Trapped lung

Trapped lung, also known as unexpandable/unexpanded lung, is a term used where there is non-expandable lung after fluid removal, often thoracentesis. It is similar to but not entirely synonymous with the term lung entrapment, which is a similar condition but caused by active pleural disease rather than pleural inflammation from remote disease.

Pathology

Trapped lung develops as a sequela of pleural space inflammation from remote disease resulting in the development of a mature, fibrous membrane that impedes the lung from re-expanding. This creates a negative pressure environment in the pleural space, which is filled with fluid creating a pleural effusion. Drainage of this pleural fluid will often result in unavoidable pneumothorax from parenchymal-pleural fistulae. 

Radiographic features

Plain radiograph

Commonly noted to be associated with post-thoracentesis pneumothorax, and may have the following features ⁴: 

• visceral pleural peel (thickening)
• basal pneumothoraces
• ipsilateral volume loss
• lobar atelectasis

Pneumothoraces, if present, typically do not appear larger on expiratory images ¹.

Differential diagnosis

General imaging differential considerations include ¹:

• obstructing bronchogenic carcinoma
• cryptogenic organising pneumonia
• organising pneumonia 

Treatment and prognosis

The definitive treatment is surgery including pleurectomy and decortication to remove the fibrosed visceral pleura from the lung to relieve pressure and allow for expansion of the trapped lung.