Mediastinum
Mediastinum(plural—mediastina)(Latin intermediate) is the middle space left in the thoracic cavity in between the lungs. Its most important content is the heart, enclosed in the pericardium in the middle part of the inferior mediastinum or the middle mediastinum. Above it lies superior mediastinum. Anterior and post- erior to the heart are anterior mediastinum and posterior mediastinum, respectively.
The mediastinum is the median septum of the thorax between the two lungs. It includes the mediastinal pleurae.
Competency achievement: The student should be able to:
AN 21.11 Mention boundaries and contents of the superior, anterior, middle and posterior mediastinum.1
SUPERIOR AND INFERIOR MEDIASTINA
Boundaries
Anteriorly: Sternum
Posteriorly: Vertebral column Superiorly: Thoracic inlet Inferiorly: Diaphragm
On each side: Mediastinal pleura.
Divisions
For descriptive purposes, the mediastinum is divided into the superior mediastinum and the inferior medi- astinum. The inferior mediastinum is further divided into the anterior, middle and posterior mediastina (Fig. 17.1).
The superior mediastinum is separated from the inferior by an imaginary plane passing through the sternal angle anteriorly and the lower border of the
Fig. 17.1: Subdivisions of the mediastinum
body of the fourth thoracic vertebra posteriorly. The inferior mediastinum is subdivided into three parts by the pericardium. The area in front of the pericardium is the anterior mediastinum. The area behind the pericardium is the posterior mediastinum. The pericardium and its contents form the middle medi- astinum.
SUPERIOR MEDIASTINUM
Boundaries
Anteriorly: Manubrium sterni (Fig. 17.1) Posteriorly: Upper four thoracic vertebrae Superiorly: Plane of the thoracic inlet
Mediastinum
DISSECTION
Reflect the upper half of manubrium sterni upwards and study the boundaries and contents of superior and three divisions of the inferior mediastinum.
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Section 2
Thorax
Inferiorly: An imaginary plane passing through the sternal angle in front, and the lower border of the body of the fourth thoracic vertebra behind.
On each side: Mediastinal pleura. Contents
1 Trachea and oesophagus
2 Muscles: Origins of (i) sternohyoid, (ii) sterno-
thyroid, (iii) lower ends of longus colli.
3 Arteries: (i) Arch of aorta, (ii) brachiocephalic artery, (iii) left common carotid artery, (iv) left subclavian
artery (Fig. 17.2).
4 Veins: (i) Right and left brachiocephalic veins,
(ii) upper half of the superior vena cava, (iii) left
superior intercostal vein.
5 Nerves: (i) Vagus, (ii) phrenic, (iii) cardiac nerves of
both sides, (iv) left recurrent laryngeal nerve.
6 Thymus
7 Thoracic duct
8 Lymph nodes: Paratracheal, brachiocephalic, and
tracheobronchial.
Fig. 17.2: Arrangement of the large structures in the superior mediastinum. Note the relationship of superior vena cava, ascending aorta and pulmonary trunk to each other in the middle mediastinum, i.e. within the pericardium. The bronchi are not shown
INFERIOR MEDIASTINUM
The inferior mediastinum is divided into—anterior, middle and posterior mediastina.
Anterior Mediastinum
Anterior mediastinum is a very narrow space in front of the pericardium, overlapped by the thin anterior borders of both lungs. It is continuous through the
Fig. 17.3: Some structures present in superior, middle and posterior mediastina
MEDIASTINUM
superior mediastinum with the pretracheal space of the neck. It 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 (Fig. 17.1)
2 Lymph nodes with lymphatics
3 Small mediastinal branches of the internal thoracic
artery
4 The lowest part of the thymus
5 Areolar tissue.
Middle Mediastinum
Middle mediastinum is 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 (see Figs 15.2 and 15.3) On each side: Mediastinal pleura
Contents
1 Heart enclosed in pericardium (Fig. 17.2)
2 Arteries: (i) Ascending aorta, (ii) pulmonary trunk,
(iii) two pulmonary arteries (Fig. 17.3)
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Section 2
Thorax
280
THORAX
3 Veins: (i) Lower half of the superior vena cava, (ii) terminal part of the azygos vein, and (iii) right and left pulmonary veins.
4 Nerves: (i) Phrenic, and (ii) deep cardiac plexus.
5 Lymph nodes: Tracheobronchial nodes.
Posterior Mediastinum
Boundaries
Anteriorly: (i) Pericardium, (ii) bifurcation of trachea, (iii) pulmonary vessels, and (iv) posterior part of the upper surface of the diaphragm.
Posteriorly: Lower eight thoracic vertebrae and intervening discs.
On each side: Mediastinal pleura.
Contents
1 Oesophagus (Fig. 17.4).
2 Arteries: Descending thoracic aorta and its branches.
3 Veins: (i) Azygos vein, (ii) hemiazygos vein, and (iii) accessory hemiazygos vein.
4 Nerves: (i) Vagi, (ii) splanchnic nerves, greater, lesser and least, arising from the lower eight thoracic ganglia of the sympathetic chain (see Fig. 15.1).
Fig. 17.4: Structures in the posterior part of the superior media- stinum, and their continuation into the posterior mediastinum. Note the relationship of the arch of the aorta to the left bronchus, and that of the azygos vein to the right bronchus
5 Lymph nodes and lymphatics:
a. Posterior mediastinal lymph nodes lying along-
side the aorta.
b. The thoracic duct (Fig. 17.4).
CLINICAL ANATOMY
• The prevertebral layer of the deep cervical fascia extends to the superior mediastinum, and is 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.
The pretracheal fascia of the neck also extends to the superior mediastinum, where it blends with the arch of the aorta. Neck infections between the pretracheal and prevertebral fasciae can spread into the superior mediastinum, and through it into the posterior mediastinum. Thus mediastinitis can result from infections in the neck.
• Thereisverylittlelooseconnectivetissuebetween the mobile organs of the mediastinum. Therefore, the space can be readily dilated by inflammatory fluids, neoplasms, etc.
• In the superior mediastinum, all large veins are on the right side and the arteries on the left side. During increased blood flow, veins expand enormously, while the large arteries do not expand at all. Thus there is much ‘dead space’ on the right side and it is into this space that tumour or fluids of the mediastinum tend to project (Fig. 17.5).
• Compression of mediastinal structures by any tumour gives rise to a group of symptoms known as mediastinal syndrome. The common symptoms are as follows:
a Obstruction of superior vena cava gives rise to engorgement of veins in the upper half of the body.
b. Pressure over the trachea causes dyspnoea, and cough.
c. Pressure on oesophagus causes dysphagia.
d. Pressure or the left recurrent laryngeal nerve gives rise to hoarseness of voice (dysphonia).
e. Pressure on the phrenic nerve causes paralysis of the diaphragm on that side.
f. Pressure on the intercostal nerves gives rise to pain in the area supplied by them. It is called intercostal neuralgia.
Section 2
Thorax
Mnemonics
1 From Medical Council of India, Competency based Undergraduate Curriculum for the Indian Medical Graduate, 2018;1:44–80.
MEDIASTINUM
281
"BATS AND TENT" Brachiocephalic veins Arch of aorta
Thymus
Superior vena cava Trachea
Esophagus
Nerves (vagus and phrenic) Thoracic duct
Fig. 17.5: Large vessels in relation to heart
g. Pressure on the vertebral column may cause erosion of the vertebral bodies.
The common causes of mediastinal syndrome are bronchogenic carcinoma, Hodgkin’s disease causing enlargement of the mediastinal lymph nodes, aneurysm or dilatation of the aorta, etc.
FACTS TO REMEMBER
• Mediastinum is the middle space between the lungs.
• It is chiefly occupied by the heart enclosed in pericardium with blood vessels and nerves.
• Unit structures in the superior mediastinum are trachea, oesophagus, left recurrent laryngeal nerve between the two tubes and thoracic duct on the left of the oesophagus.
CLINICOANATOMICAL PROBLEM
A patient presents with lots of dilated veins in the front of chest and anterior thoracic wall.
• What is the reason for so many veins seen on the anterior body wall?
• Howdoesvenousbloodgobackincirculation?
Ans: This appears to be a case of blockage of superior vena cava after the entry of vena azygos. The blood needs to return to heart and it is done through inferior vena cava. The backflow occurs:
Superior vena cava blockagebrachiocephalic veinssubclavian veinsaxillary veinslateral thoracic veins thoracoepigastric veins superficial epigastric veinsgreat saphenous veins femoral veinscommon iliac veinsinferior vena cavaright atrium of heart
Many veins open up to assist the drainage.
Superior mediastinum contents: PVT Left BATTLE
Phrenic nerve
Vagus nerve
Thoracic duct
Left recurrent laryngeal nerve (not the right) Brachiocephalic veins
Aortic arch (and its 3 branches) Thymus
Trachea
Lymph nodes
Esophagus