This time, we have an interesting CXR to examine. There are three distinct places in the image below where air is in places it shouldn’t be. Can you identify them?
Need a refresher on how to read a CXR? This post will help you out.
Scroll down further for the answer.
Image Key: Blue arrows–supraclavicular subcutaneous emphysema; Purple arrows–pneumothorax; Red arrows = pneumoperitoneum
Pneumothorax: air in the pleural space
On an upright CXR, a pneumothorax is one of the more easily identifiable pathologies in the thoracic cavity. The presence of air separates the parietal pleura and visceral pleura, resulting in the lung tissue being pushed towards midline. This results in the edge of the lung tissue being easily identifiable (purple arrows). The rest of the cavity is devoid of lung markings.
It is important to note that the size of a pneumothorax can vary greatly. Therefore even if the absence of lung markings isn’t as striking as it is in this picture, the edges of the thoracic cavity should always be closely examined to see if there is any evidence of air. On the other extreme is a tension pneumothorax, which is defined as an expanding pocket of air in the thoracic cavity, which causes half of the lung to completely collapse and shift the mediastinal structures in the contralateral direction.
Pneumoperitoneum: air in the abdominal cavity
The presence of air in the abdominal cavity comes from two major sources: outside the body or the GI tract. Air from outside the body enters into the abdominal cavity through either iatrogenic (surgery, peritoneal dialysis) or traumatic (penetrating wound) routes. Air from the GI tract enters if any segment of the bowel is perforated (most commonly secondary to a duodenal ulcer). On an upright CXR, as is shown above, the air rises to the level of the diaphragm and can be identified.
Even though the subdiaphragmatic air in this picture is clearly evident, CXR’s are not the gold standard diagnostic test for pneumoperitoneum. Abdominal CT scans can pick up much smaller amounts of air that may be difficult to visualize on a plain film.
Subcutaneous Emphysema: air in subcutaneous tissue planes
The image above has distinct areas of radiolucency in the supraclavicular area as a result of air tracking in the subcutaneous tissue, which is defined as subcutaneous emphysema. The area is patchy from the infiltration of air into soft tissues.
Similarly to pneumomediastinum, the air comes from either inside the body (secondary to pneumothorax, pneumomediastinum) or outside the body (penetrating trauma, chest tube insertion site). The air travels along fascial planes between the dermal and muscular layers. Another, more serious, cause is necrotizing fasciitis. In this case, however, it is likely that the air entered into the subcutaneous tissues as a result of trauma, which also resulted in a pneumothorax.
Author: Jaymin Patel
Image Contributor: Katren Tyler, M.D.