Along with symptoms such as breathlessness, establishing the size of a pneumothorax helps to determine management.
Pneumothoraces are managed in accordance with British Thoracic Society guidelines which define a large pneumothorax as being of greater than 2 cm width at the level of the hilum.
This chest X-ray shows a large pneumothorax (P) which is >2 cm depth at the level of the hilum.
British Thoracic Society Guidelines
Guidelines for the definition and treatment of pleural disease are subject to change. See the British Thoracic Society website for up-to-date information.
This chest X-ray shows an apical pneumothorax (P) which does not reach down to the level of the hilum.
Although size is an important factor in the management of a pneumothorax the clinical features are also considered. If a patient with a pneumothorax is breathless or has known lung disease then interventional measures can be implemented even if the pneumothorax is small.
The '2 cm rule' does not apply to a pneumothorax which is localised or which compresses the lung non-uniformly.
In this image the lung is compressed in a non-uniform manner, that is, the edge of the lung is not parallel with the chest wall. Even though the lung remains in contact with the chest wall at the level of the hilum (tick), there is still a large volume of air in the pleural cavity.
This patient had chronic lung disease and was breathless, so a chest drain was inserted.