1

Describe the boundaries and compartments of the mediastinum

The mediastinum represents the medial compartment of the thorax (Figure 1) and is bounded by the:

a)

left and right pleura (laterally);

b)

sternum and costal cartilages (anteriorly);

c)

vertebral column (posteriorly);

d)

thoracic inlet (superiorly);

e)

diaphragm (inferiorly).

The mediastinum itself can be further subdivided into compartments to aid differential diagnosis or plan surgical access:

a)

four-compartment model (Figure 2) – which is subdivided by a line drawn between the sternal angle of Louis and the inferior border of the T4 vertebrae (thoracic plane), and further by the parietal pericardium (see Chapter 35):

i)

superior mediastinum – between the thoracic inlet and thoracic plane;

ii)

anterior mediastinum – below the thoracic plane and anterior to the parietal pericardium;

iii)

middle mediastinum – below the thoracic plane and within the parietal pericardium. It also contains the carina and main bronchi;

iv)

posterior mediastinum – below the thoracic plane and posterior to the parietal pericardium;

b)

Felson’s three-compartment model (Figure 3) – where there is no superior compartment:

i)

anterior compartment – which passes from the thoracic inlet superiorly to the diaphragm inferiorly and is bounded posteriorly by the posterior wall of the trachea and anterior pericardium, and anteriorly by the posterior aspect of the sternum and costal cartilages;

ii)

middle compartment – which is bounded anteriorly by the anterior pericardium, posteriorly by the posterior pericardium and posterior wall of the trachea, and superiorly ends at the pericardial reflection;

iii)

posterior compartment – which is bounded anteriorly by the posterior aspect of the pericardium and trachea;

c)

Shield’s three-compartment model (Figure 4):

i)

prevascular compartment – which is bounded posteriorly by the anterior aspect of the pericardium and great vessels;

ii)

visceral compartment – which is bounded posteriorly by the anterior surface of the vertebral bodies and therefore occupies the thoracic inlet;

iii)

paravertebral compartment – which is the most posterior compartment and is split into the bilateral paravertebral sulci.

2

Describe the anatomical structures that are located on the thoracic plane (Figure 5)

Bony:

a)

manubriosternal junction (angle of Louis);

b)

inferior aspect of the T4 vertebrae;

c)

2nd costal cartilage.

Vascular:

a)

inferior aspect of the aortic arch;

b)

bifurcation of the main pulmonary artery;

c)

azygos vein arches over the right main bronchus and drains into the superior vena cava;

d)

ligamentum arteriosum.

Miscellaneous:

a)

bifurcation of the trachea at the carina;

b)

thoracic duct crosses from the right to the left side of the mediastinum;

c)

left recurrent laryngeal nerve loops around the aortic arch;

d)

deep and superficial cardiac plexuses.

3

Describe the anatomy of the thymus (Figure 6)

The thymus gland is a bilobed pyramidal-shaped structure, which largely lies in the superior and anterior mediastinum.

A loose capsule of connective tissue surrounds the gland and infiltrates into the cortex and medulla splitting the thymus into lobules. These infiltrations of capsule allow access for the blood and lymphatic vessels to supply the thymus.

The lower border roughly corresponds to the level of the 4th costal cartilage.

The upper border is often less defined, and the upper poles can reach the neck and occasionally the lower aspect of the thyroid gland.

Anatomical relations of the thymus include:

a)

anterior – sternum, costal cartilages, and origins of sternohyoid and sternothyroid muscles;

b)

posterior – pericardium, aortic arch, great vessels, trachea and brachiocephalic vein;

c)

lateral – pleura and phrenic nerves.

The arterial supply of the thymus includes:

a)

internal thoracic arteries;

b)

inferior thyroid arteries;

c)

superior thyroid arteries (in some cases).

The venous drainage of the thymus is via the posterior veins of Keynes which drain into the:

a)

brachiocephalic veins;

b)

internal thoracic veins;

c)

superior vena cava;

d)

inferior thyroid veins.

4

Describe the anatomy of the thoracic duct (Figure 7)

The thoracic duct is the largest lymphatic vessel in the body, with 75% of the lymphatic fluid of the body flowing via the duct into the venous system.

It has thin and beaded walls, due to the many valves and may be translucent in a starved patient.

The thoracic duct is the continuation of the cisterna chyli, which forms from the intestinal and two lumbar lymphatic trunks.

During its anatomical course, it:

a)

ascends to lie anterior to the 1st or 2nd lumbar vertebra;

b)

passes cranially through the aortic hiatus of the diaphragm to enter the posterior mediastinum;

c)

continues in the right hemithorax, lying between the descending thoracic aorta and the azygos vein;

d)

crosses into the left hemithorax at around the level of the 5th thoracic vertebra, anterior to the vertebral body and behind the oesophagus. The thoracic duct in this region is 2-3mm in diameter;

e)

travels behind the aortic arch adjacent to the left side of the oesophagus and behind the left subclavian artery;

f)

passes anteriorly, 3cm above the level of the left clavicle, crossing over the vertebral artery and vein, left sympathetic trunk, left phrenic nerve and then anterior to the subclavian artery in the anterolateral superior mediastinum.

It empties into the venous circulation at the superior aspect of the junction of the left internal jugular and subclavian veins, where it is related to the left common carotid artery and vagus nerve.

The course of the thoracic duct is variable, following the normal pattern in only 50% of patients. In some patients, two thoracic ducts exist within the mediastinum and occasionally, a single thoracic duct may empty into right-sided venous structures.

5

Describe the anatomy of the trachea (Figure 8)

The boundaries of the trachea are:

a)

superior – lower border of the cricoid cartilage in the larynx (C6 level);

b)

inferior – origin of the main stem bronchi (T4 level at end expiration to T6 at peak inspiration).

The total length of the trachea is 11-13cm and can be divided into:

a)

cervical part (approximately 5cm) – from the level of the cricoid cartilage to the level of the suprasternal notch;

b)

thoracic part (approximately 6-8cm) – below the level of the suprasternal notch.

The trachea is elliptical and flattened posteriorly.

The anterolateral 2/3rd (cartilaginous trachea) is reinforced with cartilage, whereas the posterior 1/3rd (membranous trachea) is constituted by outer longitudinal and inner transverse non-striated muscle fibres (Figure 9).

There are 18-22 tracheal rings of hyaline cartilage, which constitute the relatively fixed part of the trachea. The dynamic parts are the posterior wall and the muscles between the cartilage rings, which are responsible for change in the tracheal lumen size (Figure 10).

The trachea ends at the main carina, where it divides into the right and left main bronchi. The right main bronchus is wider, shorter and continues more vertically than the left. This makes inhalation of foreign bodies into the right side more common.

The trachea is lined by ciliated pseudostratified columnar epithelium, containing numerous goblet cells.

6

Describe the blood supply, venous and lymphatic drainage, and innervation of the trachea (Figure 11)

Arterial supply:

a)

cervical trachea – inferior thyroid arteries (shared with the oesophagus);

b)

thoracic trachea – bronchial arteries (shared with the oesophagus).

Venous drainage:

a)

entire length – inferior thyroid venous plexus, which drains into the brachiocephalic veins and then on to the superior vena cava.

Lymphatic drainage – via perivascular lymphatics and adjacent 1st lymph node groups:

a)

cervical trachea – inferior group of deep cervical lymph nodes;

b)

thoracic trachea – pretracheal, paratracheal and subcarinal lymph nodes.

Innervation:

a)

parasympathetic nerve supply – vagus nerve;

b)

sympathetic nerve supply – sympathetic trunk;

c)

rich sensory innervation – which is linked to coughing.

7

What are the important anatomical relations of the trachea (Figure 12)?

Anteriorly:

a)

cervical part:

i)

thyroid isthmus;

ii)

anastomosis of the two superior thyroid arteries;

iii)

inferior thyroid veins;

iv)

anterior jugular vein;

v)

jugular arch in the suprasternal space of Burns;

b)

thoracic part:

i)

manubrium;

ii)

remnant of thymus;

iii)

left innominate vein;

iv)

aortic arch;

v)

innominate artery;

vi)

left common carotid artery;

vii)

deep cardiac plexus.

Posteriorly:

a)

cervical part:

i)

oesophagus;

ii)

recurrent laryngeal nerves in the tracheo-oesophageal groove;

b)

thoracic part:

i)

oesophagus;

ii)

left recurrent laryngeal nerve.

Laterally:

a)

cervical part:

i)

medial surfaces of the lateral lobes of the thyroid;

ii)

inferior thyroid arteries;

iii)

internal jugular veins;

iv)

common carotid arteries;

b)

thoracic part:

i)

pleura (bilaterally);

ii)

superior vena cava (right side);

iii)

right vagus nerve (right side);

iv)

arch of azygos vein (right side);

v)

left common carotid artery (left side);

vi)

left subclavian artery (left side);

vii)

phrenic nerve (left side);

viii)

vagus nerve (left side);

ix)

aortic arch (left side).

8

Describe the anatomical course of the oesophagus (Figure 13)

The oesophagus is a muscular tube (approximately 25cm in length) that is divided into three parts:

a)

cervical oesophagus – which extends from the pharynx at the lower border of the cricoid cartilage (at the level of the C6 vertebra) to the thorax (at the level of the T1/2 vertebrae);

b)

thoracic oesophagus – which descends along the front of the vertebral column, through the superior and posterior mediastinum until it passes through the diaphragm (at the level of the T10 vertebra);

c)

abdominal oesophagus – which ends at the cardiac orifice of the stomach (at the level of the T11 vertebra).

9

Describe the anatomical layers of the oesophagus (Figure 14)

The oesophagus has four layers:

a)

adventitia – which is a bed of fat and connective tissue. Unlike the gastrointestinal tract, the oesophagus has no serosal layer except in its abdominal portion;

b)

muscular layer (tunica muscularis or muscularis propria) – which is composed of inner circular and outer longitudinal muscle fibres;

c)

submucosal or areolar layer – which contains loose connective tissue with blood vessels, nerves and mucous glands;

d)

internal mucosal layer (tunica mucosa) – which is composed of epithelium, lamina propria and muscularis mucosae in longitudinal folds that disappear on distension of the oesophagus. The oesophagus is lined by non-keratinising squamous epithelium, except after the squamocolumnar junction (also known as the Z line), after which the epithelium is glandular (lower few centimetres of the oesophagus). The mucosal layer has the strength to hold sutures and, hence, is the most important layer to close when repairing perforations of the oesophagus.

The muscular layer of the oesophagus includes:

a)

upper 1/3rd – striated muscle;

b)

middle 1/3rd – mixed striated and smooth muscle;

c)

lower 1/3rd – smooth muscle.

10

Describe the neural innervation of the oesophagus

The striated muscle part of the oesophagus is innervated by somatic neurones originating from the nucleus ambiguus, which travel within the vagus nerve.

The smooth muscle of the oesophagus is innervated by fibres originating from the dorsal motor nucleus of the vagus.

There are two neural plexi within the body of the oesophageal wall:

a)

myenteric plexus (Auerbach’s plexus) – which lies between the circular and longitudinal muscular layers and contains both parasympathetic vagal fibres and sympathetic fibres (from the cervical and thoracic sympathetic ganglia) to supply both layers of the tunica muscularis;

b)

submucosal plexus (Meissner’s plexus) – which lies within the submucosal layer of the oesophageal wall and contains only parasympathetic fibres to provide secretomotor innervation to the nearby oesophageal mucosa.

11

Describe the anatomical relations of the oesophagus (Figure 15)

Cervical oesophagus:

a)

trachea (anterior);

b)

prevertebral muscles, prevertebral fascia overlying the C6-C8 vertebrae (posterior);

c)

carotid sheath, including the common carotid artery, internal jugular vein and vagus nerve (lateral);

d)

lower poles of the thyroid gland (lateral);

e)

thoracic duct (left side at the level of the C6 vertebrae).

Thoracic oesophagus:

a)

trachea, right pulmonary artery, aortic arch, left main bronchus, left atrium, diaphragm (anterior);

b)

vertebral column, thoracic duct, descending thoracic aorta (posterior);

c)

azygos vein (right side).