Comprehensive Mediastinum Anatomy Notes

Mediastinum Anatomy Notes

Objectives

• Understand the subdivisions of the thoracic cavity and their contents.
• Describe the subdivisions of the mediastinum and their contents.
• Discuss the lymphatic drainage of the thorax and the course of the thoracic duct.
• Recognize the distribution of the vagus nerves in the thorax.
• Describe the thoracic sympathetic trunk, ganglia, and splanchnic nerves.
• Describe the relations of the esophagus in the thorax.
• Identify major structures of the heart and mediastinum on CT images at levels T4/5 and T8.

The Mediastinum

The mediastinum is the central compartment of the thoracic cavity. It contains the heart and its vessels, esophagus, trachea, phrenic and cardiac nerves, thoracic duct, thymus, and lymph nodes.

Subdivisions of the Mediastinum

The mediastinum is divided into superior and inferior mediastinum. The inferior mediastinum is further divided into anterior, middle, and posterior mediastinum.

• Superior Mediastinum: Located above the sternal angle (T4/T5).
• Anterior Mediastinum: Located between the sternum and the pericardium.
• Middle Mediastinum: Contains the pericardium, heart, ascending aorta, pulmonary trunk, terminal parts of pulmonary veins, lower half of the superior vena cava, and terminal part of the inferior vena cava.
• Posterior Mediastinum: Located behind the pericardium, contains the descending thoracic aorta, esophagus, thoracic duct, vagus nerves, sympathetic chains, azygos, and hemiazygos veins.

Superior Mediastinum Layers

The superior mediastinum can be organized into layers:

• Retromanubral Layer: Contains the remnant of the thymus gland.
• Venous Layer: Includes the right and left brachiocephalic veins and the upper part of the superior vena cava (SVC). The right phrenic nerve is also in this layer.
• Arterial Layer: Contains the aortic arch and its branches, bifurcation of the pulmonary trunk, ligamentum arteriosum, and the left vagus and phrenic nerves. Also includes the left recurrent laryngeal nerve.
• Tubal Layer: Contains the trachea, esophagus, right vagus nerve, thoracic duct, and tracheobronchial lymph nodes.
• Prevertebral Sympathetic Trunk Layer: Contains the sympathetic chain, azygos vein, and longus coli muscle.

Anterior Mediastinum

Contains:

1. Remains of the thymus.
2. Superior and inferior sternopericardial ligaments.
3. Mediastinal branches of the internal thoracic artery.

Middle Mediastinum

Contains:

1. Pericardium and structures inside it:
   • Heart
   • Ascending aorta
   • Pulmonary trunk
   • Terminal parts of right and left pulmonary veins
   • Lower half of superior vena cava
   • Terminal part of inferior vena cava

Posterior Mediastinum

Contains:

• Descending thoracic aorta
• Esophagus
• Thoracic duct
• Right and left vagi
• Right and left sympathetic chains
• Azygos, superior hemiazygos, and inferior hemiazygos veins

Thoracic Duct

• Origin: Upper end of cisterna chyli at the upper border of T12.
• Course: Enters the thorax through the aortic opening and traverses the posterior mediastinum behind the esophagus. Opposite T5, it inclines to the left and runs upwards.
• Termination: At the root of the neck, in the angle formed by the junction of the left subclavian and internal jugular veins.

Sympathetic Trunk (Chain)

• Formed by sympathetic ganglia and the communications between them.
• Present in the thorax and extends up to the neck and down to the abdomen and pelvis.
• The thoracic part contains 12 ganglia (sometimes 11 due to fusion of ganglia 11 and 12).
• The first thoracic ganglion may be fused with the inferior cervical one (75-80% of people) to form the stellate ganglion.
• Location: Ganglia are located posteriorly in the thorax, close to the heads of the ribs or at the sides of vertebral bodies.

Thoracic Sympathetic Trunks & Splanchnic Nerves

• Greater splanchnic nerve: T5-T9
• Lesser splanchnic nerve: T10-T11
• Least splanchnic nerve: T12

Horner Syndrome

Due to injury of the sympathetic chain. Causes:

1. Ptosis (eyelid drop).
2. Myosis (constriction of the pupil).
3. Anhydrosis.
4. Enophthalmos.

Arch of Aorta

• Origin, Course, and Termination:
  • Begins at the aortic orifice opposite the 3rd left intercostal space.
  • Ends behind the 2nd right sternocostal junction, becoming the arch of the aorta.
• Branches: Right and left coronary arteries.

Aortic Arch

• Origin, Course, and Termination:
  • Begins at the 2nd right sternocostal junction.
  • Passes upwards, backwards, and to the left.
  • Ends at T4 and T5, becoming the descending thoracic aorta.
• Branches:
  • Brachiocephalic artery
  • Left common carotid artery
  • Left subclavian artery
  • Thyroidea ima (may arise from the arch to supply the thyroid gland)

Descending Aorta

• Origin, Course, and Termination:
  • Begins at the disc between T4 and T5 as the continuation of the arch of the aorta.
  • Descends obliquely downwards and to the right, first on the left side of the bodies of the upper thoracic vertebrae (T5-T7), then in front of the lower thoracic vertebrae (T8-T12).
  • Ends at the lower border of T12, passing through the aortic opening of the diaphragm to continue as the abdominal aorta.
• Branches:
  1. Posterior intercostal arteries (from 3rd to 11th).
  2. Subcostal arteries.
  3. Two left bronchial arteries.
  4. Four or five esophageal branches.
  5. Small twigs to the pericardium, mediastinum, and diaphragm.

Applied Anatomy

• Patent Ductus Arteriosus (PDA)
• Aortic Aneurysm
• Coarctation of the Aorta

Clinically Important Points

1. Coarctation of the Aorta: Narrowing which may involve the segment from the left subclavian artery to the ductus arteriosus. If the obstruction is distal to the ligamentum arteriosum, collateral circulation to the lower limb is maintained via branches of the subclavian arteries and those of the descending aorta.
2. Aortic Aneurysm: Localized dilatation of the aorta which may compress the contents of the superior mediastinum, causing mediastinal syndrome. Rupture can cause severe, often fatal, hemorrhage.
3. Pulmonary Embolism: Embolism of the pulmonary trunk by a blood clot is a common cause of sudden death.

Vagus Nerve

Phrenic Nerve

• Root value: C3, 4, and 5.
• Arises in the neck and enters the thorax via the inlet.

Course of Right Phrenic Nerve (Venous)

Descends on the right side of:

• Right innominate vein
• Superior vena cava
• Right atrium (separated by the pericardium)
• Inferior vena cava
• In front of the root of the right lung

Passes through the IVC opening of the diaphragm to be distributed on the inferior surface of the diaphragm.

Course of Left Phrenic Nerve (Arterial)

Descends between left common carotid and subclavian arteries, in front of the arch of the aorta, in front of the root of the left lung, and on the side of the left ventricle. Pierces the diaphragm.

Mediastinal Syndrome

Compression of the mediastinum by a tumor, inflammatory mass, lymph nodes, or retrosternal goiter leads to:

1. Dyspnea (compression of the trachea).
2. Dysphagia (compression of the esophagus).
3. Hoarseness of voice (compression of the left recurrent laryngeal nerve).
4. Ischemia (compression of the aorta).
5. Non-pitting edema (compression of the thoracic duct).