Why is the left lateral decubitus position recommended as part of an acute abdominal series?

The term pneumoperitoneum refers to the presence of air within the peritoneal cavity. Pneumoperitoneum results from tissue ischemia, erosion, infection, mechanical injury, or thermal injury, and the differential diagnosis is wide, including cancer, iatrogenic injury, infection, and ulcerative disease. [1]  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. 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 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 the images below). 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.

Why is the left lateral decubitus position recommended as part of an acute abdominal series?
Pneumoperitoneum. Diagram of transverse section of the abdomen shows peritoneal reflection.

Why is the left lateral decubitus position recommended as part of an acute abdominal series?
Pneumoperitoneum. Diagram of a sagittal section of the female abdomen and pelvis shows peritoneal reflection.

Why is the left lateral decubitus position recommended as part of an acute abdominal series?
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.

Why is the left lateral decubitus position recommended as part of an acute abdominal series?
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.

Why is the left lateral decubitus position recommended as part of an acute abdominal series?
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.

The most common cause of a spontaneous pneumoperitoneum is the introduction of air through the female genital tract (see the images below).

Why is the left lateral decubitus position recommended as part of an acute abdominal series?
Pneumoperitoneum. Upright chest radiograph shows a large pneumoperitoneum outlining the spleen and the superior surface of the liver.

Why is the left lateral decubitus position recommended as part of an acute abdominal series?
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.

Why is the left lateral decubitus position recommended as part of an acute abdominal series?
Pneumoperitoneum. Contrast-enhanced axial CT scan through the liver shows a collection of air anterior to the liver. Also note the air surrounding the gallbladder and the leakage of water-soluble contrast material from a perforated duodenal ulcer.

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. 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.

An erect chest radiograph is the most sensitive plain radiograph for the detection of free intraperitoneal gas in an emergency setting. On chest radiography, any subdiaphragmatic free gas can be seen. [2]  Free intraperitoneal air is often missed with plain radiographyy. 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.

Hokama et al noted that a supine abdominal radiograph can show 3 important signs of pneumoperitoneum: the hyperlucent liver sign,which is the darker density of the large free gas anterior to the ventral hepatic surface replacing the brightness of the hepatic shadow; the falciform ligament sign, which is a visible longitudinal linear density of the falciform ligament outlined by gas; and the Rigler sign, also known as the double-wall sign, which is the bowel wall outlined by gas both inside and outside. [3]

With both conventional radiography 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.

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. [4, 5]  In patients with perforated viscus and pneumoperitoneum, point-of-care ultrasound can assist the emergency physician in making a rapid bedside diagnosis, which can lower mortality associated with this diagnosis. [6, 7, 8]

Compared with plain radiography, US examination also has the advantage of depicting other changes, such as free abdominal fluid and inflammatory masses. [9, 10, 11]  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. [12, 13, 14, 7, 15, 16]

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

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 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 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.

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.

Most pregnant women (90%) experience significant improvement or complete resolution of peptic ulcer disease. Complications of peptic ulcer disease such as hemorrhage and perforation are rare in pregnancy. Radiology has a role in the evaluation of suspected perforation, but the use of conventional radiography involves irradiation of the fetus. With lateral chest radiography, the fetus is excluded from the direct beam. US is readily available in most centers and can be used in the pregnant patient.

In a series of 100 patients, Woodring and Heiser found that use of upright lateral chest radiographs led to a confirmation of pneumoperitoneum in 98% of patients [17] ; by contrast, use of standard upright posteroanterior (PA) radiographs resulted in a confirmation in only 80% of patients, suggesting that upright lateral views are more sensitive than standard upright PA chest radiographs.

Negative findings support conservative management. For cases in which there is strong clinical suspicion but the radiographic findings are negative, the decision to use further imaging such as CT or to perform surgical exploration must be made on an individual basis. Although a negative lateral chest radiograph excludes a pneumoperitoneum in most cases, the physician should not hesitate to perform a full abdominal series when the index of clinical suspicion is high; in such instances, the benefit outweighs the disadvantage of the small radiation dose to the fetus.

Pneumoperitoneum has a variety of causes, including pneumatosis intestinalis. Pneumatosis intestinalis may be associated with benign causes or may occur in patients with intra-abdominal cancer. Lee and associates retrospectively and blindly reviewed 84 patients with pneumatosis intestinalis to determine the overall proportion of clinically worrisome and benign pneumatosis intestinalis occurring in patients with cancer and to evaluate associated risk factors on CT. The study revealed that benign pneumatosis intestinalis was more prevalent than clinically worrisome pneumatosis intestinalis. The study also revealed that CT features of mesenteric stranding, bowel thickening, bowel dilation, portomesenteric venous gas and ascites, and localization confined to the small bowel were worrisome signs. The location and pattern, such as linear, cystic, or both, were also important. [18]