Imaging features
Free gas most commonly accumulates in the central abdomen beneath the anterior abdominal wall or in peritoneal recesses. On standard abdominal computed tomography (CT) windows, small volumes of free gas can be difficult to distinguish. To avoid missing dangerous pathology, scans must be reviewed using wide window settings, specifically the lung window.
On an abdominal plain radiograph taken with the patient in the supine position, two classic signs of pneumoperitoneum can be identified:
- Rigler's sign (Rigler's sign): gas is visualized on both sides of the bowel wall (both inside the lumen and outside in the peritoneal cavity). Because of this, bowel loops are clearly separated from each other and acquire a characteristic "ghostlike" appearance.
- Falciform ligament contouring (falciform ligament): the ligament is clearly outlined by gas on both sides. This sign is usually visible in the epigastric region, slightly to the right of the midline.
Causes of pneumoperitoneum without peritonitis
The detection of free gas is traditionally associated with acute surgical pathology, but there is a whole range of conditions in which pneumoperitoneum is not accompanied by peritoneal inflammation (peritonitis).
| Main causes |
|---|
| Postoperative period (up to 7 days after surgery) |
| Asymptomatic ("silent") hollow viscus perforation (often in elderly, unconscious patients, or those on steroid therapy) that has walled off spontaneously |
| Rupture of a cyst in pneumatosis cystoides intestinalis (pneumatosis intestinalis) |
| Peritoneal dialysis |
| Downward spread of gas from the mediastinum in pneumomediastinum |
| Stercoral ulcer |
| Air entry through the fallopian tubes and vagina |
Differential diagnosis
Certain pathological processes can mimic free gas. A classic example is a subdiaphragmatic abscess.
On an upright chest radiograph, it may appear as an encapsulated collection of gas under the right hemidiaphragm. The key radiographic feature distinguishing it is the presence of a horizontal fluid level. Contrast-enhanced CT is used for definitive confirmation of a subdiaphragmatic abscess and reliable differentiation from true pneumoperitoneum.
Related conditions: gas in the retroperitoneum
In addition to pneumoperitoneum, gas can accumulate in the retroperitoneum. This occurs with posterior gastric or duodenal ulcer perforation, complications of sigmoid diverticular disease, and following colonoscopy (iatrogenic causes).
On a supine radiograph, free retroperitoneal gas is best seen in the flanks or around the kidneys. On CT (also in the lung window), gas is clearly visualized in the perirenal spaces and around the inferior vena cava.
Feature: retroperitoneal gas can spread cranially (into the mediastinum) and caudally (into the gluteal region and thigh). The presence of gas in the soft tissues of the left thigh is considered a classic sign of diverticular perforation.
Briefly: answers to topical questions
Which anatomical structures are damaged during spontaneous cyst rupture in pneumatosis cystoides intestinalis?
In pneumatosis cystoides intestinalis, gas cysts are located within the bowel wall, predominantly in the subserosal and submucosal layers.
Their intermittent subclinical rupture can cause pneumoperitoneum without signs of peritonitis.
Class and exam questions
In which CT window should small pockets of free intraperitoneal gas be looked for?
Free gas must be sought using wide window settings—the so-called lung window (W: 1500, L: -600)—since it is easily missed on standard abdominal scans.
What is Rigler's sign on a radiograph?
This is a sign of pneumoperitoneum in which the bowel wall is clearly visible on both sides because gas is present both inside its lumen and outside (free gas in the abdominal cavity).
Can pneumoperitoneum be normal or follow a benign course?
Yes, pneumoperitoneum without peritonitis can be observed for up to 7 days after surgery, in peritoneal dialysis, upon rupture of cysts in intestinal pneumatosis, or when air spreads from a pneumomediastinum.
What else to study on this topic
- Gas in the bowel wall and bowel infarction
- Pneumatosis cystoides intestinalis
- Emphysematous pyelonephritis and gas in the urinary tract
- Differential diagnosis of small and large bowel dilation
- Causes of gastric dilation: mechanical obstruction and volvulus
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