into the arch of the azygous v.