Introduction
Pleural diseases comprise abnormal fluid, air, thickening and tumours of the pleural space between the visceral and parietal pleura. In radiology the key tasks are to recognise whether fluid or air is present, estimate its size and character, decide whether urgent drainage is needed, and suggest the underlying cause. The normal pleura is not seen on plain film; it becomes visible only when it is separated from the lung by air or fluid, and the costophrenic angles are normally sharp.
Pleural effusion
A pleural effusion is an accumulation of fluid in the pleural space. It is the most common pleural abnormality seen on chest imaging and is classified biochemically as a transudate or an exudate.
Types of pleuritis
- Fibrinous pleuritis: there are no chest X-ray findings.
- Exudative pleuritis: the chest X-ray finding depends on the amount of exudate. Blunting of the costophrenic angle is seen because fluid collects at the bottom of the pleural space.
- Adhesive pleuritis is seen at the end stage of exudative pleuritis.
- Encapsulated pleural effusions develop where there are adhesions, because fluid absorption is difficult there; the chest X-ray shows uniform opacities with sharp outlines.
Detection and size
- Upright posteroanterior film: blunting of the costophrenic angle, which needs roughly 200 mL; a concave upper border (meniscus sign) indicates free fluid; the Damoiseau line is formed.
- Supine film: diffuse hazing of one hemithorax, apical capping and a deep costophrenic sulcus; small effusions are easily missed.
- Lateral decubitus film (laterography): free fluid layers out along the dependent side and shows even small volumes, and it confirms that the fluid is free-flowing.
- Ultrasound: the most sensitive method for small effusions; it distinguishes fluid from consolidation and guides thoracentesis.
- CT: shows loculations, pleural thickening, the lung parenchyma and the underlying cause, such as a mass.
Transudate versus exudate
Light's criteria separate the two types: an exudate is present if the pleural to serum protein ratio is above 0.5, the pleural to serum LDH ratio is above 0.6, or the pleural LDH is above two thirds of the upper limit of normal for serum.