Pneumoperitoneum on X‑ray
Pneumoperitoneum is gas within the peritoneal cavity, outside the bowel. In a patient who has not had recent surgery or a procedure, it usually means a perforated hollow viscus, most often a perforated peptic ulcer or a perforated colon (from diverticulitis, cancer or obstruction). Perforation is a surgical emergency. Free gas can be seen on an erect chest X-ray, where it collects under the diaphragm, and on a supine abdominal X-ray, where the signs are subtler and easily missed. CT is the most sensitive test, and a normal plain film does not exclude perforation. This page covers both views, the mimics, the benign causes and the next steps.
1. The erect chest X‑ray
With the patient upright, free gas rises to the highest point of the abdomen: under the hemidiaphragms.
- Look for a lucent crescent of gas under one or both hemidiaphragms. It is easiest to see on the right, where it separates the dense liver from the diaphragm.
- The diaphragm then appears as a thin, even line of soft tissue, with lung above and gas below.
- On the left, free gas has to be distinguished from the gastric bubble and the splenic flexure (see mimics below).
Key facts: volume and timing [VARIES]
- Classic teaching is that, with ideal technique and the patient upright for about 10 minutes beforehand, an erect chest X-ray can show as little as about 1–2 mL of free gas.
- In real clinical practice its sensitivity for perforation is much lower, commonly quoted at around 50–70%.
- CT is more sensitive and also shows the likely site of perforation.
If the patient cannot sit up, the chest film is taken semi-erect or supine, and the subdiaphragmatic sign may not be seen. The supine signs then matter.
See how to read a chest X-ray: looking under the diaphragm is part of the D in ABCDE.
2. Signs on the supine abdominal X‑ray
In a supine patient, free gas rises to the front of the abdomen, collecting in the midline and over the liver. It does not form a crescent; instead, it outlines structures that are not normally seen.
| Sign | What you see |
|---|---|
| Rigler's sign (double-wall sign) | Both the inner (mucosal) and outer (serosal) walls of a bowel loop are visible, because there is gas on both sides of the wall. Usually needs a moderate volume of free gas; some sources quote around 1 L for reliable visibility. [VARIES] |
| Football sign | A large oval lucency over the central abdomen, from a large volume of gas outlining the peritoneal cavity. More common in children. |
| Falciform ligament sign | A vertical line in the right upper abdomen, just to the right of the midline: the falciform ligament outlined by gas on both sides. |
| Triangle (telltale triangle) sign | A small triangular pocket of gas between three adjacent bowel loops, or between two loops and the abdominal wall. |
| Lucent liver sign | Gas lies in front of the liver, so the liver looks darker than expected. |
| Cupola sign | Gas collects under the central tendon of the diaphragm, forming a lucent arc over the lower thoracic spine. |
| Inverted V sign | The lateral umbilical ligaments in the pelvis are outlined, forming an upside-down V. |
| Urachus sign | The median umbilical ligament (urachus) is outlined as a midline line in the lower abdomen. |
| Doge's cap sign | A triangular collection of gas in Morison's pouch, under the liver. |
Pitfall: pseudo-Rigler
Two loops of gas-filled bowel lying next to each other can create a false double-wall appearance, because their touching walls form a thin line with gas on both sides. True Rigler's sign shows a wall with gas on both sides along a single loop, often over a long segment. If unsure, get an erect chest X-ray or CT.
3. The left lateral decubitus film
When the patient cannot sit up, a left lateral decubitus abdominal film (patient lying on the left side, horizontal beam) can show free gas rising between the liver and the right lateral abdominal wall, where it is not confused with the stomach. The patient should lie in position for about 10 minutes beforehand. In practice, CT has largely replaced this view in adults.
4. Mimics
- Chilaiditi sign: a loop of colon lying between the liver and the right hemidiaphragm. It looks like free gas, but haustral folds can be seen crossing the lucency, and the gas does not move freely. It is usually an incidental finding, though rarely it causes symptoms (Chilaiditi syndrome).
- Gastric bubble under the left hemidiaphragm: it has a thicker wall, may show rugal folds, and often contains a fluid level. Free gas outlines a thin diaphragm.
- Subphrenic abscess: a gas collection under the diaphragm, often with an air–fluid level, and usually a single locule that does not change shape with position. The patient is often septic, sometimes after surgery.
- Basal lung lucency or a thin rim of lung above an irregular diaphragm.
- A cupola of fat or an irregular diaphragm contour with lobulation.
5. Free gas that isn't a perforation
- After surgery: free gas after abdominal surgery is expected. It usually clears within about 3–7 days, and can persist longer after open surgery. Gas after laparoscopy usually clears faster, because carbon dioxide is absorbed quickly. [VARIES] An increasing volume of free gas on serial films, or gas appearing late, suggests a leak.
- Peritoneal dialysis is a common benign cause.
- After procedures, such as PEG insertion or endoscopic polypectomy.
- Pneumatosis cystoides intestinalis (benign gas cysts in the bowel wall) can rupture.
- Gas tracking down from the chest, such as in ventilated patients with barotrauma. See pneumomediastinum and surgical emphysema.
In each case, correlate with the clinical picture: a patient with free gas and peritonitis, sepsis or worsening pain needs urgent surgical assessment regardless of the recent history.
6. Next steps
- Free gas plus a clinical picture of perforation: urgent surgical referral. Resuscitation, analgesia and antibiotics should not wait for further imaging.
- CT abdomen and pelvis is the most sensitive test for free gas, shows small volumes not visible on plain films, and usually identifies the site and cause.
- A normal erect chest X-ray does not exclude perforation. If suspicion remains, request CT (RCR iRefer).
- Retroperitoneal perforation (for example of the duodenum or rectum) may not produce intraperitoneal gas at all; gas may instead outline the kidneys or psoas.
Red flags: act now
Escalate immediately
- Gas under the diaphragm in a patient without recent surgery or a procedure.
- Any supine sign of free gas, such as Rigler's or the football sign.
- Post-operative free gas that is increasing, or a deteriorating post-operative patient.
- Peritonism with a normal film: CT, and surgical review.
What to focus on, by role
| Role | Priorities |
|---|---|
| Medical student | Subdiaphragmatic gas on the erect chest film, Rigler's sign, and the Chilaiditi mimic. |
| Nurse | Sit the patient upright for the time needed before an erect film if safe. Escalate abdominal pain with rigidity, fever or a falling blood pressure. |
| Junior doctor | Look under the diaphragm on every erect chest film, know the supine signs, and request CT when the film is negative but the patient looks perforated. |
| Radiographer | True erect positioning after an adequate wait; include both hemidiaphragms; a decubitus view if requested and appropriate. |
| Advanced / FRCR | The full set of supine signs, post-operative interpretation and retroperitoneal gas. |
Practise on abdominal X-rays →
References
- Radiopaedia. Pneumoperitoneum. radiopaedia.org
- Radiology Masterclass. Pneumoperitoneum. radiologymasterclass.co.uk
- Royal College of Radiologists. iRefer: making the best use of clinical radiology. irefer.org.uk
For education only. Not a substitute for a formal report, senior review or your local policy. Never use this page to make decisions about a real patient.