Mediastinum

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 mediasteinum 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.

Superior mediastinum

The superior mediastinum is bounded by:

• 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

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 esophagus thoracic duct remains of the thymus some lymph glands anterior longitudinal ligament

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.

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

Structures at the level of the thoracic plane

The start and end of the aortic arch The division between the superior and inferior mediastinum The upper margin of the superior vena cava The crossing of the thoracic duct The bifurcation of the trachea The bifurcation of the pulmonary trunk The level of the sternal angle The level of Rib 2 where it attaches to the sternum via the 2nd costal cartilage The body of vertebrae T4(the disc between the vertebra T4and T5) The drainage of the azygos vein into the superior vena cava

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Inferior mediastinum

Anterior mediastinum

Bounded:

• Laterally by the pleurae
• Posteriorly by the pericardium
• Anteriorly by the sternum, the left transversus thoracis and the fifth, sixth, and seventh left costal cartilages

Contents
Contents A quantity of loose areolar tissue Some lymphatic vessels which ascend from the convex surface of the liver Two or three anterior mediastinal lymph nodes The small mediastinal branches of the internal thoracic artery Thymus (involuted in adults)

Middle mediastinum

Bounded: pericardial sac

Contents
Contents the heart enclosed in the pericardium the ascending aorta the lower half of the superior vena cava with the azygos vein opening into it the bifurcation of the trachea and the two bronchi the pulmonary artery dividing into its two branches the right and left pulmonary veins the phrenic nerves some bronchial lymphatic glands pericardiocophrenic vessels

Posterior mediastinum

It is bounded:

• anteriorly by the pericardium (in front of)
• inferiorly by the thoracic surface of the diaphragm (below).
• superiorly by the transverse thoracic plane
• posteriorally by the bodies of the vertebral column from the lower border of the fifth to the twelfth thoracic vertebra (behind).
• laterally by the mediastinal pleura (on either side)

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 elsewhence (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.

There are many diseases that can present with a widened mediastinum (usually found via a chest x-ray). The most common ones are aortic unfolding, traumatic aortic rupture, thoracic aortic aneurysm, and traumatic thoracic vertebral fracture. A widened mediastinum is a classic but rare hallmark sign of anthrax infection.

See also

• Widened mediastinum
• Mediastinum testis (unrelated structure in the scrotum)
• Mediastinal germ cell tumor
• Mediastinitis
• Anthrax
• Mediastinal tumor

References

This article incorporates text in the public domain from the 20th edition of Gray's Anatomy (1918)

Additional images

• 

  A transverse section of the thorax, showing the contents of the middle and the posterior mediastinum. The pleural and pericardial cavities are exaggerated since normally there is no space between parietal and visceral pleura and between pericardium and heart.

External links

• 1691353147 at GPnotebook
• Anatomy figure: 21:01-03 at Human Anatomy Online, SUNY Downstate Medical Center - "Divisions of the mediastinum."
• Anatomy figure: 21:02-03 at Human Anatomy Online, SUNY Downstate Medical Center - "The anatomical divisions of the inferior mediastinum."
• thoraxlesson3 at The Anatomy Lesson by Wesley Norman (Georgetown University) - "Subdivisions of the Thoracic Cavity"