Pneumatosis intestinalis, defined as gas in the bowel wall, is often first identified on abdominal radiographs or computed tomography (CT) scans. It is a radiographic finding and not a diagnosis, as the etiology varies from benign conditions to fulminant gastrointestinal disease.
Pneumatosis intestinalis is considered an ominous finding in ischemia, especially in association with portomesenteric venous gas. The disease is seen in other conditions, including chronic obstructive pulmonary disease, connective tissue disorders, infectious enteritis, celiac disease, leukemia, amyloidosis, and acquired immunodeficiency syndrome (AIDS); it is also found in association with organ transplantation, steroid use, and chemotherapy. [1, 2, 3, 4, 5, 6] (See the images below.)
Surgery should be performed in patients who are not responding to nonoperative treatment, especially those with signs of perforation, peritonitis, or abdominal sepsis.
Primary versus secondary pneumatosis intestinalis
Pneumatosis intestinalis occurs in 2 forms. Primary pneumatosis intestinalis (15% of cases) is a benign idiopathic condition in which multiple thin-walled cysts develop in the submucosa or subserosa of the colon. Usually, this form has no associated symptoms, and the cysts may be found incidentally through radiography or endoscopy. When the cysts protrude into the lumen, they may mimic polyps or carcinomas, as shown on barium enema studies. This primary form is often termed pneumatosis cystoides intestinalis. 
The secondary form (85% of cases) is associated with obstructive pulmonary disease, as well as with obstructive and necrotic gastrointestinal disease.
Microvesicular gas collections, defined as 10-100 mm cysts or bubbles within the lamina propria, are predominantly associated with primary (benign) pneumatosis intestinalis, whereas linear or curvilinear gas collections seen parallel to the bowel wall are found in secondary pneumatosis. Therefore, linear gas collections are usually an ominous sign.
Pneumatosis intestinalis is usually identified on plain radiographs of the abdomen. Occasionally, submucosal cysts may be identified during endoscopy. The cysts, which may appear similar to polyps, may be examined at biopsy for signs of inflammation. Gas may collect peripherally in the lumen of the bowel, around fecal or contrast material. This gas can simulate pneumatosis and is usually depicted on CT scans. [8, 9] Rarely, emphysematous ureteritis may simulate pneumatosis of the descending or sigmoid colon on plain radiographs. Colitis cystica profunda is an extremely rare disease in which mucin-filled cysts form in the wall of the rectum. [8, 9, 10, 11]
The patterns of the radiolucencies are seen as linear, curvilinear, small bubbles, or collections of cysts. Cystic collections of gas localized to the wall of the colon are suggestive of primary pneumatosis intestinalis.
Pneumatosis intestinalis may be complicated by pneumoperitoneum, which can be detected as free air on a simple upright or cross-table lateral view of the abdomen.
Pneumoperitoneum may represent rupture of subserosal cysts in benign primary pneumatosis, or it may occur after perforation in the setting of intestinal necrosis. Linear or curvilinear gas collections may be seen throughout the intestinal wall in secondary pneumatosis. Portal venous gas, which is tubular and peripherally located in the liver (as opposed to biliary air, which is centrally located), is an ominous finding, often occurring with ischemic bowel. (See the images below.) 
The presence of gas in the mesenteric and portal circulation is an ominous radiographic finding in bowel ischemia. Angiography can provide insight into the nature of vascular compromise.
Degree of confidence
Abdominal radiographic findings are detected in approximately two thirds of patients with pneumatosis. Radiographs are sufficient for diagnosis of pneumatosis, although additional studies, such as CT scans, ultrasonograms, or water-soluble enema studies, may be considered to delineate pneumatosis or the site of perforation. The concomitant finding of portal venous gas does not always suggest bowel ischemia. Other etiologies must be clinically correlated with the patient's history.
Rarely, emphysematous ureteritis may simulate pneumatosis of the descending or sigmoid colon on plain radiographs.
Abdominal CT scanning can depict small amounts of intramural gas not shown on routine radiographs. Depending on the morphology, distention, and thickness of the bowel loops, CT scanning helps to provide clues to the cause of pneumatosis intestinalis. With contrast enhancement, thickened bowel wall may suggest ischemia in the setting of pneumatosis. Dilated bowel loops and abnormal fluid levels suggest an obstructive cause of pneumatosis. [13, 14, 15]
Sensitivity in detecting small, gaseous inclusions in the mesenteric vein or intrahepatic branches of the portal vein is increased with abdominal CT scanning. In this circumstance, pneumatosis is one of the signs of mesenteric ischemia.
CT scans provide additional details, such as various morphologic changes (including mural wall thickening, dilatation, abnormal or absent wall enhancement), mesenteric stranding, edema or hemorrhage, vascular engorgement, ascites, and portomesenteric gas. [13, 14, 15] (See the images below.)
Degree of confidence
CT scanning is often helpful in determining the primary cause of pneumatosis intestinalis, and it can demonstrate important coexistent findings or complications. The use of multidetector-row CT scans, thin sections, and high-quality portal venous phase scans are advantageous for providing greater accuracy in the detection of ischemia, as well as for diagnosing other causes of acute abdomen, such as perforation, abscess formation, and peritonitis.
The sensitivity of CT scanning (82%) for the diagnosis of acute bowel ischemia is comparable to that of angiography (87.5%).
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) may be useful for identifying intestinal ischemia as a cause of pneumatosis. High signal intensity on T1- and T2-weighted images suggests ischemia. Gas bubbles within the bowel wall have been described on abdominal MRI scans of neonates with necrotizing enterocolitis.
Ultrasonography of the abdomen shows within the bowel wall circumferential, bright, echogenic foci that represent the gas bubbles. The pattern of gas distribution, particularly within the dependent wall, should raise the suspicion of pneumatosis intestinalis in a patient with the appropriate clinical history. [16, 12]
Degree of confidence
Misregistration artifact occurs only on the nondependent wall and usually involves the superficial wall layers, in contrast to pneumatosis intestinalis, which is typically circumferential and often has a submucosal or subserosal location.
In addition to the familiar artifacts of dirty shadowing and small reverberation artifacts, a bubble of intraluminal gas may falsely lie within the gut wall, producing an artifact called pseudopneumatosis. It is more likely seen with thickening of the gut wall.