1

Describe the anatomy of the mediastinum

The mediastinum represents the medial compartment of the thorax 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).

It is conventionally divided for the purposes of gross description, based on a hypothetical plane level with the angle of Louis, rather than an actual fascial boundary, into compartments (Figure 1):

a)

above the angle of Louis – superior mediastinum;

b)

below the angle of Louis – anterior, middle and posterior mediastinum.

Superior mediastinum – which extends from the thoracic inlet (lower edge of the 1st rib) to the angle of Louis (lower edge of the T4 vertebral body). It is further subdivided into three distinct fascial compartments, which are important in understanding the spread of infection (such as descending mediastinitis) or malignancy:

a)

prevascular space;

b)

retrovascular or pretracheal space;

c)

peripharyngooesophageal space.

Beneath the angle of Louis, the mediastinum is subdivided into three compartments (by the parietal pericardium), bounded inferiorly by the diaphragm:

a)

anterior mediastinum – which lies between the sternum and costal cartilages anteriorly, and parietal pericardium posteriorly;

b)

middle mediastinum – which lies within the confines of the parietal pericardium;

c)

posterior mediastinum – which lies between the parietal pericardium anteriorly and vertebral bodies posteriorly, extending laterally into the paravertebral sulci.

2

What are the contents of the mediastinal compartments?

Superior mediastinum:

a)

aortic arch, and brachiocephalic, left common carotid and left subclavian arteries;

b)

superior vena cava and brachiocephalic veins;

c)

vagus, left recurrent laryngeal and phrenic nerves;

d)

oesophagus;

e)

trachea;

f)

thoracic duct;

g)

thymus;

h)

lymph nodes.

Anterior mediastinum:

a)

thymus;

b)

lymph nodes;

c)

mediastinal fat.

Middle mediastinum:

a)

heart;

b)

ascending aorta;

c)

superior vena cava;

d)

main pulmonary artery (bifurcating into the left and right pulmonary arteries);

e)

left and right pulmonary veins;

f)

carina and main bronchi;

g)

phrenic nerves;

h)

lymph nodes.

Posterior mediastinum:

a)

descending aorta;

b)

azygos, hemiazygos and accessory hemiazygos veins;

c)

sympathetic chain, vagus nerve, splanchnic nerves and spinal nerve roots;

d)

oesophagus;

e)

thoracic duct;

f)

lymph nodes.

3

What are the commonest causes of a mediastinal mass in each compartment in adults?

Superior mediastinum:

a)

aortic arch aneurysm;

b)

lymphadenopathy – lymphoma, sarcoidosis, tuberculosis;

c)

thymic lesion – thymoma, thymic carcinoma;

d)

germ cell tumour;

e)

upper oesophageal lesion – carcinoma or cyst;

f)

retrosternal goitre;

g)

parathyroid adenoma.

Anterior mediastinum:

a)

thymic tumour;

b)

lymphadenopathy – lymphoma, sarcoidosis, tuberculosis;

c)

germ cell tumour;

d)

retrosternal goitre;

e)

parathyroid adenoma.

Middle mediastinum:

a)

lymphoma;

b)

bronchogenic cyst;

c)

pleuropericardial cyst;

d)

proximal aortic aneurysm (aortic root or ascending aorta);

e)

tracheal tumour.

Posterior mediastinum:

a)

neurogenic tumour;

b)

enteric cyst;

c)

oesophageal lesion – carcinoma or cyst;

d)

diaphragmatic hernia – hiatus hernia or Bochdalek hernia;

e)

descending aortic aneurysm;

f)

meningocoele.

In adults, the commonest cause of a mediastinal mass (in all compartments) is:

a)

nerve sheath tumour – 21%;

b)

thymoma – 19%;

c)

cyst – 18%;

d)

lymphoma – 13%;

e)

germ cell tumour – 10%.

4

What are the clinical features of a patient with a mediastinal mass?

Non-specific symptoms including cough, chest pain, dyspnoea and malaise.

Localising features of compression may include superior vena cava syndrome, dysphagia, dyspnoea, Horner’s syndrome, voice hoarseness or paralysis.

Clinical features of myasthenia gravis, including clumsiness, diplopia, dysarthria and dysphagia later in the day.

Clinical features of lymphoma, including palpable axillary, cervical, supraclavicular or inguinal lymph nodes.

Testicular mass, which may be present in a patient with lymphoma or a germ cell tumour.

In addition, the patient may present with symptoms attributable to hormone secretion, such as parathyroid hormone, catecholamines or vasoactive peptide.

5

Which investigations should be performed in a patient with a mediastinal mass?

Blood tests:

a)

tumour markers – to detect a germ cell tumour, including:

i)

alpha fetoprotein (αFP);

ii)

beta-human chorionic gonadotrophin (βhCG);

b)

myasthenia serology – to detect a thymoma, including:

i)

antibodies against the nicotinic acetylcholine receptor (AChR);

ii)

anti-MuSK (muscle specific tyrosine kinase);

iii)

titin;

iv)

ryanodine;

c)

serum calcium levels – to detect a parathyroid adenoma;

d)

blood film – to detect lymphoma.

Radiology:

a)

chest radiograph (CXR, Figure 2) – which can demonstrate the size, location, extent and density of the lesion;

b)

contrast-enhanced thoracic computed tomography (CT) scan (Figure 3) – which is used to further delineate the characteristics of the lesion and define its relationship with adjacent anatomical structures;

c)

CT abdomen and pelvis – in patients with a suspected diagnosis of lymphoma;

d)

magnetic resonance imaging (MRI) scan (Figure 4) – to better delineate a nerve sheath tumour and to determine whether it has extended into the neural foramen.

Histology may not be required in patients with a definite radiological diagnosis, including thymoma and neural sheath tumours. If necessary, histological confirmation should be obtained by the least invasive means possible, including:

a)

CT-guided core biopsy;

b)

endobronchial ultrasound fine needle aspiration (EBUS-FNA);

c)

surgical biopsy.

6

What is lymphoma?

Lymphoma is a malignant neoplasm originating from cells of the lymphatic system, including lymphocytes, which are found in lymph nodes, spleen, bone marrow, thymus and the digestive tract.

Lymphoma is subclassified as:

a)

Hodgkin’s lymphoma (HL) – which is characterised by the presence of Reed-Sternberg cells (abnormally large Type B lymphocytes that are usually found in enlarged lymph nodes). Mediastinal lymph nodes are involved in 66% of patients, with other sites of disease present in the majority (95%), including peripheral lymph nodes. It is subdivided into:

i)

classic HL – which constitutes 95% of cases, and includes the nodular sclerosis, mixed cellularity, lymphocyte-rich and lymphocyte-depleted types;

ii)

nodular lymphocyte predominant HL (NLPHL) or popcorn cell tumour – which accounts for the remaining 5% of cases;

b)

non-Hodgkin’s lymphoma (NHL) – which is 5-10 times more common than HL, mainly affects adults and carries an overall mortality of 25-30%. It is subclassified according to the World Health Organisation (WHO) based on pathological appearance, chromosomal phenotype and surface antigens for differentiation. The subtypes commonly seen within the mediastinum include:

i)

large B-cell lymphoma;

ii)

lymphoblastic lymphoma.

Primary mediastinal lymphoma is a purely mediastinal form of lymphoma (as opposed to the systemic diseases of HL and NHL). It represents 10% of cases of patients with lymphoma in the mediastinum.

7

What are the clinical features of mediastinal lymphoma?

Local symptoms including:

a)

lymphadenopathy;

b)

distended abdomen;

c)

chest pain;

d)

dyspnoea or cough.

Systemic features (also known as B symptoms):

a)

persistent fever (>38°C);

b)

drenching night sweats;

c)

weight loss (>10% of body weight over 6 months).

Other possible symptoms of HL include:

a)

itching skin;

b)

fatigue;

c)

loss of appetite;

d)

alcohol-induced pain, which is rare but pathognomonic.

8

Which investigations are performed in the diagnosis of mediastinal lymphoma?

Histology – from biopsies of enlarged lymph nodes or other abnormal areas. In NHL, it is more common to require a mediastinal biopsy, whereas patients with HL usually have peripheral sites that are amenable for biopsy. The diagnosis and pathological subtype definition can be extracted from very small tissue samples, with fine needle aspiration (FNA) or core biopsy often being sufficient.

Blood tests, including:

a)

alkaline phosphatase – which is elevated in 50% of patients with HL;

b)

white blood cell count – where a neutrophilia is common in HL but rare in NHL.

Chest radiograph (CXR) – which may demonstrate the presence of enlarged hilar or mediastinal lymph nodes.

Computed tomography (CT) scan of the chest, abdomen and pelvis – which can be used to stage the disease and identify sites for diagnostic biopsy (Figure 5).