Practical 5: Posterior Mediastinum 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 & hemiazygos veins, thoracic duct, esophagus, sympathetic trunk, intercostal vessels (posterior), and vagus nerves.

Azygos System of Veins

• The arrangement of azygos veins is variable.
• The azygos system connects to thoracic, abdominal, and back veins.
• These connections provide alternative drainage routes if the superior vena cava (SVC) or inferior vena cava (IVC) are blocked.
• Accessory hemiazygos and hemiazygos veins cross over vertebrae T7 and T9 to join the azygos vein.

Thoracic Duct

• Lymph from the right thorax, upper limb, head, and neck drains into the right lymphatic duct.
• The rest of the body's lymph drains into the thoracic duct.
• Lymph trunks drain into corresponding venous angles (junction of internal jugular and subclavian veins).
• Chyle (fatty lymph) from the intestines enters the cisterna chyli at the L1 level, then drains into the thoracic duct.
• The thoracic duct ascends through the aortic hiatus, anterior to the vertebrae, between the azygos vein and the esophagus/aorta.
• It veers to the left at T4/5 level and drains into the left venous angle.

Sympathetic Chain

• The sympathetic chain originates from T1 to L2 spinal nerves.
• It's needed at every spinal level to innervate tissues.
• Paravertebral sympathetic ganglia run parallel to the spinal cord, alongside the vertebrae.
• Sympathetic nerves enter at T1-L2, travel up and down, and connect with other spinal nerves to reach their targets.
• Nerves leave the T1-L2 ventral rami and join the chain via white rami communicantes.
• Nerves leave the chain and join spinal nerves via gray rami communicantes.
• White rami are myelinated, and grey rami are unmyelinated.
• There are only three pairs of cervical ganglia due to the merging of the original eight cervical ganglia.
• The most inferior ganglion is called the ganglion impar.
• This ganglion fuses with its counterpart in the midline.

Descending Thoracic Aorta

• The descending thoracic aorta runs from T4 to T12, posterior to the root of the left lung and esophagus.
• It descends toward the midline and passes through the aortic hiatus of the diaphragm at the T12 level.

Descending Thoracic Aorta: Branches

• Costocervical trunk (branch of subclavian artery) gives off a superior intercostal branch.
• Posterior and subcostal arteries run from T3 to T11.
• Bronchial arteries arise indirectly from posterior intercostal arteries (right side).
• Unpaired branches arising anteriorly supply esophageal, mediastinal tissues, and lymph nodes (pericardial).
• Superior phrenic arteries, paired, run on the superior surface of the diaphragm.

Thoracic Aortic Aneurysms & Aortic Dissection

• An aneurysm is a ballooning of a weakened part of the aorta.
• Thoracic aneurysms are less common than abdominal aortic aneurysms (AAAs).
• Older men are more likely to have AAAs (6% risk) than older women (1% risk).
• Risk factors include atherosclerosis, high blood pressure (hypertension), trauma, and connective tissue disorders like Marfan's syndrome.

Aortic Dissection

• Aortic dissection involves a tear in the tunica intima, allowing blood into the intima-media space.
• The tear propagates, creating a rapidly expanding false lumen.
• This is often accompanied by sudden, tearing chest pain.

Pancoast Tumour and Compression of Sympathetic Trunk

• Lung cancer in the apex of the lung (Pancoast tumor) can compress the brachial plexus and upper T1 sympathetics.
• This can cause shoulder pain and potential upper limb pain/weakness.
• Compression of upper sympathetics can lead to Horner's syndrome (miosis, ptosis, anhidrosis).

Oesophagus

• The esophagus is located posterior to the trachea, pericardium, and left atrium.
• It descends toward the left and then deviates to the left.
• The esophageal plexus is formed by vagus nerves (parasympathetic) and sympathetic nerves from the sympathetic trunk.
• These nerves form anterior and posterior vagal trunks that pass through the esophageal hiatus at the T10 level.

Dysphagia (difficulty swallowing)

• Dysphagia is investigated via fluoroscopy and barium swallow.
• Problems like tumors, ulcers, or esophageal webs can cause compression or narrowing of the esophagus, making swallowing difficult.
• Hiatal hernias occur when a part of the stomach herniates through the diaphragm.

Tracheoesophageal Fistula & Oesophageal Atresia

• Developmental issues in the septum separating tracheal & esophageal development can cause the conditions.
• These conditions often involve abnormal connections (fistula) and/or blocked passages (atresia).
• These conditions may be fatal without early surgical intervention.