An air rim of 2 cm means that the pneumothorax occupies about 50% of the hemithorax. British professional guidelines have traditionally stated that the measurement should be performed at the level of the hilum (where blood vessels and airways enter the lung) with 2 cm as the cutoff, while American guidelines state that the measurement should be done at the apex (top) of the lung with 3 cm differentiating between a "small" and a "large" pneumothorax. The latter method may overestimate the size of a pneumothorax if it is located mainly at the apex, which is a common occurrence. The various methods correlate poorly but are the best easily available ways of estimating pneumothorax size. CT scanning (see below) can provide a more accurate determination of the size of the pneumothorax, but its routine use in this setting is not recommended.Not all pneumothoraces are uniform; some only form a pocket of air in a particular place in the chest. Small amounts of fluid may be noted on the chest X-ray (hydropneumothorax); this may be blood (hemopneumothorax). In some cases, the only significant abnormality may be the "deep sulcus sign", in which the normally small space between the chest wall and the diaphragm appears enlarged due to the abnormal presence of fluid. Computed tomography
A CT scan is not necessary for the diagnosis of pneumothorax, but it can be useful in particular situations.
The lungs are fully inflated within the cavity because the pressure inside the airways (intrapulmonary pressure) is higher than the pressure inside the pleural space (intrapleural pressure). Despite the low pressure in the pleural space, air does not enter it because there are no natural connections to an air-containing passage, and the pressure of gases in the bloodstream is too low for them to be forced into the pleural space. Therefore, a pneumothorax can only develop if air is allowed to enter, through damage to the chest wall or damage to the lung itself, or occasionally because microorganisms in the pleural space produce gas. Once air enters the pleural cavity, the intrapleural pressure increases resulting in the difference between the intrapulmonary pressure and the intrapleural pressure (defined as the transpulmonary pressure) to equal zero which cause the lungs to deflate in contrast to a normal transpulmonary pressure of ~4 mm Hg.Chest-wall defects are usually evident in cases of injury to the chest wall, such as stab or bullet wounds ("open pneumothorax"). In secondary spontaneous pneumothoraces, vulnerabilities in the lung tissue are caused by a variety of disease processes, particularly by rupturing of bullae (large air-containing lesions) in cases of severe emphysema. Areas of necrosis (tissue death) may precipitate episodes of pneumothorax, although the exact mechanism is unclear.