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66. Pleural and mediastinal disorders

Pleural diseases

Like pulmonary hypertension, plural diseases are most commonly secondary complications of underlying pulmonary or cardiac diseases, and not a primary disease itself.

Pleural effusion

Pleural effusion is the abnormal presence of fluid in the pleural space. The fluid can either be transudate or exudate.

Transudative pleural effusion (sometimes called hydrothorax) is most frequently caused by increased capillary hydrostatic pressure in the pleural vessels due to heart failure or any disease that causes hypoproteinaemia, like liver disease or nephrotic syndrome. Transudates are often reabsorbed if the underlying cause is resolved.

Pleural exudate can be serous, fibrinous, haemorrhagic or purulent (suppurative). Pus accumulation in the pleural cavity is also called pleural empyema. Pleural exudate is commonly caused by one of these processes:

  • Parapneumonic effusion – Occurs in response to pneumonia, either with or without bacterial invasion of the pleura (bacterial pleuritis)
  • Cancer
  • Pulmonary embolism
  • Autoimmune disease

Cancer-related pleural exudate is often due to lung cancer, mesothelioma or metastatic breast cancer. They’re often large and haemorrhagic and contain cancer and inflammatory cells.

Fibrinous, haemorrhagic and purulent exudates may lead to fibrinous organization, which can cause adhesions and fibrous thickening of the pleura.

Pneumothorax

Pneumothorax (PTX) refers to the presence of air in the pleural space. It may happen spontaneously or secondary to emphysema, lung abscess or a fractured rib. Secondary pneumothorax occurs when there is a rupture of the lung close to the pleural surface that allows inhaled air to enter the pleural cavity.

The pressure in the pleural cavity is normally negative to allow the lungs to expand in it. When it suddenly becomes filled with air the pressure in the pleural cavity will become equal with atmospheric pressure, causing the lung to collapse.

We can distinguish multiple types of pneumothorax:

  • Spontaneous pneumothorax – air enters through a defect in the lung without trauma
  • Closed pneumothorax – air enters through a defect in the lung after trauma
  • Open pneumothorax – air enters through a defect in the chest wall after trauma
  • Tension pneumothorax – air enters the pleural cavity during inspiration but cannot leave during expiration, due to the formation of a one-way valve

In tension pneumothorax, the pressure inside the pleural cavity increases with each inspiration, which compresses structures in the chest. This causes mediastinal structures to be shifted away from the side of the pneumothorax on x-ray. Any type of pneumothorax can progress into tension pneumothorax. Tension PTX is an emergency.

Pneumothorax may be primary or secondary. In primary pneumothorax, there is no known underlying lung condition, while in secondary pneumothorax, there is. Primary pneumothorax occurs due to the rupture of air-filled so-called subpleural bullae. These bullae are asymptomatic in most cases, but because they are thin walled, they’re predisposed to rupture. Bullae form due to overload of elastic fibres, which occurs in tall and thin people, and in weed smokers.

Primary pneumothorax risk factors:

  • Family history
  • Male
  • Slim, tall stature
  • Smoking
  • Marfan syndrome

Secondary pneumothorax risk factors:

  • Emphysema/COPD
  • Cystic fibrosis
  • TB
  • Blunt trauma
  • Penetrating injury
  • Iatrogenic pneumothorax
  • Mechanical ventilation

Haemothorax, chylothorax and pleural empyema

Haemothorax is the collection of blood (rather than bloody exudate) in the pleural cavity. It may occur due to trauma, pneumothorax, or ruptured intrathoracic aortic aneurysm.

Chylothorax is the collection of lymphatic fluid in the pleural cavity. The lymphatic fluid contains lipids, so it’s milky in appearance. It occurs due to obstruction of major lymphatic ducts, most commonly due to cancer.

Pleural empyema is a complication of pneumonia or lung abscess.

Mesothelioma

Mesothelioma is a cancer of mesothelial cells that usually occurs in the parietal or visceral pleura but can also arise from the peritoneum and pericardium. The biggest risk factor is asbestos exposure, but it takes 25-40 years from the initial exposure until the cancer develops. Cigarette smoking further increases the risk.

Asbestos are mineral fibres, not particles, so they’re impossible for macrophages to get rid of. They remain in the lung for life. The cancer itself is usually preceded by extensive pleural fibrosis. Asbestos fibres generate reactive oxygen species, which cause DNA damage leading to cancer. Psamomma bodies are found in mesothelioma.

Mesotheliomas rarely metastasize to distant locations, but they do invade the thoracic wall and subpleural lung parenchyme. The prognosis is very poor, with a mean survival of 1 year after diagnosis.

Disorders of the mediastinum

Mediastinitis

Mediastinitis is a severe condition. It can be either acute or chronic. Acute mediastinitis is associated with cardiothoracic surgery, or perforation of mediastinal structures like the oesophagus. Chronic mediastinitis is idiopathic.

Mediastinitis causes retrosternal pain, and subcutaneous emphysema of the neck and face.

Thymoma

Thymomas are epithelial tumours of the thymus. They can be benign or malignant, but both types are rare. This mediastinal mass can cause chest pain, cough, or superior vena cava syndrome.

Thymoma is associated with a neuromuscular disorder called myasthenia gravis. 10% of patients with myasthenia gravis have a thymoma.