Superior, Anterior & posterior mediastinum

subdivisions of mediastinum

  • The superior mediastinum is located superior to the transverse thoracic plane passing through the sternal angle and the T4/5 IV disc.

  • Inferior part is further divided into three parts:

    • Anterior, Middle, Posterior

anterior mediastinum

  • The anterior mediastinum contains the inferior portion of the thymus gland in infants & children.

    • Lymph nodes

    • Sternopericardial ligaments

  • It is anterior to the pericardium and posterior to the body of the sternum

contents of superior mediastinum

  • from anterior to posterior

  • Thymus

  • Great vessels

    • Brachiocephalic veins and SVC

    • Arch of aorta and roots of:

    • Brachiocephalic trunk

    • Left common carotid artery

    • Left subclavian artery

  • Phrenic nerves and vagus nerves

  • Cardiac nerve plexus

  • Left recurrent laryngeal nerve

  • Trachea and esophagus with nerves

  • Thoracic duct and lymphatic trunks

thymus

  • thymus occupies the most anterior part of the superior mediastinum, just posterior to the sternum.

  • After puberty, it gradually degrades and is replaced by fat.

  • Blood supply is from branches of the internal thoracic.

veins of the superior mediastinum

  • The brachiocephalic veins are formed by the union of the subclavian and internal jugular veins bilaterally.

    • They unite at the 1st costal cartilage to form the superior vena cava.

    • Left brachiocephalic vein is twice as long as the right, passes anterior to the arch of the aorta and great vessels.

aortic arch branches & angiogram

  • Aortic arch begins posterior to the 2nd right sternocostal joint (level of sternal angle). It arches superiorly, posteriorly to the left, and then inferiorly.

  • In most (65%) individuals, the typical branches arising from the aortic arch are the brachiocephalic trunk, the left common carotid and left subclavian arteries.

great vessels

CT scan of thorax: superior mediastinum

superior mediastinum nerves

  • The phrenic nerves lie on the anterior surface of the anterior scalene muscles as they pass inferiorly into the thorax.

  • vagus nerves, and sympathetic nerves, form the carotid, pulmonary, and esophageal plexuses.

recurrent laryngeal nerves

  • Left recurrent laryngeal nerve arises from the vagus nerve and wraps underneath the aortic arch just lateral to the ligamentum arteriosum, ascends to the larynx between the trachea and esophagus.

  • Clinical note: aortic aneurysm, bronchogenic or esophageal carcinoma may compress the left recurrent laryngeal nerve causing a hoarse voice.

  • Left recurrent laryngeal is in the superior mediastinum.

  • right recurrent laryngeal nerve arises from the right vagus nerve and wraps underneath the right subclavian artery before ascending to the larynx.

    • It does not descend to the superior mediastinum

a deep dissection of superior mediastinum

contents of posterior mediastinum

  • Thoracic aorta

  • Thoracic duct

  • Posterior mediastinal lymph nodes (tracheobronchial)

  • Azygos and hemiazygos veins

  • Esophagus and plexus

anterior view of thoracic aorta

  • Thoracic aorta begins at lower border of T4 body on left side. Approaching median plane as it descends to T12 vertebrae and passes through aortic hiatus.

  • Note the mediastinal, bronchial and esophageal, and paired branches.

  • Numbers 1-12 identify the paired posterior intercostal arteries and paired subcostal arteries.

esophagus & nerve plexus

  • Esophagus is fibromuscular tube from pharynx to the stomach. Lies anterior to vertebral bodies. Passes through superior to posterior mediastinum.

  • Esophageal plexus: Branching fibers from both left and right vagus nerves mix in the esophageal plexus and then form trunks.

  • The anterior vagal trunk arises from the left vagus nerve.

  • The right vagus nerve gives rise to the posterior vagal trunk.

esophageal nerves

  • Parasympathetic innervation is from vagus nerves.

  • Sympathetic innervation is from upper thoracic ganglia.

axygous venous system

  • Azygos vein forms a collateral pathway between SVC and IVC, and drains posterior wall of thorax and abdomen.

  • Forms from junction of subcostal and ascending lumbar vein at the L1 or L2 vertebral level.

    • Receives intercostal, accessory, and hemiazygos veins.

    • Arches over root of right lung and drains to the SVC.

  • Accessory hemiazygos: Crosses at T8 to empty into the azygos.

  • Hemiazygos: crosses at T9 to empty into the azygos.

  • Clinical note: azygos vein can provide a route for venous return to the heart if either SVC or IVC is obstructed.

thoracic duct

  • The thoracic duct (largest lymphatic vessel) begins at the cisterna chyli (L2 vertebral level).

  • Appears beaded due to valves.

  • Ascends to the right of midline between azygos vein and the aorta.

  • Traverses the diaphragm via the aortic hiatus.

  • At vertebral level T4, T5, or T6 it crosses to the left of midline and enters the superior mediastinum.

  • Empties on the left side at the junction of the internal jugular and left subclavian veins.

  • Clinical note: thoracic duct injury can lead to chylothorax (lymph in pleural cavity)

thoracic & right lymphatic duct

  • right lymphatic duct empties into the junction of the internal jugular and subclavian.

  • The right lymphatic duct drains right upper limb, head, neck, thorax.

  • Thoracic duct carries lymph drained from the rest of the body.

sympathetic trunks

  • Splanchnic nerves pass into the abdominal cavity carrying preganglionic sympathetic fibers to prevertebral ganglia.

  • Sympathetic ganglia and trunks. The ganglia are not in the posterior mediastinum.