Posterior mediastinum Flashcards
What is the role of the pulmonary ligament
To allow expansion of the lung
Summarise the difference between the paths of the phrenics and the vagi
Phrenics- in front of lung hilum
Vagi- behind lung hilum
Why can we not see the right recurrent laryngeal at T3 in cross-section
it loops from vagus at the right subclavian artery- which has not yet formed
we can see the left however as this loops from the vagus at the ligamentum arteriosum
List the contents of the posterior mediastinum
Oesophagus Descending aorta Thoracic duct Azygos venous system Posterior mediastinal lymph nodes Thoracic sympathetic trunks Splanchnic nerves
Summarise the course of the oesophagus
Begins at level of C7 vertebra Ends at stomach, level of T11 vertebra Bends more anteriorly at T7 Is right of aorta above T7 Deviates to left at T7 Progressively anterior to aorta below T7 Passes through diaphragm at T10 Has constrictions at four locations
Describe the course of the oesophagus
Muscular tube passing between the pharynx in the neck and the stomach in the abdomen
Begins at the inferior border of the cricoid cartilage and ends at the cardiac opening of the stomach- opposite T11
Descends on anterior aspect of bodies, generally in a midline position as it moves through the thorax
as it approaches the diaphragm, it moves anteriorly and to the left, crossing from the right side of the thoracic aorta to assume a position anterior to it
it then passes through the oesophageal hiatus at T10
Ultimately has a slight anterior-to-posterior curvature that parallels the thoracic portion of the vertebral column and is secured superiorly by its attachment to the pharynx and inferiorly by its attachment to the diaphragm
Describe the relations to the oesophagus
Right side- mediastinal part of parietal pleura
posterior- thoracic duct on the right inferiorly but crosses to the left more superiorly
also posterior and to the left is the thoracic aorta
anterior- below the level of tracheal bifurcation are the right pulmonary artery and the left main bronchus
the oesophagus then passes immediately posterior to the left atrium, separated from it only by the pericardium
other structures that pass posterior include portions of the hemiazygos veins, the right posterior intercostal vessels and near the diaphragm, the thoracic aorta.
Describe the constrictions of the oesophagus
Thin, flexible, muscular tube that can be compressed or narrowed by surrounding structures at four locations:
junction of oesophagus with pharynx in the neck
in the superior mediastinum- where the esophagus is crossed by the arch of the aorta
posterior mediastinum- oesophagus is compressed by the left main bronchus and at the oesophageal hiatus
Describe the clinical consequences of these constrictions
A swallowed object is more likely to lodge at a constricted area
An ingested corrosive substance would move more slowly through a narrowed region, causing more damage at this site than elsewhere
Also, constrictions present problems during the passage of instruments
Describe arterial supply to the oesophagus
Involves many vessels
esophageal arteries from the thoracic aorta, bronchial arteries, and ascending branches of the left gastric artery in the abdomen.
Describe venous drainage of the oesophagus
Involves small vessels returning to the azygos vein, hemiazygos vein and oesophageal branches of the left gastric vein in the abdomen
lymphatic drainage of the oesophagus returns to the posterior mediastinal and left gastric nodes
Summarise the azygos system of veins
A series of longitudinal veins on each side that drain blood from the body wall and move it superiorly to empty into the SVC
Blood from thoracic viscera may also enter the system and can form anastomotic connections with abdominal veins
the longitudinal vessels may or may not be continuous and are connected to each other from side to side at various points
serves as an important site capable of returning venous blood from the lower part of the body to the heart if the IVC is blocked
Describe the course of the azygos vein
Arises opposite L1 or L2 at the junction between the right ascending lumbar vein and the right subcostal vein
May also arise as a direct branch of the IVC
Enters thorax through the aortic hiatus or enters posterior or through the right crus of the diaphragm
Ascends through the posterior mediastinum, usually to the right of the thoracic duct
At T5, it arches anteriorly over the root of the right lung to join the SVC before it enters the pericardial sac
Describe the tributaries of the azygos vein
right superior intercostal vein ( a single vessel formed by the junction of the second, third, and fourth intercostal veins)
fifth to eleventh right intercostal arteries
hemiazygos veins
accessory hemiazygos vein
esophageal, right bronchial, pericardial and mediastinal veins
Describe the course of the hemiazygos vein
Usually arises at the junction of the left ascending lumbar vein and the left subcostal vein- can arise form either of these veins alone and often has a connection to the left renal vein
the hemiazygos vein usually enters the thorax through the left crus of the diaphragm but may enter through aortic hiatus.
Ascends on the left side of the posterior mediastinum, to around T9
Here it crosses the vertebral column posterior to the thoracic aorta, oesophagus, and thoracic duct, to reach the azygos vein
List the tributaries of the hemiazygos vein
lowest four or five posterior intercostal veins
esophageal veins
mediastinal veins
Describe the course of the accessory hemiazygos vein
Descends from the left side from the superior portion of the superior mediastinum to approximately T8.
At this point, it crosses the vertebral column to join the azygos vein, or ends in the hemiazygos vein, or has a connection to both
Usually, it has a connection superiorly to the left superior intercostal vein
Which vessels drain into the accessory hemiazygos
fourth to eight left posterior intercostal veins
sometimes, the left bronchial veins
Describe the oesophageal plexus
After passing posteriorly to the root of the lungs, the right and left vagus nerves approach the esophagus
As they reach the oesophagus, each nerve divides into several branches that spread over the structure, forming the oesophageal plexus
There is some mixing of fibres from the two vagus nerves as the plexus continues inferiorly on the oesophagus toward the diaphragm
Just above the diaphragm- the fibres converge to form two trunks
Describe the two trunks of the oesophageal plexus
Anterior vagal trunk- anterior surface- mainly from fibres originally in the left vagus nerve
Posterior vagal trunk- posterior surface of the oesophagus- mainly from fibres originally in the right vagus nerve
The vagal trunks continue in the surface of the oesophagus as it passes through the diaphragm and into the abdomen.
Describe the innervation of the oesophagus
Striated muscle fibres in the superior portion of the oesophagus originate from the brachial arches and are innervated by brachial efferents from vagus nerves
Smooth muscle fibres are innervated by components of the PSNS part of the autonomic division of the PNS, visceral efferents from vagus nerves
These are preganglionic fibres that synapse in the myenteric and submucosal plexuses of the enteric nervous system
Describe sensory afferents from the oesophagus
Involves visceral afferent fibres originating in the vagus nerves, sympathetic trunk and splanchnic nerves
visceral afferents from vagus nerve- relaying info back to the CNS about normal physiological processes and reflex activities- not involved in relay of pain
visceral afferents from SNS trunk and splanchnic nerves- primary participants in detection of oesophageal pain and transmission of this information to various levels of the CNS
Summarise the vagus nerves in the posterior mediastinum
branches to chest and abdomen are parasympathetic (control smooth and cardiac muscle and glands of gut/airways); also sensory from gut/lungs; recurrent laryngeal nerve NOT parasympathetic - run back up to neck to supply most skeletal muscles of larynx
Summarise the sympathetic trunk in the posterior mediastinum
receive branches from T1-L2, distributing sympathetic nerves to smooth muscle and glands throughout body; nerves to wall synapse in ganglia, and nerves to viscera synapse in local ganglia; also bring pain fibres from viscera to CNS; T5-T12 reach abdomen in splanchnic nerve bundles
Describe the branches of the vagus nerve in the posterior mediastinum
Branches to chest and abdomen are parasympathetic (control smooth and cardiac muscle + glands of gut and airways)
Also large sensory from gut and lungs
Recurrent laryngeal nerve not parasympathetic – run back up neck to supply most skeletal muscles of larynx
Summarise the phrenic nerves in the posterior mediastinum
Phrenics (C3, 4, 5) pass anterior to lung roots and cross pericardium to diaphragm
Outline the role of the sympathetic trunks in the posterior mediastinum
Receive branches from spinal nerves T1 - L2
Distribute sympathetic nerves to smooth muscle and glands throughout body
Nerves to body wall synapse in ganglia of trunks
Nerves to internal organs (viscera) synapse in local ganglia
Also bring pain fibres back to CNS from viscera
Fibres from lower T5 - T12 reach abdomen in bundles called splanchnic nerves
Describe the arrangement of the sympathetic trunks in the posterior mediastinum
Superior portion- anterior to the neck of the ribs
Inferiorly, they become more medial in position until they lie on the lateral aspect of the vertebral bodies
The sympathetic trunks leave the thorax by passing posterior to the diaphragm under the medial arcuate ligament or through the crura of the diaphragm
Throughout their course- the trunks are covered by parietal pleura
Describe the two types of medial branches given off by the ganglia
first five- postganglionic sympathetic fibres which supply the various thoracic viscera
next seven- preganglionic sympathetic fibres which supply the abdominal and pelvic viscera- splanchnic nerves
Summarise the thoracic duct in the posterior mediastinum
Lymph duct returning lymph from lower limbs, pelvis, abdomen & left thoracic wall to blood
Begins below diaphragm at cisterna chyli
Starts between oesophagus and aorta on right
Crosses behind oesophagus to left side between T7 and T4
Drains into left brachiocephalic vein
What does the thoracic duct drain
confluence of lymph trunks in abdomen
descending thoracic lymph trunks draining the lower six or seven intercostal spaces on both sides
upper intercostal lymph trunks draining the upper five or six intercostal spaces
ducts from posterior mediastinal nodes
ducts from posterior diaphragmatic nodes
Receives left jugular and left subclavian trunk before emptying into the junction between the left subclavian and left internal jugular veins
Describe coarctation of the aorta
Congenital abnormality in which the aortic lumen is constricted just distal to the origin of the left subclavian artery
At this point, the aorta becomes significantly narrowed and the blood supply to the lower limbs and abdomen is diminished
Over time, collateral vessels develop around the chest wall and abdomen to supply the lower body
The coarctation also affects the heart, which has to pump blood at higher pressures to maintain the peripheral perfusion- this may produce cardiac failure
