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 gastroscopy1,2,3,4,5,6,7,8,9
- 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
More on Pneumoperitoneum |
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Further Reading
Keywords
perforated abdominal viscus, air in the peritoneal cavity, peritoneal air, perforated ulcer, peptic ulcer, pneumatosis coli
