Abstract

The mediastinum (from Medieval Latin "mediastinus", "midway") is the central compartment of the thoracic cavity surrounded by loose connective tissue, as an undelineated region that contains a group of structures within the thorax. The mediastinum contains the heart and its vessels, the esophagus, trachea, phrenic and cardiac nerves, the thoracic duct, thymus and lymph nodes of the central chest.

Structure

The mediastinum lies within the thorax and is enclosed on the right and left by pleurae. It is surrounded by the chest wall in front, the lungs to the sides and the spine at the back. It extends from the sternum in front to the vertebral column behind, and contains all the organs of the thorax except the lungs. It is continuous with the loose connective tissue of the neck.

The mediastinum can be divided into an upper (or superior) and lower (or inferior) part:

- The superior mediastinum starts at the superior thoracic aperture and ends at the thoracic plane.

- The thoracic plane separates the superior and inferior mediastinum. It is a plane at the level of the sternal angle, and the intervertebral disc of T4–T5.

- The inferior mediastinum from this level to the diaphragm. This lower part is subdivided into three regions, all relative to the pericardium – the anterior mediastinum being in front of the pericardium, the "middle mediastinum" contains the pericardium and its contents, and the "posterior mediastinum" being behind the pericardium.

Anatomists, surgeons, and clinical radiologists compartmentalize the mediastinum differently. For instance, in the radiological scheme of Felson, there are only three compartments (anterior, middle, and posterior), and the heart is part of the anterior mediastinum.

Structure | Superior mediastinum

The superior mediastinum is bounded:

- "superiorly" by the thoracic inlet, the upper opening of the thorax;

- "inferiorly" by the transverse thoracic plane;

- "laterally" by the pleurae;

- "anteriorly" by the manubrium of the sternum;

- "posteriorly" by the first four thoracic vertebral bodies.

- Contents

- muscles

- origins of the Sternohyoidei and Sternothyreoidei

- lower ends of the Longi colli

- arteries

- aortic arch

- brachiocephalic artery

- thoracic portions of the left common carotid and the left subclavian

- veins

- brachiocephalic veins and

- upper half of the superior vena cava

- left highest intercostal vein

- nerves

- vagus nerve

- cardiac nerve

- superficial and deep cardiac plexuses

- phrenic nerve

- left recurrent laryngeal nerve

- trachea with paratracheal and tracheobronchial lymph nodes

- esophagus

- thoracic duct

- remains of the thymus

- some lymph glands

- anterior longitudinal ligament

Structure | Thoracic plane

A number of structures occur at the level of the thoracic plane, which divides the superior and inferior mediastinum:

Structure | Inferior mediastinum

Is bounded:

Bounded: pericardial sac – It contains the vital organs and is classified into the serous and fibrous pericardium.

Is bounded:

Clinical significance

The mediastinum is frequently the site of involvement of various tumors:

- "Anterior mediastinum": substernal thyroid goiters, lymphoma, thymoma, and teratoma.

- "Middle mediastinum": lymphadenopathy, metastatic disease such as from small cell carcinoma from the lung.

- "Posterior mediastinum": Neurogenic tumors, either from the nerve sheath (mostly benign) or elsewhere (mostly malignant).

Mediastinitis is inflammation of the tissues in the mediastinum, usually bacterial and due to rupture of organs in the mediastinum. As the infection can progress very quickly, this is a serious condition.

Pneumomediastinum is the presence of air in the mediastinum, which in some cases can lead to pneumothorax, pneumoperitoneum, and pneumopericardium if left untreated. However, that does not always occur and sometimes those conditions are actually the cause, not the result, of pneumomediastinum. These conditions frequently accompany Boerhaave's syndrome, or spontaneous esophageal rupture.

Clinical significance | Widened mediastinum

"Widened mediastinum/mediastinal widening" is where the mediastinum has a width greater than 6 cm on an upright PA chest X-ray or 8 cm on supine AP chest film.

A widened mediastinum can be indicative of several pathologies:

- aortic aneurysm

- aortic dissection

- aortic unfolding

- aortic rupture

- hilar lymphadenopathy

- anthrax inhalation - a widened mediastinum was found in 7 of the first 10 victims infected by anthrax ("Bacillus anthracis") in 2001.

- esophageal rupture - presents usually with pneumomediastinum and pleural effusion. It is diagnosed with water-soluble swallowed contrast.

- mediastinal mass

- mediastinitis

- cardiac tamponade

- pericardial effusion

- thoracic vertebrae fractures in trauma patients.