A pneumothorax (pneumo- + thorax; plural pneumothoraces) is an abnormal collection of air or gas in the pleural space that separates the lung from the chest wall. Like pleural effusion (liquid buildup in that space), pneumothorax may interfere with normal breathing. It is often called collapsed lung, although that term may also refer to atelectasis.A primary pneumothorax is one that occurs without an apparent cause and in the absence of significant lung disease, while a secondary pneumothorax occurs in the presence of existing lung pathology. In a minority of cases, the amount of air in the chest increases markedly when a one-way valve is formed by an area of damaged tissue, leading to a tension pneumothorax. This condition is a medical emergency that can cause steadily worsening oxygen shortage and low blood pressure. Unless reversed by effective treatment, these sequelae can progress and cause death.Pneumothoraces can be caused by physical trauma to the chest (including blast injury), or as a complication of medical or surgical intervention. Symptoms typically include chest pain and shortness of breath. Diagnosis of a pneumothorax by physical examination alone can be difficult or inconclusive (particularly in smaller pneumothoraces), so a chest radiograph or computed tomography (CT) scan is usually used to confirm its presence.Small spontaneous pneumothoraces typically resolve without treatment and require only monitoring. This approach may be most appropriate in subjects who have no significant underlying lung disease. In larger pneumothoraces, or when there are marked symptoms, the air may be extracted with a syringe or a chest tube connected to a one-way valve system. Occasionally, surgical interventions may be required when tube drainage is unsuccessful, or as a preventive measure, if there have been repeated episodes. The surgical treatments usually involve pleurodesis (in which the layers of pleura are induced to stick together) or pleurectomy (the surgical removal of pleural membranes).