A pneumothorax is defined as air inside the thoracic cavity but outside the lung.   A spontaneous pneumothorax (PTX) is one that occurs without an obvious inciting incident. Some causes of spontaneous PTX are; idiopathic, asthma, COPD, pulmonary infection, neoplasm, Marfan's syndrome, and smoking cocaine.  However, most pneumothoraces are iatrogenic and caused by a physician during surgery or central line placement.  Trauma, such as a motor vehicle accident is another important cause.  A tension PTX is a type of PTX in which air enters the pleural cavity and is trapped during expiration usually by some type of ball valve-like mechanism.  This leads to a buildup of air increasing intrathoracic pressure.  Eventually the pressure buildup is large enough to collapse the lung and shift the mediastinum away from the tension PTX.  If it continues, it can compromise venous filling of the heart and even death.

On CXR, a PTX appears as air without lung markings in the least dependant part of the chest.  Generally, the air is found peripheral to the white line of the pleura.  In an upright film this is most likely seen in the apices.  A PTX is best demonstrated by an expiration film.  It can be difficult to see when the patient is in a supine position.  In this position, air rises to the medial aspect of the lung and may be seen as a lucency along the mediastinum.  It may also collect in the inferior sulci causing a deep sulcus sign.

A hydropneumothorax is both air and fluid in the pleural space. It is characterized by an air-fluid level on an upright or decubitus film in a patient with a pneumothorax. Some causes of a hydropneumothorax are trauma, thoracentesis, surgery, ruptured esophagus, and empyema.