eMedicine Specialties > Radiology > Gastrointestinal

Pneumoperitoneum

Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR, Consultant Radiologist and Honorary Professor, North Manchester General Hospital Pennine Acute NHS Trust, UK
Sumaira MacDonald, MBChB, PhD, MRCP, FRCR, Lecturer, Sheffield University Medical School; Endovascular Fellow, Sheffield Vascular Institute; Muthusamy Chandramohan, MBBS, DMRD, FRCR, Consultant Radiologist, Bradford Teaching Hospitals, UK

Updated: Jun 18, 2008

Introduction

Background

The term pneumoperitoneum refers to the presence of air within the peritoneal cavity. The most common cause is a perforation of the abdominal viscus—most commonly, a perforated ulcer, although a pneumoperitoneum may occur as a result of perforation of any part of the bowel; other causes include a benign ulcer, a tumor, or trauma. The exception is a perforated appendix, which seldom causes a pneumoperitoneum.

The presence of a pneumoperitoneum does not, however, always imply a perforation, because a number of other (mostly nonsurgical) conditions are associated with pneumoperitoneum (see Pathophysiology section, below). Likewise, not every bowel perforation results in a pneumoperitoneum; some perforations seal over, allowing little gas to escape. A pneumoperitoneum is common after abdominal surgery; it usually resolves 3-6 days after surgery, although it may persist for as long as 24 days after surgery.

The most common cause of a spontaneous pneumoperitoneum is the introduction of air through the female genital tract.

Pneumoperitoneum. Upright chest radiograph shows a large pneumoperitoneum outlining the spleen and the superior surface of the liver.

Pneumoperitoneum. (Left) Sagittal sonogram through the liver shows a comet-shaped artifact due to free air in the anterior subphrenic space, which causes shadowing. Also note the free peritoneal fluid. (Right) A transverse oblique sonogram through the midabdomen shows dilated loops of small bowel with a streak of free fluid between the bowel loops.

Pneumoperitoneum. Contrast-enhanced axial CT scan through the liver shows a collection of air anterior to the liver, as depicted in the diagram in Image 15. Also note the air surrounding the gallbladder and the leakage of water-soluble contrast material from a perforated duodenal ulcer.

Pathophysiology

Causes of pneumoperitoneum

Air may arise within the peritoneal cavity through several causes. In some cases, pneumoperitoneum is associated with peritonitis, requiring urgent abdominal surgery; in other cases, pneumoperitoneum may be an incidental finding for which only observation is required.

The causes of a pneumoperitoneum are as follows:

Ruptured hollow viscus — Perforated peptic ulcer, ruptured or perforated Meckel sigmoid and jejunal diverticula, necrotizing enterocolitis, toxic megacolon, inflammatory bowel disease, and, in premature infants, idiopathic gastric perforation
Infection of the peritoneal cavity with gas-forming organisms and/or rupture of an adjacent abscess
Iatrogenic factors — Recent abdominal surgery, abdominal trauma, leaking surgical anastomosis, misplaced thoracentesis or pleural drainage tube, endoscopic perforation, injury caused by the tip of an enema catheter, percutaneous needle biopsy, peritoneal catheter placement, peritoneal dialysis, paracentesis, diagnostic or therapeutic procedures, instrumental perforation of uterus or vagina, culdocentesis, ruptured urinary bladder, Rubin test for tubal patency, pelvic examination, intercourse, orogenital insufflation, douching, knee-to-chest movements in females (as may occur during skiing, horseback riding, and squatting), perforating foreign body, and application of compressed air directed toward the anus and overdistention of the stomach with gas during gastroscopy^{1,2,3,4,5,6,7,8,9}
Bowel obstruction caused by a neoplasm, imperforate anus, Hirschsprung disease, or meconium ileus (in which case gas may permeate through the bowel wall)
Pneumatosis intestinalis
Chemoembolization of liver tumors
Extension from the chest — Pneumomediastinum and bronchopleural fistula
Perforated peptic ulcer and Meckel diverticulum — Most common causes in older children
Necrotizing enterocolitis and spontaneous perforation of the stomach — First and second most common causes of organ perforation in newborns and infants, respectively

Regarding the last item above, idiopathic bowel perforation may also affect the small bowel, the large bowel, and the appendix, in addition to the stomach. These are usually single perforations that occur on the antimesenteric border; they are smaller than 1 cm. Bowel necrosis is minimal at the perforation site. A suggested etiologic factor is ischemic necrosis as a result of localized vascular accident. Other causes of perforation in the neonatal period include atresias, meconium ileus, and Hirschsprung disease.

Anatomic locations of abdominal gas collections

Abnormal abdominal gas collections are classified according to the anatomic location, which is often the key to the differential diagnosis.

Extraluminal gas

Extraluminal gas may be involved in pneumoperitoneum or gas within an abscess or fistulous tract. Gas within a pelvic abscess usually indicates that the abscess is of GI origin. Gas within an abscess of pelvic inflammatory disease (PID) is unusual. Gas within the paracolic gutter is usually associated with GI perforation. Diverticulitis may produce extraluminal gas trapped within the adjacent mesentery.

Intraluminal gas

Intraluminal gas may be normal or abnormal. The gas may be intratumoral (within a neoplasm in association with infection or bowel communication), intramural, within a paralyzed loop of bowel, within an obstructed Meckel diverticulum (secondary infection), or within the biliary tree. Normal intraluminal gas can be differentiated by the presence of gas within the bowel lumen in association with peristalsis that is visible on fluoroscopy or ultrasonography (US).

Intraparenchymal gas

Within the portal vein, intraparenchymal gas may sometimes be seen on real-time US as gas microbubbles moving through the liver or as linear collections of hyperlucent branching gas at the periphery of the liver. Gas may be seen in a liver abscess. The differential diagnosis between liver microabscesses and microcalcification may be difficult to make with US. In most other organs, intraparenchymal gas usually indicates an abscess.

Intratumoral gas

Intratumoral gas typically occurs in a gastric leiomyoma or leiomyosarcoma; in such cases, the gas may be seen extending from the lumen of the stomach into the tumor. Intratumoral gas may also be seen in hepatic tumors after chemoembolization; in such cases, differentiation of the gas from an abscess may be difficult with the use of images alone.

Intramural gas

Intramural gas may be related to ischemia. US features that are distinctive of infection include high-amplitude echoes that do not change with the patient's position or with peristalsis. Adjacent bowel wall thickening is often present. Crohn disease and cytomegalovirus (CMV) infection are less common causes of intramural bowel gas.

Pneumatosis coli is often better shown with CT than with US. Acute emphysematous cholecystitis, which often occurs in diabetic patients and the elderly, shows evidence of intramural gas on US. Confusion may occur with mural calcification, which is often curvilinear but which does not have the characteristic ring-down artifact associated with air bubbles. Adenomyosis of the gallbladder may cause a comet-tail artifact.

The presence of free intra-abdominal gas usually indicates a perforated abdominal viscus. The most common cause is perforation of a peptic ulcer. Patients with such conditions need urgent surgery. Occasionally, patients with vague abdominal symptoms have unequivocal features of a pneumoperitoneum, but there is little clinical evidence of peritonism. These patients have a pneumoperitoneum without peritonitis. They are treated expectantly and do not require surgery.

Causes of pneumoperitoneum without peritonitis

Causes of pneumoperitoneum without peritonitis include the following:

Silent perforation that has sealed, as in patients with diabetes mellitus, patients receiving corticosteroids, the elderly, severely ill patients, and comatose patients
Laparotomy
Peritoneal dialysis
Entry through the female genital tract (as occurs during skiing and after squatting)
Perforated jejunal diverticulosis
Leakage through distended bowel (as occurs after endoscopy)
Perforation of a stercoral ulcer
Tracking from a pneumomediastinum
Therapeutic embolization
Dissection of benign gastric emphysema
Ruptured pneumatosis intestinalis cysts
Status epilepticus (associated with a benign pneumoperitoneum)

Frequency

United States

There are no reports of the total incidence of pneumoperitoneum of any cause. The incidence of perforation in association with duodenal ulcer is less then 10%.

International

No demographic data for the international incidence or prevalence of pneumoperitoneum are available.

Mortality/Morbidity

The mortality and morbidity rates associated with pneumoperitoneum depend on the etiology.

Pneumoperitoneum without peritonitis (eg, silent perforation, in which the perforation has sealed) and benign pneumoperitoneum (see Pathophysiology , above) have good prognoses and respond well to expectant management. Pneumatosis intestinalis can lead to a pneumoperitoneum that persists for months; this usually requires no treatment. Similarly, benign gastric emphysema can lead to the dissection of gas into surrounding tissues. The gas may eventually escape into the peritoneum, causing a benign pneumoperitoneum. In a perforated abdominal viscus, mortality and morbidity depend on the degree of peritoneal contamination, the development of localized or generalized peritonitis, and the interval between presentation and surgery.

Overall mortality and morbidity rates in patients with a perforated duodenal ulcer are 9.6% and 28%, respectively. The 2 most important factors that determine mortality rates are age and the time between the onset of symptoms and surgery. Elderly patients have a highly unfavorable prognosis because of their advanced age and the presence of comorbidities, which lead to poorer surgical outcomes. Long-term patients operated on for peptic disease, particularly those with perforated ulcer, have a shortened life expectancy, as compared with that of the general population. Independent predictors of death in this group of patients include age, male sex, emergency surgery, gastric ulcer, and cigarette smoking.

The mortality rate after surgical repair of a perforated gastric ulcer is higher than that after repair of duodenal ulcer because patients with gastric ulcers are often old, and their perforation is often large, allowing for considerable contamination of the peritoneal cavity with gastric juice.

The incidence of endoscopic retrograde cholangiopancreatography (ERCP)–related perforations is low, at 0.35%. Gastric and duodenal perforations usually require surgery, but sphincterotomy and guidewire-related perforations rarely require surgery. Factors associated with an increased risk of perforation include suspected dysfunction of the sphincter of Oddi, older age, a dilated bile duct, sphincterotomy, and longer duration of the procedure.¹⁰

Left colonic perforation has a high mortality rate in patients with septic complications. Most colonoscopic perforations (65%) require surgery; the others may be managed conservatively.

Perforations occurring as a complication of a barium enema examination have a high mortality rate, particularly with an intraperitoneal spill of barium. Barium sulfate itself can cause acute peritonitis. The combination of feces and barium sulfate appears to be more toxic than either feces or barium sulfate by itself. Perforations caused by a barium enema study are more lethal than those caused by colonoscopy; the mortality rate is 50%. Barium contamination of the soft tissues initially results in an inflammatory reaction, with eventual fibrosis and barium granuloma formation. The fibrosis around these granulomas may be exuberant. The barium sulfate tends to remain in the soft tissues throughout the patient's lifetime.

Because of marked pericolonic fibrosis, free perforation in the peritoneal cavity is rare in colonic diverticulitis. Diverticulitis of the small bowel is rare, but if it occurs, it may lead to perforation and abscess formation within the mesentery.^11,12

Race

No racial or ethnic predilection is reported.

Sex

No sex predilection exists, although pneumoperitoneum without peritonitis is common in females. This observation is related to the Rubin test for tubal patency; pelvic examination; intercourse; orogenital insufflation; douching; and knee-to-chest movements such as those performed while skiing, horseback riding, and squatting.

Age

Pneumoperitoneum from a perforated abdominal viscus can occur in persons of any age. The age at presentation depends on the cause. Pneumoperitoneum without peritonitis occurs more often in adults than in children.^7,13,14

Anatomy

The peritoneum is a thin, serous membrane that lines the abdominal cavity. It has parietal and visceral layers, the latter being reflected over the abdominal viscera. A thin layer of serous fluid, which acts as a lubricant, separates the 2 layers. Several intra-abdominal organs are invaginated by visceral peritoneum to such an extent that they are almost completely covered by peritoneum; they have double layers of peritoneum within them as mesenteries and ligaments (see Images below and Images 1-2 in Multimedia).

Pneumoperitoneum. Diagram of transverse section of the abdomen shows peritoneal reflection.

Pneumoperitoneum. Diagram of a sagittal section of the female abdomen and pelvis shows peritoneal reflection.

Pneumoperitoneum. Diagrams of the right upper quadrant show the location of the oblong collection of air in the right subhepatic space seen on a plain supine abdominal radiograph.

Pneumoperitoneum. Diagram of the right upper quadrant shows a triangle-shaped collection of air in the Morison pouch, as seen on a plain supine abdominal radiograph. This collection is usually bound by the 11th rib, and it may be triangular (doge's cap), crescent shaped, or semicircular.

Pneumoperitoneum. Diagram of the right upper quadrant shows the location of a circular collection of air projected over the liver interposed between the anterior liver surface and the anterior thoracic and abdominal wall seen on a plain supine abdominal radiograph.

Male versus female anatomy

In males, the peritoneal cavity is completely closed, but in females, the peritoneal cavity communicates with the genital tract via the fallopian tubes, which provide a potential pathway for suction of air into the peritoneal cavity. The fallopian tubes also constitute a possible pathway of infection from the genital tract.

Peritoneal reflections and ligaments

Various peritoneal reflections and ligaments interrupt the peritoneal cavity.

At the level of the umbilicus, the obliterated fetal urachus (median umbilical cord) forms a shallow ridge that extends cephalad from the dome of the urinary bladder to the umbilicus. The median umbilical cords, which represent the obliterated umbilical arteries, extend from the internal iliac arteries to the umbilicus in the shape of an inverted V.

The falciform ligament is a double layer of peritoneum that forms anteriorly near the midline between the umbilicus and the esophagus. It passes backward and splits to enclose the liver. Superiorly, peritoneal layers form the triangular and coronary ligaments, which enclose the bare area of the liver. The layers of peritoneum investing the liver unite on its visceral surface to form the lesser omentum, which passes from the liver to the esophagus, the stomach, and the first part of the duodenum. The free edge of the lesser omentum between the porta hepatis and duodenum contains the portal vein; the hepatic artery and common bile duct lie anteromedially and anterolaterally, respectively.

The layers of the lesser omentum split to enclose the stomach and then reunite to form the greater omentum and gastrosplenic and lienorenal ligaments. In the free edge of the falciform ligament lies the ligamentum teres (the obliterated fetal left umbilical vein), which passes into the groove between the quadrate lobe and the left lobe of the liver.

The transverse colon is enclosed by the transverse mesocolon, which is attached posteriorly to the anterior aspect of the pancreas.

The lesser sac lies behind the lesser omentum and the stomach. The spleen, which is attached by the gastrosplenic and lienorenal ligaments, forms the left wall of the lesser sac. The right of the sac communicates with the main peritoneal cavity via the foramen of Winslow.

These reflections and peritoneal spaces are important radiologically because it is here that air accumulates in a pneumoperitoneum.

Presentation

The clinical features depend on the cause of the pneumoperitoneum. Pneumoperitoneum that has a benign cause and that is unaccompanied by peritonitis is usually asymptomatic. In such cases, pneumoperitoneum may be an incidental finding, although occasionally, vague abdominal symptoms may occur.

The symptoms from a perforated hollow abdominal viscus depend on the development of peritonitis. Peritonitis may be diffuse or localized. The severity of peritonitis depends on the type of GI contents released into the peritoneal cavity.

Perforation of a peptic ulcer is usually a sudden event that causes excruciating pain starting in the epigastrium and spreading throughout the abdomen. Pain may be referred to the shoulder tip because of diaphragmatic irritation. Shock and collapse often occur, but initially, the pulse rate is characteristically slow, and vomiting occurs once or twice. The abdomen is exquisitely tender, and it may be rigid and boardlike. Bowel sounds may be absent because of ileus secondary to peritonitis. The patient may lie still because every movement provokes pain. If the perforation is walled off, the pain and tenderness may be confined to the epigastrium. Rarely, the pain and tenderness are maximal in the right iliac fossa, mimicking appendicitis, because the gastric contents may reach the right iliac fossa via the right paracolic gutter. Symptoms from a perforated abdominal viscus are often alarming; they may be insidious in the elderly, in those with diabetes, and in the very young.

Perforation of an appendix usually occurs distal to the site of obstruction. The sudden discharge into the peritoneal cavity of highly infective material may lead to shock and signs of diffuse peritonitis, although more often, the peritonitis is more localized. A pneumoperitoneum is rare with a perforated appendix.

Colonic perforation is usually a complication of diverticulitis. Free perforation into the peritoneum from a colonic carcinoma may occur, though this is relatively rare. Patients with diverticulitis may present with pain in the left iliac fossa, a tender palpable mass, and fever. One third of the patients have a history of recurrent episodes of similar attacks.

Preferred Examination

CT is regarded as the criterion standard for the detection of a pneumoperitoneum; it provides exquisite scans, and it is theoretically more sensitive than plain abdominal radiography. However, CT is not always required when a pneumoperitoneum is suspected. Despite the contrary consensus, the accuracy of supine abdominal radiography closely approximates CT when the entire abdomen is imaged.

US is usually the first investigation performed in emergent patients. US is a noninvasive test that is widely available and is particularly valuable in children, pregnant women, and individuals of reproductive age. Some studies have reported sensitivities greater than that of plain abdominal radiography in the diagnosis of a pneumoperitoneum.^15,16 Compared with plain radiography, US examination also has the advantage of depicting other changes, such as free abdominal fluid and inflammatory masses.^17,18,19

Limitations of Techniques

Free intraperitoneal air is often missed with plain radiology. The failure to detect free air is more a function of lacking standardization and of inadequate technique. In most institutions, a kidney, ureter, bladder (KUB) image is used instead of other images in cases of suspected pneumoperitoneum. These radiographs are insufficient for the diagnosis of a pneumoperitoneum because the uppermost portion of the peritoneal cavity, which reveals important signs, may be excluded from the examination.

If proper technique is applied, contrast-enhanced studies and CT scanning can be avoided. Although a CT scan is considered a criterion standard in the diagnosis of a pneumoperitoneum, it is expensive in terms of both radiation burden and cost.

With both conventional radiology and CT, oral contrast material is used to opacify the lumen of the GI tract and to demonstrate a bowel leak. The leak may be too small, or it may have sealed, and extravasation of the contrast material may not occur. When a distal small or large bowel perforation is suspected, one major limitation of the use of oral contrast material is that several hours may be required to opacify the bowel. Thus, the randomness of bowel opacification, the difficulty encountered in securing the cooperation of a sick patient, and the relative clinical urgency for diagnosis limit the value of oral contrast enhancement.

In addition, oral contrast material may obscure relevant clinical information, such as the presence of an appendicolith and bowel hemorrhage, although these may not be relevant in terms of bowel perforation. Although US is a noninvasive and relatively inexpensive test, it remains operator dependent, and it has limitations in patients who are obese and in those with a large amount of intra-abdominal gas.^20,21

Differential Diagnoses

Appendicitis	Gastric Carcinoma
Bowel, Trauma	Gastric Ulcer
Carcinoid, Gastrointestinal	Gastrointestinal Stromal Tumors - Leiomyoma/Leiomyosarcoma
Colitis, Ischemic	Pneumatosis Intestinalis
Colon, Adenocarcinoma
Colon, Diverticulitis
Duodenum, Ulcers

Radiography

Findings

Optimal radiographic technique is important with a suspected abdominal perforation. At least 2 radiographs should be obtained, including a supine abdominal radiograph and either an erect chest image or a left lateral decubitus image. The patient should remain in position for 5-10 minutes before a horizontal-beam radiograph is acquired. A lateral chest x-ray has been found to be even more sensitive for the diagnosis of pneumoperitoneum than an erect chest x-ray. The images below depict radiographic technique.

Pneumoperitoneum. Plain radiograph of the right upper quadrant shows a tiny streak of air under the diaphragm due to a pneumoperitoneum.

Pneumoperitoneum. Upright chest radiograph shows a large collection of air under both hemidiaphragms due to perforated duodenal ulcer.

Pneumoperitoneum. Upright chest radiograph shows a large pneumoperitoneum outlining the spleen and the superior surface of the liver.

Pneumoperitoneum. Images in a 24-year-old man known to have Crohn disease who presented with acute abdominal pain. (Left) Supine radiograph of the right upper quadrant shows a vague lucency overlying the liver. (Right) Lateral decubitus radiograph shows an obvious pneumoperitoneum. Note also the air-fluid levels within the bowel due to associated ileus. At surgery, a perforated terminal ileum secondary to Crohn disease was diagnosed.

Pneumoperitoneum. This elderly patient was knocked down by a car in a motor vehicle accident. Her main complaint was hip pain. Plain abdominal radiograph of the pelvis confirms the presence of a fracture of the neck of the left femur, but also note a bowel relief sign (arrow). At surgery, a perforation of the small bowel secondary to blunt abdominal trauma was confirmed.

Pneumoperitoneum. Coned view of the lower abdomen shows the lateral umbilicus sign (arrow), which is a sign of a large pneumoperitoneum on a plain abdominal radiograph. Note also the bowel relief sign.

Pneumoperitoneum. Coned view of the lower abdomen shows the urachus sign (arrows), which is another sign of a large pneumoperitoneum on a plain abdominal radiograph.

Pneumoperitoneum. Diagrams of the right upper quadrant show the location of the oblong collection of air in the right subhepatic space seen on a plain supine abdominal radiograph.

Pneumoperitoneum. Supine abdominal radiograph in the same patient as in Image 7 shows an elliptical collection of air within the subhepatic space. Note also the bowel relief sign.

Pneumoperitoneum. Diagram of the right upper quadrant shows a triangle-shaped collection of air in the Morison pouch, as seen on a plain supine abdominal radiograph. This collection is usually bound by the 11th rib, and it may be triangular (doge's cap), crescent shaped, or semicircular.

Pneumoperitoneum. Plain abdominal radiograph of a patient with a pneumoperitoneum shows a triangular collection of air in the Morison pouch (solid arrows). Also note the bowel relief sign (open arrows).

Pneumoperitoneum. (Left) Supine chest radiograph shows a tiny collection of air under the diaphragm (arrow) in a patient with pneumoperitoneum. (Right) Supine abdominal radiograph shows a triangular collection of air in the Morison pouch (arrow).

Pneumoperitoneum. Diagram of the right upper quadrant shows the location of a circular collection of air projected over the liver interposed between the anterior liver surface and the anterior thoracic and abdominal wall seen on a plain supine abdominal radiograph.

Pneumoperitoneum. A 66-year-old man was admitted to the hospital with urinary retention and was being examined for prostatic pathology. While he was in the ward, he had a sudden onset of acute abdominal pain. Findings on this supine radiograph were interpreted as being normal, but note the pear-shaped lucency projected over the liver indicative of a pneumoperitoneum (see also Images 19-20).

Pneumoperitoneum. A 49-year-old man was admitted to the hospital with acute abdominal pain. Findings from the initial plain abdominal radiographs were interpreted as being normal. Because the cause of his abdominal pain was not clear, an upper GI series performed with water-soluble contrast material was requested. (Left) Radiograph obtained early in the study shows no leakage, but note the triangular collection of air within the Morison pouch. (Right) When this earlier plain radiograph was interpreted, the collection of air within the Morison pouch was seen; this had escaped detection earlier (arrow).

Pneumoperitoneum. Supine abdominal radiograph in a 26-year-old man with known Crohn disease who presented with acute abdominal pain. Findings on the initial plain radiographs were interpreted as normal. Radiograph shows barium within the stomach, but note air within the lesser sac and in the Morison pouch. At surgery, a perforated duodenal ulcer was confirmed.

Pneumoperitoneum. Supine abdominal radiograph shows a falciform ligament (arrow).

Pneumoperitoneum. (Left) Upper GI barium series in a patient who presented with acute abdominal pain. Note the duodenal ulcer crater and air within the ligamentum teres (arrow). (Right) Follow-up barium study shows that the barium leak and air within the ligamentum teres (arrow) persists.

Pneumoperitoneum. Plain abdominal radiograph in a 24-year-old man who presented with acute abdominal pain 24 hours after undergoing an upper GI series with barium. Radiography was performed to evaluate peptic ulcer disease. Note that barium has been released into the anterior subphrenic space (arrows). Note also the delineation of the falciform ligament of the escaped barium. Also seen is barium within the grooves of mesenteric vessels (arrows). The bowel relief sign is obvious.

Pneumoperitoneum. Left, Posteroanterior (PA) chest radiograph in a patient receiving long-term steroid therapy who presented with breathlessness but no abdominal symptoms. A large pneumoperitoneum is present, and because of the lack of abdominal signs, she was observed. (Right) Two weeks later, a repeat anteroposterior (AP) chest radiograph was obtained because the patient felt vaguely unwell, although she had no abdominal signs. Note the air-fluid level to the left of the upper lumbar spine and left basal pleural effusion. At surgery, a retroperitoneal abscess secondary to a colonic perforation was diagnosed. Patients who are diabetic or those taking steroids are prone to silent perforations.

Pneumoperitoneum. This patient was unwell after endoscopic retrograde cholangiopancreatography (ERCP). Plain abdominal radiograph shows a falciform ligament (arrow) and the bowel relief sign. The patient was treated conservatively because of a lack of abdominal signs. He recovered fully.

Pneumoperitoneum. Plain abdominal radiograph in a patient in whom a retropneumoperitoneum developed after endoscopic retrograde cholangiopancreatography (ERCP).

Pneumoretroperitoneum. Chest radiograph (left) and plain radiograph (right) show surgical emphysema and retroperitoneal air secondary to a retroperitoneal bowel perforation.

Pneumoperitoneum. Image shows bowel perforation after bowel infarction. Note the large pneumoperitoneum and air within the portal venous radicals.

Pneumoperitoneum, mimics. Pneumatosis coli secondary to necrotizing enterocolitis.

Pneumoperitoneum, mimics. Plate atelectasis at the right lung base mimics a small pneumoperitoneum.

Pneumoperitoneum, mimics. Large bulla at the base of the right lung mimics a large pneumoperitoneum.

Pneumoperitoneum, mimics. Image shows colonic interposition. Note the haustra.

Pneumoperitoneum, mimics. Air-containing bowel loops within a Morgagni hernia.

Pneumoperitoneum, mimics. Air within the portal venous radicals secondary to bowel infarction is an ominous sign in adult patients.

Pneumoperitoneum, mimics. Image shows air within the biliary tree after papillotomy.

Some authors suggest a complete free-air series, which includes the acquisition of a left lateral decubitus image after the patient is in the proper position for 20 minutes and the acquisition of an upright radiograph after 5 minutes and a supine radiograph after 1 minute. The total examination time is therefore 26 minutes, which becomes cumbersome for patients who are ill and in pain.

The plain radiographic signs of a pneumoperitoneum have been classified into those of a small pneumoperitoneum and those of a large pneumoperitoneum associated with more than 1000 mL of free air.^22,23,24,25

Signs of a large pneumoperitoneum

Signs of a large pneumoperitoneum include the following:

The football sign, which usually represents a large collection of air within the greater sac. The air seems to outline the entire abdominal cavity. Some authorities apply the term football sign to the air surrounding the falciform ligament, which looks like the laces of a football.
The gas-relief sign, the Rigler sign, and the double-wall sign are all terms applied to the visualization of the outer wall of bowel loops caused by gas outside the bowel loop and normal intraluminal gas.²⁶ Free intraperitoneal gas and intraperitoneal fluid in excess of 1000 mL are usually required to elicit this sign.
The urachus is a vestigial peritoneal reflection not normally seen on a plain abdominal radiograph. It has the same opacity as other soft tissue intra-abdominal structures, but when a pneumoperitoneum occurs, air outlines the urachus. The urachus is then seen as a thin midline linear structure in the lower abdomen proceeding cephalad from the dome of the urinary bladder. The base of the urachus may be slightly thicker than the apex.
The lateral umbilical ligaments, which contain the inferior epigastric vessels, may become visible as an inverted V sign in the pelvis as a result of a large pneumoperitoneum.
A telltale triangle sign represents a triangular pocket of air between 2 loops of bowel and the abdominal wall.
Scrotal air may be seen in children as a result of peritoneal intrascrotal extension (through patent process vaginalis).
Free air under the diaphragm may depict the diaphragmatic muscle slips as arcuate soft tissue bands, arching parallel to the diaphragmatic dome.
Gas within the lesser sac may be present, particularly with a perforation of the posterior wall of the stomach.
Air may be present around the spleen.
Signs of partial large bowel obstruction with a sigmoid diverticulum perforation may occur in association with signs of a pneumoperitoneum.
On a left lateral decubitus radiograph, free air is apparent around the inferior edge of the liver, which forms the least dependent part of the abdomen in that position. In obese patients, particularly women, the least dependent part may be overlying the hips, a point at which free air may be present.

Right upper quadrant gas

Menuck et al published an important report in 1976 describing the importance of right upper quadrant gas, which is best seen in a small pneumoperitoneum on supine radiographs.²⁷

An oblong saucer-shaped or cigar-shaped collection of air may be present in the subhepatic space inferior to the lower edge of the liver.
A triangular collection of air may be seen in the Morrison pouch, which is bound by the left 11th rib. The configuration of this air collection varies and may be semicircular, crescent shaped, or triangular. This has been likened to a doge's cap.
A round, oval, or pear-shaped collection of air may be projected over the liver shadow between the ventral liver surface and the anterior thoracic or abdominal wall. This collection may be solitary or present in several smaller locules. The liver shadow normally has no gas overlying it, though such gas does occur in association with the following conditions: colonic interposition, subphrenic abscess, liver abscess with gas-forming organisms, the presence of portal venous gas, the presence of biliary gas, and as an effect of chemoembolization.
Parahepatic gas bubbles may be seen lateral to the right edge of the liver.
The cupola sign (saddlebag or moustache sign) represents gas trapped under the central tendon of the diaphragm.
Small collections of air around the periduodenal area normally occur with a retroperitoneal perforation in the second part of the duodenum, but it has also been described with a pneumoperitoneum.
The falciform ligament is a linear soft tissue opacity coursing vertically between the umbilicus and the ligamentum teres notch in the inferior surface of the liver. The falciform ligament may be thin and of uniform diameter, but it is occasionally a linear lobulated structure that may be several millimeters thick.
Gas within the ligamentum teres notch may be seen as an inverted V–shaped collection on the undersurface of the liver at the junction of the right and left lobes.
Gas within the ligamentum teres is seen as a vertical slitlike or oval lucency lying between the 11th and 12th right ribs and 2.5-4 cm lateral to the spinal edge. The gas collection may be 2-7 mm wide and 6-20 mm long.
Air in the gallbladder fossa is a recently described sign that is better demonstrated with CT than with radiography.

Use of contrast medium in the evaluation of suspected perforation

Not infrequently, patients with an acute abdomen and suspected perforation have no free gas, as assessed on plain radiographs. The differential diagnosis usually includes acute cholecystitis, pancreatitis, and a perforated ulcer.

To aid in the examination, about 50 mL of water-soluble contrast agent is given orally or via a nasogastric tube with the patient lying right-side down. Fluoroscopy may be used to examine the patient; spot images are obtained after the patient stays in the right lateral decubitus position.

In patients with a perforated ulcer, contrast material may leak into the peritoneum. Fluoroscopy is not always essential, and plain abdominal radiography may be performed. Patients with pancreatitis may also be examined with this technique; in these patients, an edematous, stretched duodenal loop may be visualized. The use of ionic water-soluble contrast medium should be avoided because patients may inadvertently inhale it.

Degree of Confidence

Plain radiography remains the mainstay in imaging an acute abdomen, including a perforated abdominal viscus. As little as 1 mL of free gas can be detected on a plain radiograph—either an erect chest image or a left lateral decubitus abdominal image.²⁸ Pneumoperitoneum is detectable in 56% of patients by using a supine abdominal image. In approximately one half of patients with a pneumoperitoneum, gas overlies the right upper quadrant.

False Positives/Negatives

Mimics of a pneumoperitoneum include the following: