Essential respiratory medicine. Shanthi Paramothayan

Chapter 16. Disorders of the mediastinum

Learning objectives

 To understand the basic anatomy of the mediastinum

 To understand the diagnostic pathway for patients presenting with a mediastinal mass

 To understand the differential diagnosis and management of a mass in the anterior mediastinum

 To understand the differential diagnosis and management of a mass in the middle mediastinum

 To understand the differential diagnosis of mediastinal lymphadenopathy

 To understand the differential diagnosis and management of a mass in the posterior mediastinum

 To understand the aetiology and management of acute and chronic mediastinitis

 To understand the aetiology and management of a pneumomediastinum

Abbreviations

AFP alpha-feta protein

β-hcg β-human chorionic gonadotrophin

CXR chest X-ray

CT computed tomography

FNA fine needle aspiration

LDH lactate dehydrogenase

MEN multiple endocrine neoplasia

MG myasthenia gravis

MRI magnetic resonance imaging

PA postero-anterior

PET-CT positron emission tomography with computed tomography

SVCO superior vena cava obstruction

VATS video assisted thoracoscopic surgery

WHO World Health Organisation

Anatomy of the mediastinum

The anatomy and physiology of the lungs are discussed in Chapter 2. The mediastinum is the central part of the thorax with the lungs on either side, the thoracic inlet above, the vertebral bodies behind, and the diaphragm below. The mediastinum contains the heart, trachea, oesophagus, thoracic duct, thymus, lymph nodes, aorta, pulmonary arteries, pulmonary veins, azygous vein, superior vena cava, inferior vena cava, phrenic nerves, sympathetic chain and parasympathetic chain. These structures are held together by connective tissue and fatty tissue. The mediastinum is divided into three areas: the anterior (or antero-superior), the middle and the posterior (Table 16.1). These are not anatomical divisions as there are no tissue planes separating them, but are arbitrary radiological divisions used to facilitate the classification of masses within the mediastinum.

Table 16.1 Structures in the mediastinum.

Anterior mediastinum Middle mediastinum

Posterior mediastinum

Thymus

Tracheal bifurcation

Sympathetic ganglia

Lymph nodes

Oesophagus

Spinal nerve roots

Thyroid

Lymph nodes

Lymph nodes

Ascending aorta

Part of azygous vein

Parasympathetic chain

Pulmonary artery

Inferior vena cava

Oesophagus

Phrenic nerves

Posterior heart

Lower half of superior vena cava

Aortic arch

Pulmonary artery

Pulmonary vein

Thoracic duct

Descending thoracic aorta

Vertebrae

Diagnosis of a mediastinal mass

Some 75% of mediastinal masses are benign, and more likely to be so in an adult. Thymomas, thyroid masses, lymph nodes, and benign cysts are the commonest mediastinal masses in adults. In children, over 80% of masses are neurogenic tumours, germ cell tumours, or foregut cysts.

A slow-growing, benign, mediastinal mass may be asymptomatic and found incidentally on a chest X-ray. As the mass enlarges, it can cause symptoms of cough, chest pain, and breathlessness. If the mass compresses adjacent structures, such as the trachea, oesophagus, or superior vena cava, it can cause stridor, dysphagia, and superior vena cava obstruction (SVCO). The patient may also develop systemic symptoms, depending on the mass.

History and examination

The patient should be asked about symptoms of fatigue, night sweats, fevers, and weight loss. Examination should look for lymphadenopathy, signs of ptosis, ophthalmoplegia and inability to maintain upward gaze suggestive of myasthenia gravis (MG) (Box 16.1). The testes should be examined in men.

Box 16.1 Investigations of a mediastinal mass.

 Chest X-ray (CXR): postero-anterior (PA) and lateral

 CT thorax, abdomen and pelvis with contrast

 MRI scan

 PET-CT scan

 Other scans as indicated: barium swallow, angiography, sestamibi parathyroid scintigraphy, radioiodine uptake scan

 Tumour markers as indicated

 CT-guided percutaneous fine-needle aspiration (FNA) or biopsy

____

Box 16.2 Tumour markers.

Alpha-fetoprotein (AFP) levels are elevated in some germ cell tumours β-human chorionic gonadotrophin (β-hcg) levels are elevated in germ cell tumours Acetylcholine receptor antibody levels are elevated in thymoma associated with myasthenia gravis

Lactate dehydrogenase (LDH) levels are elevated in several conditions, including lymphoma

____

Chest X-ray: PA and lateral

It should be possible, with a combination of a PA and lateral chest X-ray, to determine whether the mass is in the mediastinum, and then to locate it within the anterior, middle, or posterior compartments. A spiculated or nodular mass is likely to be within the lung and may contain air bronchograms, whereas a mediastinal mass will have a broad base, a smooth edge and will not contain air bronchograms. The right superior mediastinal border is formed of the right brachiocephalic vein and the superior vena cava, and is usually straight and vertical. A mediastinal mass will cause widening of the upper mediastinum, and the right superior mediastinal border will become distorted and indistinct. The left mediastinal border is formed of the left carotid artery, left subclavian artery, left brachiocephalic vein, and left jugular vein. When there is a mediastinal mass on the left side, the aortic knuckle may be poorly defined.

On the lateral CXR, the anterior and middle compartments can be divided by an imaginary line anterior to the trachea and posterior to the inferior vena cava. The middle and posterior compartments can be divided by an imaginary line passing 1 cm posterior to the anterior border of the vertebral bodies. A two-dimensional CXR will not, however, give sufficient detail about the structure or location of the mass and a CT scan with contrast is required for that.

CT thorax with contrast is essential to determine the exact anatomical structure and position of the mass and any possible invasion into surrounding tissues. The radiologist will consider whether the mass contains predominantly fat, fluid, or solid components, and whether it enhances after intravenous contrast. As different types of lesions have specific radiological characteristics, it may be possible to make a clear diagnosis without histology, for example, with a thymoma. Fluid- containing lesions are usually cysts or necrotic lymph nodes. Solid components increase the likelihood of the lesion being malignant. Fat-containing lesions are usually benign, and include teratomas and lipomas.

An MRI scan will be required to assess a posterior mediastinal mass to see if there is any tumour extension into the spinal canal, and is essential prior to surgery. Angiography is recommended prior to any invasive procedure if a vascular lesion is suspected. PET-CT may be helpful if a malignant mass is suspected, and may be required prior to surgery.

Tumour markers (Box 16.2) can be helpful in narrowing the differential diagnosis of a mediastinal mass and in monitoring response to treatment.

Anterior mediastinal mass

The anterior mediastinum is behind the sternum and in front of the pericardium (Figure 16.1, Figure 16.2, Figure 16.3). On a CXR, the hilum overlay sign (one can see the hilar vessels through the mass), displacement of the anterior junction line, obliteration of the retrosternal space and a hazy cardio-phrenic angle suggest an anterior mediastinal mass (Box 16.3).

Thymoma is a tumour of epithelial cells arising in the thymus. It is commonest in men over 50 years and rare in patients younger than 20 years. It is associated with myasthenia gravis in 30—40% of cases, and 20% of patients presenting with myasthenia gravis (MG) are found to have a thymoma. Patients with MG have positive acetylcholine receptor autoantibodies which bind to acetylcholine receptors at the post-synaptic motor endplate causing nerve fatigue. Patients with thymoma and MG complain of pain, dyspnoea, dysphagia, and muscle weakness with repeated contraction, and the inability to maintain an upward gaze.

Figure 16.1 Outline of the mediastinum.

Figure 16.2 Compartments of the mediastinum on a lateral CXR.

Box 16.3 Differential diagnosis of anterior medistinal mass.

 The common mnemonic used for an anterior mediastinal mass is ‘the Four T’s’: thymus, teratoma (germ cell tumour), thyroid and terrible lymphoma

 Thymoma

 Thymic cyst

 Thymic carcinoid

 Germ cell tumour (includes teratoma/ dermoid cyst)

 Lymphoma

 Thyroid goitre

 Parathyroid adenoma

 Ascending aortic aneurysm

 Pleuropericardial cyst

 Pericardial fat pad

 Morgagni anterior diaphragmatic hernia (congenital)

____

Figure 16.3 Common mediastinal masses in the anterior, middle, and posterior mediastinum.

Thymomas contained within the thymic capsule tend to be relatively benign. If a thymoma is suspected radiologically, the case should be discussed with a thoracic surgeon and oncologist at the lung multidisciplinary meeting prior to a fine needle aspiration of the mass.

The treatment is with thymectomy, usually through a median sternotomy scar. A VATS procedure may be possible for small thymomas. A transcervical thymectomy may also be indicated in patients with myasthenia gravis without a thymoma as this can improve symptoms in many and result in complete remission in 30—40% of cases. Best results are obtained in younger patients with detectable acetylcholine receptor antibodies who present with early disease. Symptoms of myasthenia gravis should also be treated with pyridostigmine and with immunosuppressants, such as prednisolone or azathioprine.

Malignant thymomas extend outside the capsule and spread by ‘seeding’, invading local structures and spreading to the pleural space (Figure 16.4, Figure 16.5). The most widely used staging system for thymoma is the Masaoka system, which is based on the degree of capsular invasion (Table 16.2). The WHO system, which is based on the microscopic appearance of the cells, has subtypes A, B and C. Type C tumours are thymic carcinomas, which have the worst prognosis.

Thymic cysts can be congenital or acquired secondary to inflammation. They are asymptomatic unless large and cause symptoms of compression, in which case they should be excised. Thymic lipoma and thymic hyperplasia can also appear as an anterior mediastinal mass. Thymic carcinoids can behave aggressively, with local invasion and distant metastases. These tumours are not associated with myasthenia gravis but may be associated with Cushing’s syndrome. Treatment is with surgery, chemotherapy, and octreotide.

Germ cell tumours are a diverse group of benign and malignant tumours which account for 10—15% of anterior mediastinal neoplasms in adults. They result from a failure of immature germ cells to migrate during embryogenesis, with the mediastinum being the commonest extragonadal site. Benign, mature cystic teratomas (dermoid cysts) are the commonest germ cell tumours in adults, accounting for 60—70% of cases, and occurring in the third decade. These well-differentiated tumours contain fat, skin, hair, eyes, nails, sweat glands, cartilage, and teeth. They usually present with symptoms of compression and rarely with expectoration of hair (trichoptysis), sebum or fluid from a connection that forms between the tumour and the airways. Tumour markers are negative with a teratoma. CXR and CT thorax will reveal a well-circumscribed, multi-loculated, cystic mass with a fat-fluid level and calcification. Teeth and hair may be present. The prognosis is excellent with surgery.

Figure 16.4 CXR of thymoma.

Figure 16.5 CT thorax showing thymoma.

Table 16.2 Masaoka system.

Cure rates

Stage Extent of tumour invasion Management (%)

1

No tumour invasion into capsule

Thymectomy

90-95

11

Tumour invasion into the fatty tissue around thymus and microscopic capsular invasion

Thymectomy + radiotherapy

85-90

111

Tumour invasion into surrounding organs

Chemotherapy or chemo-radiation followed by thymectomy

50-70

1VA

Tumour extension to the pleura and pericardium

Chemo-radiation + thymectomy in some cases

20-50

1VB

Spread to lymph nodes +/or distant metastases by haematogenous spread

Chemo-radiation + thymectomy in some cases

20

The majority of malignant mediastinal germ cell tumours occur in men who will be symptomatic. Serum levels of β-hcg and/or AFP will be elevated in 80—85% of cases. Seminomas account for about half of malignant germ-cell tumours and affect men in their twenties and thirties. The CXR will depict a large, lobulated, well-defined anterior mediastinal mass. Local invasion of mediastinal structures is uncommon, although lymph node, lung, and bone metastases can occur. Treatment is with radiotherapy, chemotherapy and surgery, and the prognosis has improved significantly in the past decade. Measurement of tumour markers can be helpful in monitoring the disease.

Other malignant germ cell tumours affecting young men include choriocarcinoma, embryonal cell carcinoma, endodermal sinus tumour, and mixed germ-cell tumour. These too may secrete AFP and β-hcg. Some 20% of these men have Klinefelter’s syndrome, and the tumour may also be associated with haematological malignancies. These lesions appear as a large, irregular, heterogeneous mass with central necrosis, haemorrhage, and cyst formation. Invasion of adjacent structures with pleural and pericardial effusions can occur. Distant metastases occur at an advanced stage. Treatment is with chemotherapy and surgery, and the five-year survival rate is 50%.

Lymphomas can arise from lymph nodes in the anterior or middle mediastinum. B-cell Hodgkin’s lymphoma is the commonest type of lymphoma. Patients may present with B symptoms (fever, night sweats, and weight loss), and LDH levels may be elevated significantly. A surgical biopsy is recommended to confirm the histological diagnosis. The patient should be referred urgently to the haemo-oncologist for chemotherapy and further management.

A large, retrosternal thyroid goitre can appear as an antero-superior mediastinal mass. Patients are usually euthyroid, but may experience symptoms of dyspnoea and dysphagia because of compression of the trachea and oesophagus. The CXR and CT thorax may show tracheal deviation (Figure 16.6). The contrast CT scan may reveal enhancement of an encapsulated mass with haemorrhagic and cystic changes, and possible calcification. Further imaging with radioactive iodine (123I or 131I) scan and a thyroid ultrasound will be required, as well as thyroid function tests and measurement of thyroid antibodies.

Figure 16.6 CXR showing retrosternal thyroid.

Surgery should be considered if the patient is symptomatic.

Parathyroid adenomas occur in elderly women and should be considered in those who have persistent hyperparathyroidism and hypercalcaemia despite parathyroidectomy. These benign functioning ectopic adenomas occur in the anterior mediastinum near the thymus. They may be too small to be detected on a CXR. A contrast CT thorax will show an encapsulated mass and there will be increased uptake with 99mTc sestamibi scintigraphy. Management is with surgical excision.

Middle mediastinal mass

The middle mediastinum is the area between the anterior and posterior mediastinum. A middle mediastinal mass may appear in the aorto-pulmonary window with widened paratracheal stripes, displacement of the azygo-oesophageal recess on the right, and a pseudoparavertebral line on the left (Box 16.4).

The commonest mass in the middle mediastinum is due to lymph node enlargement (Figure 16.7), which can be due to a variety of aetiologies (Box 16.5).

Box 16.4 Differential diagnosis for middle mediastinal mass.

 Lymphadenopathy

 Foregut duplication cysts: bronchogenic cyst, oesophageal duplication cyst

 Pericardial cyst

 Vascular anomalies: aortic arch anomalies

 Foramen of Morgagni diaphragmatic hernia

____

Figure 16.7 CT thorax with contrast showing lymphadenopathy.

Box 16.5 Causes of lymphadenopathy in mediastinum.

 Infectious granulomatous disease: mycobacterium tuberculosis, histoplasmosis, coccidiodomycosis

 Non-infectious granulomatous disease: sarcoidosis, silicosis

 Lymphoma: Hodgkin’s lymphoma or non-Hodgkin’s lymphoma

 Metastases to lymph node from lung, breast, renal cell carcinoma, gastrointestinal malignancy, mesothelioma, and prostate

 Reactive hyperplasia from viral or bacterial infection

 Amyloidosis

 Castleman’s disease (giant lymph node hyperplasia)

 Drugs: phenytoin, methotrexate

____

Sarcoidosis commonly presents with symmetrical, bilateral, hilar lymphadenopathy. The other conditions listed can present with asymmetric lymph node enlargement. Calcification of lymph nodes can occur with tuberculosis and histoplasmosis. Silicosis is associated with ‘eggshell calcification’.

Foregut duplication cysts account for 20% of mediastinal masses and are commoner in children. They are usually asymptomatic when small, but may cause symptoms of compression if they enlarge. Diagnosis can be made from characteristic radiological features, although biopsy or percutaneous CT-guided FNA may be necessary.

Some 50—60% of these are bronchogenic cysts which are the result of abnormal budding of the ventral foregut in embryogenesis. They can occur in adults of all ages and equally in men and women. They are asymptomatic unless they become infected or bleed. These occur most commonly in the subcarinal and paratracheal regions, although some can occur in the pulmonary parenchyma. CXR will show a well-circumscribed, spherical, homogeneous mass with fluid and calcification. There will be no enhancement with contrast on a CT scan. Bronchogenic cysts are lined with pseudostratified, columnar, ciliated respiratory epithelium and can contain fluid, mucus, milky fluid with calcium, blood, or purulent material. This material can be analysed from a FNA.

Oesophageal duplication cysts and neuroenteric cysts are commoner in children and originate from the dorsal foregut. They account for 10—15% of all cysts. Oesophageal cysts are located close to the distal oesophagus on the right, are lined by squamous or enteric epithelium and can contain gastric mucosa or pancreatic tissue. Neuroenteric cysts may also occur in the posterior mediastinum and contain neural tissue. These may be associated with spina bifida and other vertebral abnormalities.

Foregut duplication cysts should be followed up clinically and radiologically. Surgery should be considered if symptoms develop.

Pericardial cysts (also called spring water or clear water cysts) are rare, usually asymptomatic, and found incidentally in middle-aged adults. They appear as well-circumscribed, fluid-containing lesions abutting the heart, diaphragm, and the anterior chest wall, typically in the right cardio- phrenic angle. The cyst does not enhance on a contrast CT scan. Management is conservative unless symptoms arise (Figure 16.8).

Figure 16.8 CXR showing a pericardial cyst.

Vascular anomalies originate from the arterial or venous parts of the systemic or pulmonary circulation and account for 10% of all mediastinal masses. It is advisable to confirm that the lesion is vascular with angiography prior to any attempt at a biopsy to avoid catastrophic consequences.

Diaphragmatic hernias are common and can be due to congenital defects in the diaphragm. A foramen of Morgagni diaphragmatic hernia appears as a mass in the right cardiophrenic angle. CT thorax, combined with barium studies, can usually confirm the diagnosis.

Posterior mediastinal mass

The posterior mediastinum is the area behind the pericardium and in front of the vertebral bodies (see Figures 16.1—16.3). On a CXR, the mass extends above the superior clavicle (cervicothoracic sign) with widening of the paravertebral stripes (Box 16.6).

Most neurogenic tumours (90%) occur in the posterior mediastinum and account for 75% of primary, posterior mediastinal neoplasms. They constitute 15—20% of posterior mediastinal masses in adults, most of which are benign. They make up 40% of posterior mediastinal tumours in children, and 50% will be malignant. Neurogenic tumours can be divided into peripheral nerve sheath tumours, sympathetic ganglia tumours and paragangliomas.

Peripheral nerve sheath tumours include neurofibromas and schwannomas which are the commonest cause of a posterior mediastinal mass, and are usually benign in adults. Neurofibromas are slow-growing and arise from a posterior spinal nerve root and can involve any nerve in the thorax. They occur equally in men and women in their twenties and thirties and are asymptomatic when small. In 10% of cases, neurofibromas are multiple. 30—45% of neurofibromas occur in individuals with neurofibromatosis (von Recklinghausen’s disease), who are at increased risk of malignant transformation of a pre-existing neurofibroma. If a neurofibroma increases in size, it can cause pain, pressure in the back, with erosion of ribs, vertebral bodies and neural foramina, and, in rare cases, results in spinal cord compression. If the neuroma is large enough to cause symptoms, surgery should be conducted after an MRI scan to ensure that there is no intraspinal extension. Symptoms of pain and a rapidly enlarging mass should alert the clinician to the possibility of a malignant transformation.

Box 16.6 Differential diagnosis of posterior mediastinal mass.

 Neurogenic tumour

 Lipoma

 Descending aortic aneurysm

 Bochdalek posterior diaphragmatic hernia (congenital)

 Foregut duplication cyst

 Lateral thoracic meningocele

____

A neurofibroma appears as a round or lobulated, paravertebral, posterior, mediastinal mass, spanning one or two vertebral bodies. As it increases in size, it can appear as a dumb-bell structure straddling the intervertebral foramen. A CT scan demonstrates a heterogeneous mass which may contain calcification and low areas of attenuation.

Malignant tumours of the nerve sheath/neuro- sarcomas are a rare group of spindle cell sarcomas occurring equally in men and women in their twenties to their forties. Some 50% occur in patients with neurofibromatosis (Figure 16.9, Figure 16.10). They can invade locally and metastasize.

Sympathetic chain ganglia tumours include neuroblastomas, which occur in young children, and are the commonest neurogenic tumour in this age group. Ganglioneuromas and ganglioneuro- blastomas occurs in older children. These are rare in adults. Some 50% arise from adrenal glands, with one-third in the mediastinum, the most common extra-abdominal location. Treatment is with surgery and chemotherapy.

Figure 16.9 CXR showing a neurofibroma.

Paragangliomas are rare neuroendocrine tumours that occur in men in the third and fourth decade; 75% of these occur sporadically, but 25% are hereditary and may be associated with a mutation of the gene for succinate dehydrogenase, or be part of Type 2a or 2b multiple endocrine neoplasia (MEN) syndrome. Paragangliomas usually arise from the adrenal gland, but 10% are extra-adrenal, occurring in the head, neck, thorax, or abdomen. Less than 2% of these are intra-thoracic, but these are more likely to be malignant. Only 3% are malignant and spread to distant sites; 1—3% of tumours secrete catecholamines and the clinical presentation is like that of a phaeochromocytoma. Management is surgical resection.

Lateral thoracic meningocele is a rare lesion that consists of redundant meninges.

Other mediastinal conditions

Conditions such as mediastinitis, haematoma, vascular lesions, and malignancies are can occur in any part of the mediastinum.

Mediastinitis can be caused by infection or inflammation (Box 16.7).

Acute mediastinitis can be secondary to bacterial infection or iatrogenic secondary to endoscopic surgical procedures. It is usually rapidly progressive and often fatal. Patients with acute mediastinitis are systemically unwell and present with fever, rigors, chest wall tenderness, dyspnoea, and dysphagia. A CT thorax may show mediastinal widening, emphysema, pneumomediastinum, mediastinal air-fluid level, and pleural effusions. Treatment is that of the underlying cause and includes surgical drainage, debridement, and intravenous antibiotics.

Figure 16.10 CT thorax showing a neurofibroma.

Box 16.7 Causes of acute mediastinitis.

 Oesophageal rupture

 Tracheo-bronchial perforation

 Penetrating chest injury

 Post-operative sternal wound infection

 Oro-pharyngeal infection

 Paravertebral abscess

 Vertebral abscess

 Radiotherapy

 Anthrax

 Malignancy

____

Chronic (fibrosing) mediastinitis results from long-standing inflammation and the formation of dense fibrous tissue in the carinal, paratracheal, and hilar regions (Box 16.8). This results in significant compression of the mediastinal structures, causing dysphagia, breathlessness, and superior vena cava obstruction. CXR, contrast CT thorax, MRI scan, perfusion scintigraphy and biopsy may be required to exclude malignancy and infection, and to make the diagnosis. Unfortunately, no effective treatment is available, although surgery may relieve symptoms of compression.

Box 16.8 Causes of chronic mediastinitis (fibrosing mediastinitis).

 Mycobacterium tuberculosis

 Histoplasmosis

 Sarcoidosis

 Silicosis

 Autoimmune disease: retroperitoneal fibrosis, Riedel’s thyroiditis, systemic lupus erythematosus, rheumatoid arthritis, sclerosing cholangitis

 Drugs: methysergide, practolol

 Mediastinal haematoma

 Radiation therapy

____

Pneumomediastinum can result from rupture of the airways, causing air to track into the pulmonary interstitium and the soft tissues of the neck. It can be the caused by penetrating injury, for example, gunshot wounds, or oesophageal or tracheobronchial perforation. A pneumomediastinum can occur secondary to violent vomiting, acute airway obstruction (for example, in acute asthma) or severe coughing. Increased alveolar pressure from the Valsalva manoeuvre, the Heimlich manoeuvre, and mechanical ventilation can also result in a pneumomediastinum, as can iatrogenic causes such as mediastinal surgery, mediastinoscopy, tonsillectomy, or thyroidectomy. Pneumomediastinum can also be secondary to cervical emphysema or pneumoperitoneum that tracks into the mediastinum, often secondary to infection with gas-forming organisms. Patients with abnormal lungs are at greater risk.

The clinical symptoms include pleuritic chest pain, neck pain, dyspnoea, and dysphagia. Clinical signs include subcutaneous emphysema with crepitations in the neck and Hammans sign, which is a mediastinal crunch on auscultation. A CXR will show air outlining the mediastinal structures, and a lateral chest X-ray will reveal retrosternal air which will not move with a change in position (Figure 16.11, Figure 16.12).

There is a risk of developing a tension pneumothorax or a tension pneumopericardium, as well as mediastinitis. The prognosis depends on the underlying cause, but a pneumomediastinum usually resolves within a week.

Figure 16.11 CXR showing a pneumomediastinum.

Figure 16.12 CT thorax showing a pneumomediastinum.

 The mediastinum is an area in the thorax between the lungs, vertebral bodies, thoracic inlet, and diaphragm.

 The mediastinum can be artificially separated into the anterior, middle, and posterior mediastinum.

 A contrast CT scan is necessary to understand the structure and exact location of a mediastinal mass.

 Masses in the mediastinum are often asymptomatic when small, causing symptoms of compression when they enlarge.

 The differential diagnoses of mediastinal masses varies in adults and children.

 Children are more likely to have a posterior mediastinal mass which is more likely to be malignant.

 Common causes of an anterior mediastinal mass in adults include thymoma, thyroid mass, teratoma (germ cell tumour), and lymphadenopathy.

 A thymoma is usually benign but may be associated with myasthenia gravis.

 Surgical resection of a thymoma is usually curative and relieves the symptoms of myasthenia gravis in a significant proportion of patients.

 Thymectomy may improve the symptoms of myasthenia gravis even in those without a thymoma.

 The majority of germ cell tumours in adults (teratomas) are benign.

 Seminomas are the commonest germ cell tumours in men and may secrete AFP and P-hcg.

 There are several causes of mediastinal lymphadenopathy, including infections, malignancies, sarcoidosis, and drugs.

 Bronchogenic cysts are developmental anomalies which are benign.

 Neurogenic tumours are the commonest cause of a posterior mediastinal mass.

 Neurofibromas are usually benign, but can become malignant in individuals with neurofibromatosis.

 Acute mediastinitis is a serious condition caused by infection and inflammation of the mediastinum, often secondary to trauma or procedures.

 Chronic (fibrosing) mediastinitis occurs from long-standing infection or inflammation, resulting in the formation of fibrous tissue.

 Chronic fibrosis can compress mediastinal structures and cause dysphagia and SVCO. There is no effective treatment for this.

 Pneumomediastinum can occur after penetrating chest injury, endoscopic procedures, barotrauma, or due to gas-forming bacteria from the peritoneum.

MULTIPLE CHOICE QUESTIONS

16.1 Which of the following statements about the mediastinum is true?

A The anterior mediastinum lies behind the pericardium

B The oesophagus lies within the anterior mediastinum

C The thymus is within the anterior mediastinum

D The middle mediastinum contains the sympathetic ganglia

E The posterior mediastinum contains the ascending aorta

Answer: C

The thymus lies in the anterior mediastinum, which is in front of the pericardium and behind the sternum. The oesophagus runs through the middle and posterior mediastinum. The ascending aorta is in the anterior mediastinum, while the posterior mediastinum contains the descending aorta and the sympathetic ganglia.

16.2 Which statement is true of most mediastinal masses?

A In adults they are malignant

B In children they are duplication cysts

C They have a poor prognosis

D In children they are congenital

E They secrete hormones

Answer: D

Overall, most mediastinal masses are benign, although they are more likely to be malignant in children. The commonest mediastinal tumours in children are neurogenic tumours. Only some germ-cell tumours and paragangliomas secrete hormones.

16.3 Which statement is true of a thymoma?

A It occurs most commonly in young women

B It can be associated with myasthenia gravis in 90% of cases

C It usually metastasises early

D It often transforms to a malignant thymoma

E It has a good prognosis

Answer: E

Thymomas are common in middle-aged men and are associated with myasthenia gravis in 30-40% of cases. They are usually benign and spread to local structures by breaching the capsule and seeding. Thymomas have a good prognosis if detected early. They do not commonly transform to a malignant thymoma.

16.4 Which of the following statements about germ cell tumours is true?

A Teratomas are the commonest germ cell tumour in adults

B Germ cell tumours account for 50% of anterior mediastinal masses

C Seminomas occur in elderly men

D Teratomas metastasise to the lungs and the heart

E The five-year survival with seminoma is less than 10%

Answer: A

Teratomas (dermoid cysts) are the commonest germ cell tumours in adults and are benign. Germ cell tumours account for 10-15% of anterior mediastinal masses in adults, most commonly in young men. Advances in treatment have resulted in significant improvement in survival.

16.5 What is the commonest cause of a middle mediastinal mass in an adult?

A Bronchogenic cyst

B Retrosternal thyroid

C Lymphadenopathy

D Pericardial cyst

E Diaphragmatic hernia

Answer: C

Lymphadenopathy (various causes) is the commonest middle mediastinal mass in an adult.

16.6 Which statement is true of bronchogenic cysts acquired secondary to infection?

A They can contain blood and mucus

B They are much commoner in women than in men

C They are lined with lung parenchymal cells

D They enhance with contrast CT scan

E They are associated with Neurofibromatosis

Answer: A

Bronchogenic cysts are development anomalies which occur equally in men and women. They are lined with pseudostratified, columnar, ciliated respiratory epithelium and can contain fluid, blood, and mucus. They do not enhance with contrast.

16.7 Which statement is true of posterior mediastinal masses?

A They are commoner in children than adults

B They are more likely to be malignant in adults

C They commonly present with spinal cord compression

D They usually secrete catecholamines

E They cause narrowing of the paravertebral stripe on a chest X-ray

Answer: A

The commonest posterior mediastinal masses are neurogenic tumours which are more common in children, and more likely to be malignant in children. They rarely cause s pinal cord compression and only a small percentage of paragangliomas secrete catecholamines. On a chest X-ray there is widening of the paravertebral stripes.

16.8 Which of the following statements about neurogenic tumour is true?

A Neurogenic tumours grow rapidly

B They arise from the sympathetic ganglia

C They can occur in von Recklinghausen’s disease

D They may secrete catecholamines

E They may be part of MEN Type 2a

Answer: C

Neurogenic tumours are slow-growing tumours that arise from peripheral nerves or nerve sheaths. Some 30% of individuals with neurofibroma have neurofibromatosis (von Recklinghausen’s disease) and present with multiple neurofibromas. Paragangliomas are associated with MEN Types 2a and 2b and secrete catecholamines.

16.9 Which of the following statements is true of chronic mediastinitis?

A It can present with superior vena cava obstruction

B It can be treated effectively with intravenous steroids

C It can be caused by oesophageal rupture

D It can be treated with immunosuppression

E It can improve with cessation of the drug causing it

Answer: A

Chronic fibrosing mediastinitis results from long-standing inflammation secondary to infection, drugs, sarcoidosis, or autoimmune disease. When advanced, the fibrosis can cause compression of organs, including SVCO. There is no effective treatment for this.

16.10 Which statement is true of pneumomediastinum?

A It has a mortality rate of 80%

B It can be caused by the Valsalva manoeuvre

C It can be managed with a small chest drain

D It can improve with hyperbaric oxygen

E It always requires thoracic surgery

Answer: B

A pneumomediastinum can occur from trauma or alveolar over-distension and usually has a reasonable prognosis, resolving within seven days. This does, however, depend on the underlying cause. Chest drain, surgery or hyperbaric oxygen are not indicated.

FURTHER READING

Armstrong, P, Wilson, A., Dee, P., and Hansell, D. (1995). Imaging of Diseases of the Chest, 2nde.

St. Louis, MO: Mosby. Hill, N.S. (1999). Noninvasive mechanical ventilation. In: Comprehensive Respiratory Medicine (ed. R.R. Albert, S.C. Spiro and J.R. Jett), 12.1-12.10. London: Mosby.

Strollo, D.C., Rosado de Christenson, M.L., and Jett, J.R. (1997). Primary mediastinal tumors. Part 1: tumors of the anterior mediastinum. Chest 112 (2): 511-522.



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