Pneumothorax necessitans is a rare complication of pneumothorax.
Clinical features:
(1) presentation with lobulated or localized subcutaneous emphysema
(2) presence of pneumothorax
(3) expansion of the subcutaneous mass when coughing
Mechanism: defect in the pleural surface, with dissection of air into extrathoracic soft tissues and forming a subcutaneous air pocket
Risk factors:
(1) pleural empyema
(2) osteomyelitis
(3) bronchopleural fistula
(4) prolonged tube thoracostomy
Differential diagnosis:
(1) empyema necessitans
