Mediastinum PDF Lecture Notes

Mediastinum  The mediastinum: a median partition between the 2 pleural sacs which extends from the sternum in front to the vertebral column behind and from the thoracic inlet above to the diaphragm below.  Divisions of the mediastinum: ❖ The mediastinum is divided by an imaginary line...

Mediastinum  The mediastinum: a median partition between the 2 pleural sacs which extends from the sternum in front to the vertebral column behind and from the thoracic inlet above to the diaphragm below.  Divisions of the mediastinum: ❖ The mediastinum is divided by an imaginary line passed between the sternal angle of Louis anteriorly and the disc between T4 and T5 vertebrae posteriorly into two main compartments: I) Superior mediastinum: Boundaries ▪ Anterior: manubrium sterni. ▪ Posterior: upper 4 thoracic vertebrae. ▪ Superior: thoracic inlet. ▪ Inferior: an imaginary line between the angle of Louis and the disc between T4 and T5. Contents: A) Retromanubrial structures: (fatty lymphatic & venous layers) ▪ These are structures lies immediately behind the manubrium sterni and arranged from anterior to posterior as follows:  1) Remains of thymus gland: After puberty it undergoes gradual involution and replaced by fibro-fatty tisuues. 2) Fat and anterior mediastinal lymph nodes. 3) Right braciocephalic vein: descends vertically behind the right margin of the manubrium. 4) Left braciocephalic vein: runs obliquely from left to right above the arch of the aorta, crossing the origins of its 3 branches. 5) Upper ½ of SVC: formed by the union of the right and left brachiocephalic veins, descends vertically behind the right margin of the manubrium. 6) Right phrenic nerve on the right side of right brachio-cephalic vein & superior vena cava. B) Intermediate structures:( 2 arches & 4 nerves) 1) Arterial layer: Arch of aorta and its 3 branches. (brachiocephalic, left common carotid and left subclavian arteries). 2) Left phrenic & left vagus nerves: descend on the left side of aortic arch. 3) Left recurrent laryngeal nerve: it arises from the left vagus nerve and curves around the lower aspect of the arch of the aorta. 4) Superficial cardiac plexus below arch of aorta. 5) Arch of azygos. C) Prevertebral structures:(3 tubes, 1 nerve) 1) Trachea with its right vagus on its right side. It descends behind the arch of aorta and its branches. 2) Oesophagus : It descends posterior to the trachea. 3) Thoracic duct: ascends on the left border of oesophagus. 4) Left recurrent laryngeal nerve ascends in between the trachea and oesophagus.  Structures of superior mediastinum can be classified from anterior to posterior into fatty lymphatic layer, venous layer, arterial layer, tubular layer.  Contents of the superior mediastinum II) Inferior mediastinum: ❖ Lies below a line drawn from the angle of Louis to the disc between T4 and T5 vertebrae. ❖ It is divided into: a) Anterior mediastinum: It is a narrow space between body of sternum and pericardium. It contains: (2 lymphatic tissues & 2 ligaments) Fat, lymph nodes, lower part of thymus gland Superior and inferior sternopericardial ligaments. b) Middle mediastinum: It lies between anterior and posterior mediastinum. Contents: 1) Pericardium and structure inside it: heart, ascending aorta, pulmonary trunk, lower 1/2 of SVC, terminal parts of IVC and 4 pulmonary veins. 2) Structures outside pericardium: Bifurcation of trachea into two bronchi. Deep cardiac plexus in front of tracheal bifurcation Tracheobronchial lymph nodes around tracheal bifurcation. Arteries: right and left pulmonary arteries.  Veins: 2 right and 2 left pulmonary veins. Phrenic nerves on each side of pericardium. c) Posterior mediastinum: Lies between pericardium and vertical part of diaphragm anteriorly, and lower 8 thoracic vertebrae posteriorly. Contents: 1) Oesophagus: on right side of descending thoracic aorta till the level of T7 vertebra where it crosses in front of descending thoracic aorta. 2) 2 vagi forming oesophageal nerve plexuses. 3) Descending thoracic aorta and its branches: It descends on left side of esophagus till the level of T7 vertebra then crosses behind it. 4) Azygos vein and 2 hemiazygos veins cross from left to right. 5) Thoracic duct: Passes behind the right border of oesophagus. 6) Posterior mediastinal lymph nodes. Posterior mediastinum  Superior & Posterior mediastinum Mediastinum (right lateral view) Mediastinum (Left lateral view)