Abdominal X-ray - Abnormal bowel gas pattern Free gas/pneumoperitoneum
Key points
Bowel perforation is a surgical emergency
An ERECT chest X-ray should be requested if perforation is suspected
Be familiar with Rigler's sign
Free gas, or pneumoperitoneum, is gas or air trapped within the peritoneal cavity, but outside the lumen of the bowel. Pneumoperitoneum can be due to bowel perforation, or due to insufflation of gas (CO2 or air) during laparoscopy. Both these causes have identical X-ray appearances, but very different clinical significance.
Bowel perforation
Bowel perforation is a surgical emergency. Medical students and doctors must be familiar with the X-ray appearances of pneumoperitoneum in the clinical context of an acute surgical abdomen.
Erect chest X-ray
Patients presenting with an acute surgical abdomen should be investigated with an ERECT chest X-ray, as well as the standard supine abdominal X-ray. The patient should be positioned sitting upright for 10-20 minutes prior to acquiring the erect chest X-ray image. This allows any free intra-abdominal gas to rise up, forming a crescent beneath the diaphragm. It is said that as little as 1ml of gas can be detected in this way.
Bowel perforation is a favourite of the finals radiology OSCE. If you are shown an abdominal X-ray you should request to see the erect chest X-ray.
This patient has a large volume of free gas under the diaphragm. Dark crescents have formed separating the thin diaphragm from the liver on the right, and bowel on the left.
If perforation is suspected you must look very closely. In this patient, only a very thin crescent has formed under only the right hemidiaphragm.
Pneumoperitoneum due to insufflation of gas at laparoscopy has identical appearances.
Abdominal X-ray
Although the erect chest X-ray is a much more sensitive investigation for pneumoperitoneum, there are several signs that may be useful in detecting free gas on an abdominal X-ray.
Rigler's/double wall sign
Rigler's sign (also known as the double wall sign) is the appearance of lucency (gas) on both sides of the bowel wall.
