Trapped lung is a type of unexpandable lung, and the term is used when a thick, fibrotic visceral pleural rind prevents lung expansion. This is commonly post-pneumonic, occurring when active pleural inflammation heals with fibrosis. This differs from the situation of actively inflamed and thickened pleura complicating pneumonia which also limits lung expansion and is called lung entrapment.
In trapped lung, pleural pressure is abnormally low and this causes a pleural transudate. Thoracentesis lowers the intra-pleural pressure further because of the rigidity of the chest wall and the inability of the lung to expand. In lung entrapment however, there is active inflammatory or malignant pleural thickening which is associated with an exudate.
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Pathology
Trapped lung typically develops as a sequela of prior pleural space inflammation with incomplete resolution. This results in a mature, fibrous visceral pleural membrane enclosing the lung which prevents re-expansion. Pleural drainage causes a steep drop in the already low pleural pressure, and this may cause pain as well as pneumothorax due to micro tears in the visceral pleura.
Radiographic features
Plain radiograph
Small hemithorax with adaptive changes in the chest wall, mediastinum and/or diaphragm. Low volume lung with pleural collection. Following thoracentesis there is commonly a pneumothorax. The following features may be seen 4:
visceral pleural peel (thickening)
basal pneumothorax
ipsilateral volume loss
Pneumothorax if present, typically does not appear larger on expiratory images 1.
Differential diagnosis
General imaging differential considerations include 1:
obstructing lung cancer
Treatment and prognosis
The definitive treatment is surgical pleurectomy and decortication to enable re-expansion of the trapped lung.
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