Posterior Mediastinum Anatomy Review

Practical 5: Posterior Mediastinum & Review

• Objectives: Identify and describe the azygos system of veins, the anatomy and course of the thoracic duct, and the sympathetic chain.
• Posterior Mediastinum Contents: Descending thoracic aorta, azygos and hemiazygos veins, thoracic duct, esophagus, sympathetic trunk, and intercostal vessels (posterior). Vagal nerves are also mentioned.

Azygos System of Veins

• Arrangement: The arrangement of azygos veins is highly variable.
• Connections: The azygos system connects to thoracic, abdominal, and back veins, providing alternate drainage routes if the superior vena cava (SVC) or inferior vena cava (IVC) are obstructed.
• Accessory Hemiazygos Vein: Accessory and hemiazygos veins cross over the vertebrae at T7 and T9, connecting to the azygos vein.

Thoracic Duct

• Drainage Route: Lymph from most of the body drains to the thoracic duct. The right lymphatic duct drains lymph from the right thorax, upper limb, head, and neck.
• Drainage Vessels: Lymph trunks drain into corresponding venous angles (junction of internal jugular vein and subclavian vein).
• Chyle: Fat and lymph (chyle) from the intestine, collected in the cisterna chyli at the L1 level, drains superiorly into the thoracic duct.

Sympathetic Chain

• Structure: A pair of ganglia runs along the sympathetic chain at every spinal level except for three paired cervical ganglia which have fused into one.
• Levels: Originates from spinal nerves T1 to L2.
• Paths: Nerves enter at T1-L2, travel up or down the chain, exiting and joining other spinal nerves.
• White & Gray Rami Communicantes: Nerves leave the T1-L2 ventral rami and join spinal nerves through white rami communicantes, which are myelinated. Nerves leave the sympathetic chain and join the spinal nerves through gray rami communicantes, which are unmyelinated.

Pancoast Tumor & Compression of Sympathetic Trunk

• Location: Lung cancer (Pancoast tumor) in the apex of the lung can compress the brachial plexus and the upper thoracic sympathetic chain (T1).
• Signs: This compression might cause shoulder pain, upper limb pain, and weakness.
• Horner's Syndrome: Compression symptoms might include ptosis (drooping eyelid), miosis (constricted pupil), and anhidrosis (lack of sweating).

Descending Thoracic Aorta

• Course: Runs posterior to the root of the left lung and esophagus, descending toward the midline through the aortic hiatus of the diaphragm at the T12 level.
• Branches: Costocervical trunk, posterior intercostals, subcostals, bronchial, esophageal, mediastinal, and superior phrenic arteries.

Aortic Aneurysms and Dissections

• Aneurysms: Weak spots in the aorta (ballooning) can be asymptomatic until rupture, dissection, or compression of nearby structures.
• Types: Aortic root and ascending aorta, aortic arch, and descending aorta are common types. Abdominal aortic aneurysms (AAAs) are more prevalent than thoracic.
• Risk Factors: Atherosclerosis, hypertension, trauma, and connective tissue disorders like Marfan's syndrome can affect the aorta.
• Dissections: A tear in a vessel's inner wall, allowing blood flow into the middle layer, can cause sudden, tearing chest pain.

Other Pleural Cavity Masses

• Fluid Types: Pleural fluid can involve blood (hemothorax), pus (empyema or pyothorax), fluid rich in proteins and fibrin (serothorax), abdominal organs in the chest (enterothorax), pleural fibrosis (fibrothorax), oil (oleothorax), or urine (urinothorax). Faeces as a mass in the pleural cavity is also mentioned.
  • Causes for some pleural cavity masses include trauma, malignancy, anticoagulant medication, aortic dissection.
• Combines: Some pleural conditions might combine different causes such as pyopneumothorax (pus and air).

Oesophagus

• Position: Located posterior to the trachea, pericardium, and left atrium; it deviates to the left and travels to the T10 level before passing through the oesophageal hiatus.
• Plexuses: Vagus nerves form anterior and posterior oesophageal plexuses by interacting with sympathetic branches. These plexuses deliver sensory and motor innervation to the esophagus, impacting pain sensation.

Tracheoesophageal Fistula & Atresia

• Septum: The trachea and esophagus are separated by a septal development that occurs during fetal development. Abnormalities in this can lead to atresia, a failure to develop a passage between structures, and/or fistulae, where a passage has developed improperly between two structures. There are different types of fistula depending on which structures are abnormally connected.
• Malformations: These congenital malformations were often fatal before surgical intervention.

Chylothorax

• Cause: Injury to the thoracic duct, whether from malignancy, infection, trauma or iatrogenic surgery, can lead to leakage of lymphatic fluid into the pleural cavity, causing a chylothorax.
• Symptoms: This can cause compression and collapse of the lung, hypovolemia, and immunosuppression. Management might be by conservative means with the condition potentially resolving spontaneously, or surgically by ligating the duct to stop the leak.