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Pneumoperitoneum (gas in the peritoneal cavity)

Pneumoperitoneum literally means free gas in the peritoneal cavity. It usually indicates bowel perforation. Free gas may also be seen up to 3 weeks after abdominal surgery and in trauma (e.g. stabbing).

Main causes of pneumoperitoneum:

1. Perforated peptic ulcer
2. Perforated appendix/bowel diverticulum
3. Post-surgery
4. Trauma

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Note: An abdominal radiograph and an erect chest radiograph are requested together when looking for a pneumoperitoneum. This is because an erect chest radiograph is very sensitive for detecting free abdominal gas since it can detect as little as 2–3 ml. On an erect chest X-ray the free gas is seen as a rim of blackness beneath and very closely opposed to the curve of the diaphragm.

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Figure 17: Two identical erect radiographs of the lower chest. The lower radiograph shows the pneumoperitoneum marked in turquoise.

The radiological signs of a pneumoperitoneum are as follows:

• Rigler’s sign: Also known as the double-wall sign, this is seen when gas is present on both sides of the intestinal wall (i.e. gas within the bowel and free gas in the peritoneal cavity).

  Normally the bowel wall is only just visible, outlined by the gas within the bowel and peritoneal fat outside of the bowel. With a pneumoperitoneum the bowel wall is easily seen as it is outlined by gas within the bowel and gas outside of the bowel.

Figure 18: 1. Diagrammatic representation of normal appearances of the bowel wall. The lumen of the bowel contains gas. You can see the bowel wall, but there is little contrast between the bowel wall and the peritoneal fat outside of the bowel. 2. Diagrammatic representation of Rigler’s sign (double-wall sign). The lumen of the bowel contains gas, and there is also gas within the peritoneal cavity. The bowel wall is therefore clearly seen outlined by the gas either side.

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Note: Rigler’s sign is completely different to Rigler’s triad. Please try not to confuse them. Rigler’s triad refers to the three findings seen in a gallstone ileus (see page 33).

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Note: Sometimes two loops of bowel lying next to each other can appear similar to Rigler’s sign. Please be aware of this and look for loops of bowel, which can usually be identified by the presence of haustra (see page 34) or valvulae conniventes (see page 30).

Figure 19: Two identical radiographs showing two loops of bowel adjacent to each other. This is not Rigler’s sign because you can see the haustra within both loops of bowel. The right radiograph shows the bowel loops marked in brown.

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• Gas outlining the liver: The liver edge may become easily visible due to surrounding free intra-peritoneal gas. Normally the liver (light grey) is outlined by peritoneal fat (dark grey). However, if there is a pneumoperitoneum, the liver is outlined by gas (black) giving a much greater contrast and therefore better visualisation of the liver edge.

Figure 20: Diagrammatic representation of gas outlining the liver. When free gas is present in the peritoneal cavity, the liver edge is seen much more easily. The position of the liver edge is shown by the white arrows.

• Falciform ligament sign: The falciform ligament is a ligament attaching the liver to the anterior abdominal wall (a remnant of the umbilical vein). Normally it is not visible; however, the ligament may become visible if outlined by free intra-peritoneal gas either side of it in a supine patient.

Figure 21: Diagrammatic representation of the falciform ligament sign. When free gas is present in the peritoneal cavity and the patient is lying supine, the falciform ligament becomes visible in the right upper quadrant as an opaque line extending inferiorly from the liver. This line appears in the position as shown by the white arrows.

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Note: If an abdominal radiograph and erect chest radiograph have been performed and there is still uncertainty about the diagnosis of pneumoperitoneum, a computed tomography (CT) scan of the abdomen should be requested. A CT scan gives a much higher radiation dose to the patient, but will clearly show the presence of free gas (white arrow) and may diagnose the underlying cause. Nowadays, most unwell patients with suspected pneumoperitoneum will go straight for a CT scan.

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Figure 22: A CT slice through the abdomen showing a pneumoperitoneum. The gas is marked with an arrow.

Example 1

Figure 23: Two identical abdominal radiographs showing a pneumoperitoneum. There are loops of bowel with gas outlining both sides of the bowel wall in keeping with Rigler’s sign. The right radiograph shows in turquoise and brown the areas where Rigler’s sign is most clearly seen. The lumen of the bowel is marked in brown and the free gas outlining the bowel wall marked in turquoise. The best example of Rigler’s sign is marked with a white circle. An area of normal appearing bowel wall is marked with a white dashed circle for comparison. (You can also see dilated loops of large bowel.)

Example 2

Figure 24: Two identical abdominal radiographs showing a large pneumoperitoneum. There are loops of bowel with gas outlining both sides of the bowel wall in keeping with Rigler’s sign. The right radiograph shows in turquoise the areas where the pneumoperitoneum is most clearly seen. Where Rigler’s sign is most clearly seen, the lumen of the bowel is marked in brown. The best example of Rigler’s sign is marked with a white circle. You can also see gas outlining the liver as shown by the white line.

Example 3

Figure 25: Two identical abdominal radiographs showing a pneumoperitoneum. There is a dilated loop of bowel with gas outlining both sides of the bowel wall in keeping with Rigler’s sign. The right radiograph shows in turquoise and brown the areas where Rigler’s sign is most clearly seen. The lumen of the bowel is marked in brown and the free gas outlining the bowel wall marked in turquoise. The best example of Rigler’s sign is marked with a white circle.

Example 4

Figure 26: Two identical abdominal radiographs of a young child showing a pneumoperitoneum. There are loops of bowel with gas outlining both sides of the bowel wall in keeping with Rigler’s sign, and there is gas outlining the falciform ligament in keeping with the falciform ligament sign. The right radiograph shows in turquoise and brown the areas where Rigler’s sign is most clearly seen. The lumen of the bowel is marked in brown and the free gas outlining the bowel wall marked in turquoise. The position of the falciform ligament is shown with white arrows. The best example of Rigler’s sign is marked with a white circle. (You can also see dilated loops of bowel.)

Example 5

Figure 27: Two identical abdominal radiographs of the upper abdomen showing a pneumoperitoneum. There is gas outlining the falciform ligament in keeping with the falciform ligament sign and there is also gas outlining the liver. The right radiograph shows in turquoise the areas where the pneumoperitoneum is most clearly seen. The position of the falciform ligament is shown with white arrows and the outline of the liver edge is shown by the white lines.

Example 6

Figure 28: Two identical abdominal radiographs taken in the left lateral decubitus position showing a large pneumoperitoneum. The patient is lying on their left side. You can see the bony pelvis on the left of the image, and the dark area on the top right of the image is the base of the patient’s right lung. There are loops of bowel with gas outlining both sides of the bowel wall in keeping with Rigler’s sign and there is also gas outlining the liver. The right radiograph shows in turquoise the areas where the pneumoperitoneum is most clearly seen. Where Rigler’s sign is most clearly seen, the lumen of the bowel is marked in brown. The best example of Rigler’s sign is marked with a white circle. You can also see gas outlining the liver as shown by the white line. The right lung is marked in blue.

Pneumoretroperitoneum (gas in the retroperitoneal space)

Pneumoretroperitoneum literally means gas in the retroperitoneal space. It is rarely seen but is always abnormal.

The retroperitoneal space is a potential space within the abdominal cavity retro (behind) to the peritoneum. It contains the kidneys, ureters, adrenal glands, aorta, inferior vena cava (IVC), most of the pancreas and duodenum, and the ascending and descending colon.

Main causes of retroperitoneal gas:

1. Bowel perforation
   • Posterior duodenal perforation (e.g. peptic ulcer perforation/post-ERCP [endoscopic retrograde cholangio pancreatography] or post-sphincterotomy)
   • Ascending or descending colon perforation (e.g. carcinoma/diverticulitis/ischaemic colitis)
   • Rectal perforation (e.g. post-surgery/post-endoscopy/foreign body insertion)
2. Post-surgical (e.g. residual air from urological/adrenal/spinal surgery)

On an abdominal radiograph the gas outlines retroperitoneal structures such as the kidneys, psoas muscles and retroperitoneal bowel (duodenum, ascending colon, descending colon and rectum). At first glance a pneumoretroperitoneum can appear similar to a pneumoperitoneum as both give increased gas (blackness) on an abdominal radiograph.

The key to identifying a pneumoretroperitoneum is to look for gas (blackness) surrounding all or part of the kidneys.

Figure 29: Diagrammatic representation of the appearance of retroperitoneal gas outlining the kidneys. When gas is present in the retroperitoneal space the kidney edges are seen much more easily. The position of the kidney edges are shown by the white arrows.

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Note: It is possible to have both a pneumoretroperitoneum and pneumoperitoneum at the same time. Remember the key to identifying retroperitoneal gas is to look for gas surrounding the kidneys. In pneumoperitoneum alone you will not see the kidneys.

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Example 1

Figure 30: Two identical abdominal radiographs showing gas in the retroperitoneal space. There are patchy areas of blackness (gas) seen outlining both kidneys either side of the spine. The right radiograph shows the retroperitoneal gas marked in turquoise, clearly outlining both kidneys.

Example 2

Figure 31: Two identical abdominal radiographs of the upper abdomen showing gas in the retroperitoneal space. There are patchy areas of blackness (gas) seen outlining both kidneys either side of the spine. The right radiograph shows the retroperitoneal gas marked in turquoise, clearly outlining both kidneys.

Pneumobilia (gas in the biliary tree)

Pneumobilia is gas in the biliary tree. It appears as branching dark lines in the centre of the liver, usually larger and more prominent towards the hilum. Sometimes you can also see gas in the common bile duct.

Figure 32: Diagrammatic representation of the appearance of gas in the biliary tree on a plain abdominal radiograph. The gas appears as a linear branching pattern (like a tree) and is seen in the centre of the liver, becoming more prominent towards the hilum.

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Note: Pneumobilia can appear very similar to portal venous gas as both give a branching gas pattern within the liver. The way to tell them apart is to look at the location of the gas. Gas in the biliary tree (pneumobilia) is seen in the centre (hilum) of the liver, not the periphery. Portal venous gas is seen in the periphery of the liver because blood in the portal vein flows from the centre (hilum) towards the periphery.

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There are many causes of pneumobilia, not all of which are pathological. The main causes are as follows:

1. Recent ERCP/incompetent sphincter of Oddi (e.g. post-sphincterotomy)
2. External biliary drain insertion/biliary stent insertion
3. Biliary-enteric connection (abnormal connection between biliary tree and bowel)
   • Surgical anastomosis (e.g. Whipple’s procedure)
   • Spontaneous (e.g. gallstone ileus)
4. Infection (rare)
   • Emphysematous cholecystitis (acute cholecystitis with gas-forming organism)

Example

Figure 33: Two identical abdominal radiographs of the upper abdomen showing gas within the biliary tree. There are branching dark lines (gas) projected over the centre of the liver, larger and more prominent towards the hilum. There is also a biliary stent projected over the midline (arrows). This is situated within the common bile duct and explains why gas is easily able to travel from the duodenum into the biliary system. The presence of pneumobilia indicates that the stent is probably patent. The right radiograph shows the gas within the biliary tree marked in dark blue.

Portal venous gas (gas in the portal vein)

Gas in the portal vein appears as branching dark lines within the periphery of the liver on a plain abdominal radiograph. In adults, it indicates serious intra-abdominal pathology and is associated with a very high mortality rate. In infants it is a finding of far less consequence.

Main causes of gas in the portal vein:

1. Ischaemic bowel (most common)
2. Necrotising enterocolitis (NEC) (most common in an infant)
3. Severe intra-abdominal sepsis (diverticulitis/pelvic abscess/appendicitis)

Figure 34: Diagrammatic representation of the appearance of portal venous gas on a plain abdominal radiograph. The gas appears as a linear branching pattern in the periphery of the liver (1). This is because the portal venous blood flows from the portal vein towards the periphery of the liver. If there is a large amount of gas in the portal vein, then it may be seen extending from the periphery to the centre of the liver and even within the splenic vein (2).

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Note: Remember pneumobilia can appear very similar to portal venous gas as both give a branching gas pattern within the liver. The way to tell them apart is to look at the location of the gas. Gas in the biliary tree (pneumobilia) is seen in the centre (hilum) of the liver, not the periphery. Portal venous gas is seen in the periphery of the liver because blood in the portal vein flows from the centre (hilum) towards the periphery.

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Example

Figure 35: Two identical abdominal radiographs of a child showing gas in the portal venous system. There are branching dark lines (gas) projected over the periphery of the liver. In this case the gas is so extensive that it is also seen in splenic vein. The right radiograph shows the gas within the portal venous system marked in dark blue. Gas in the splenic vein is marked in light blue. (You can also see dilated loops of large bowel.)