Mediastinum

Thorax Anatomy

Mediastinum

Definition, boundaries, contents, divisions, superior, anterior, middle and posterior mediastinum, with clinical anatomy.

1

Definition

Mediastinum (plural—mediastina) (Latin: intermediate) is the middle space left in the thoracic cavity in-between the lungs.

2

Boundaries of Mediastinum

Anterior
Sternum
Posterior
Vertebral column
Superior
Thoracic inlet
Inferior
Diaphragm
On each side (lateral)
Mediastinal pleurae
3

Contents of Mediastinum

Viscera

Thymus, heart enclosed in pericardial sac, trachea, oesophagus

Vessels

Thoracic aorta, pulmonary trunk, superior and inferior vena cava

Lymphatics

Lymph nodes, thoracic duct

Nerves

Vagus nerves, sympathetic trunks, and phrenic nerves

Other

Mediastinal pleurae

4

Divisions of Mediastinum

For descriptive purposes, the mediastinum is divided into the superior mediastinum and the inferior mediastinum by an imaginary plane passing through:

  • Sternal angle anteriorly
  • Lower border of the body of the 4th thoracic vertebra posteriorly

The inferior mediastinum is further divided into the anterior, middle, and posterior mediastina by the pericardium:

Anterior mediastinum

Area in front of the pericardium

Middle mediastinum

Pericardium and its contents

Posterior mediastinum

Area behind the pericardium

5

Superior Mediastinum

Boundaries

Anteriorly
Manubrium sterni
Posteriorly
Upper four thoracic vertebrae
Superiorly
Plane of the thoracic inlet
Inferiorly
An imaginary plane passing through the sternal angle in front, and the lower border of the body of the 4th thoracic vertebra behind
On each side
Mediastinal pleura

Contents

1Trachea and oesophagus

2Arteries

  • Arch of aorta
  • Brachiocephalic artery
  • Left common carotid artery
  • Left subclavian artery

3Veins

  • Right and left brachiocephalic veins
  • Upper half of the superior vena cava
  • Left superior intercostal vein

4Nerves

  • Vagus
  • Phrenic
  • Cardiac nerves of both sides
  • Left recurrent laryngeal nerve

5Thymus

6Muscles

  • Sternohyoid
  • Sternothyroid
  • Lower ends of longus colli

7Thoracic duct

8Lymph nodes

Paratracheal, brachiocephalic, and tracheobronchial

6

Anterior Mediastinum

  • Very narrow space in front of the pericardium, overlapped by the thin anterior borders of both lungs.
  • Continuous through the superior mediastinum with the pretracheal space of the neck.
  • Contains areolar tissue and part of thymus gland.

Boundaries

Anteriorly
Body of sternum
Posteriorly
Pericardium
Superiorly
Imaginary plane separating the superior mediastinum from the inferior mediastinum
Inferiorly
Superior surface of diaphragm
On each side
Mediastinal pleura

Contents

1

Sternopericardial ligaments

2

Lymph nodes with lymphatics

3

Small mediastinal branches of the internal thoracic artery

4

The lowest part of the thymus

5

Areolar tissue

7

Middle Mediastinum

Occupied by the pericardium and its contents, along with the phrenic nerves and the pericardiacophrenic vessels.

Boundaries

Anteriorly
Sternopericardial ligaments
Posteriorly
Oesophagus, descending thoracic aorta, azygos vein
On each side
Mediastinal pleura

Contents

1Heart enclosed in pericardium

2Arteries

  • Ascending aorta
  • Pulmonary trunk
  • Two pulmonary arteries

3Veins

  • Lower half of the superior vena cava
  • Terminal part of the azygos vein
  • Right and left pulmonary veins

4Nerves

  • Phrenic
  • Deep cardiac plexus

5Lymph nodes

Tracheobronchial nodes

8

Posterior Mediastinum

Boundaries

Anteriorly
  • Pericardium
  • Bifurcation of trachea
  • Pulmonary vessels
  • Posterior part of the upper surface of the diaphragm
Posteriorly
Lower eight thoracic vertebrae and intervening discs
On each side
Mediastinal pleura

Contents

1Oesophagus

2Arteries

Descending thoracic aorta and its branches

3Veins

  • Azygos vein
  • Hemiazygos vein
  • Accessory hemiazygos vein

4Nerves

  • Vagi
  • Splanchnic nerves: Greater, lesser and least, arising from the lower eight thoracic ganglia of the sympathetic chain

5Lymph nodes and lymphatics

  • Posterior mediastinal lymph nodes lying alongside the aorta
  • The thoracic duct
9

Clinical Anatomy

Prevertebral layer of deep cervical fascia

  • Extends to the superior mediastinum, attached to the fourth thoracic vertebra.
  • An infection present in the neck behind this fascia can pass down into the superior mediastinum but not lower down.

Pretracheal fascia of neck

  • Extends to the superior mediastinum, where it blends with the arch of the aorta.
  • Neck infections between pretracheal and prevertebral fasciae can spread into superior mediastinum, and through it into posterior mediastinum.
  • Thus, mediastinitis can result from infections in the neck.

Mediastinum — as a potential dead space

  • There is very little loose connective tissue between mobile organs of mediastinum; space can be readily dilated by inflammatory fluids, neoplasms, etc.

Superior Mediastinum

  • In superior mediastinum, all large veins are on the right side and arteries on the left side.
  • During increased blood flow, veins expand enormously, while large arteries do not expand at all.
  • Thus, there is much “dead space” on the right side; tumours or fluids of mediastinum tend to project into this space.

Widening of Mediastinum

  • Observed in chest radiographs.
  • Causes: Haemorrhage from lacerated blood vessels, enlarged lymph nodes, enlarged heart (hypertrophy), and so on.
Mediastinum — Study Notes