Intraabdominal air can be seen on conventional abdominal x-rays, computed tomography scans of the abdomen or ultrasonography imaging. Intraabdominal air can be physiologic or pathologic. The most common cause of normal intraabdominal air is air within the gastrointestinal tract, i.e. the stomach bubble. Pathologic air can be further divided into the extraluminal, intraluminal, intraparenchymal or intramural air. Extraluminal pathologic air can be a pneumoperitoneum or free gas within the retroperitoneum. The most common cause of intramural air is pneumatosis intestinalis.
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Image: “Double wall sign. This is a secondary sign of pneumoperitoneum. Patient is supine, and air within the abdomen and lumen of the bowel accentuate both sides of the bowel wall.” by Mikael Häggström. License: CC BY-SA 3.0


Pneumoperitoneum

Background

Pneumoperitoneum is defined as the presence of free-air within the abdominal peritoneal cavity. Pneumoperitoneum can be a complication of intraabdominal surgery or can be caused by the perforation of a viscus. Peritoneal dialysis and mechanical ventilation are two medical procedures known to cause pneumoperitoneum in a significant number of inpatients. Iatrogenic causes of pneumoperitoneum include mechanical perforation of a viscus after a colonoscopy or an upper gastrointestinal tract endoscopy.

The causes of pneumoperitoneum in children are different from adults. The most common causes in children include necrotizing enterocolitis, Hirschsprung’s disease, and meconium ileus. Iatrogenic causes of pneumoperitoneum in children include rectal perforation after the introduction of a rectal thermometer or mechanical ventilation induced pneumoperitoneum.

Clinical Presentation of Pneumoperitoneum

Patients with an acute pneumoperitoneum typically present with acute abdominal pain, vomiting, distension, fever and symptoms and signs suggestive of peritoneal irritation. Tachycardia, hypotension, a decreased urine output, and tachypnea are also commonly seen in patients with acute viscus perforation and pneumoperitoneum. The recent history of severe peptic ulcer disease, appendicitis or trauma can be elicited in a significant number of cases.

Radiographic Evaluation of Pneumoperitoneum

An erect chest x-ray is the imaging modality of choice for screening for pneumoperitoneum. The examination is very sensitive for the detection of intraperitoneal gas in an emergency setting where more sophisticated imaging modalities might be considered as harmful due to delayed treatment. Nowadays, erect chest x-rays are done in all cases of acute abdomen. The following signs can be seen on the x-ray.

A cupola or mustache sign which is seen on supine abdominal x-rays. Air is trapped underneath the central tendon of the diaphragm.

Image:
“Pneumoperitoneum on chest X-ray.” by Clinical_Cases. Licence: CC BY-SA 2.5

Rigler’s sign shows an outlining of both sides of a bowel wall. The reduction of liver opacity due to the accumulation of air anterior to the liver results in what is known as a Lucent liver sign. The free intra-abdominal air can outline the whole abdomen, i.e. football sign, or the falciform ligament of the liver, i.e. Silver’s sign. The triangular collection of gas within Morison pouch is known as Doge’s cap sign.

Radiographic sign Patient position
Air under the diaphragm Upright position
Visualization of both sides of the bowel wall (Rigler sign) Any position
Visualization of the falciform ligament (football sign) Supine position

In most cases, the emergency physician simply sees sub-diaphragmatic free air on an erect chest x-ray which is a sign considered as specific for intra-abdominal free air.

When the amount of air is quite small but the risk of viscus perforation is very high, i.e. severe penetrating trauma to the abdomen, an abdominal computed tomography scan is indicated. Small free peritoneal air can be easily detected on abdominal computed tomography scans.

Retroperitoneal Free Gas

Background

Injury to the retroperitoneal structures such as the coccygeal region, kidneys, ureters, or post renal transplantation might cause accumulation of air within the retroperitoneum. Retroperitoneal free gas is more difficult to recognize on routine abdominal or chest x-rays and can be associated with severe injuries to vital retroperitoneal structures such as the lumbosacral plexus.

Clinical Presentation of Retroperitoneal Free Gas

The medical or surgical history of the patient can raise the suspicion of the possibility of free gas within the retroperitoneal. In most cases, however, the patient presents with a history of a severe trauma such as a motor-vehicle accident and retroperitoneal free gas is discovered by chance during the radiographic evaluation of the patient. The discovery of retroperitoneal free gas is very alarming to the treating physician as it indicates severe injury to the retroperitoneum with the possibility of the kidney, ureters, major blood vessels or nerve injuries.

Radiographic Evaluation of Retroperitoneal Free Gas

Pelvic x-rays are very useful in the radiographic evaluation of a patient suspected to have free gas within the retroperitoneum. Gas might be seen near the psoas muscles. Air that is trapped and fixated near the edge of the psoas muscles is considered as a specific sign for retroperitoneal free gas and not intraperitoneal gas.

Transabdominal ultrasonography can be also used for the evaluation of small amounts of free gas within the retroperitoneum with good sensitivity and specificity. When the risk of retroperitoneal injury is high, a pelvic computed tomography scan is of tremendous value to the treating physician. A computed tomography scan can exclude free gas within the retroperitoneum, injury to vital pelvic organs and spinal injuries.

Pneumatosis Intestinalis

Background

Pneumatosis intestinalis is defined as the presence of air within the wall of the gastrointestinal tract. The accumulation of gas within the bowel wall can be caused by benign or life-threatening conditions. The benign causes of pneumatosis intestinalis include asthma, scleroderma, and pyloric stenosis. Life-threatening causes of pneumatosis intestinalis include necrotizing enterocolitis, bowel ischemia, malignant neoplasms, bowel obstruction and trauma. Whenever pneumatosis intestinalis is seen on radiography, it should be noted that this is merely an imaging finding and not a diagnosis. The cause of pneumatosis intestinalis needs to be determined as this has a huge impact on the treatment plan and prognosis of the patient.

Causes Pathology
Bowel necrosis Mucosal damage caused by lack of blood flow such as from volvulus or superior mesenteric artery/vein thrombosis
Post-endoscopy Mucosal disruption and increased luminal pressure
COPD Alveolar rupture with air dissecting into the mediastinum and extending to the bowel wall through the diaphragmatic hiatus
Steroids and autoimmune Increased bowel permeability and decreased immunity in bacterial gas entering the bowel wall

Clinical Presentation of Pneumatosis Intestinalis

The clinical presentation is dependent on the cause. Patients with asthma might present with an acute and severe exacerbation of asthma that is complicated by abdominal pain. The incidental finding of pneumatosis intestinalis can be later seen in the radiographic evaluation of the patient. On the other hand, patients with bowel ischemia or necrotizing enterocolitis are generally very sick, septic, complain of severe abdominal pain and can be in shock. The finding of pneumatosis intestinalis in these patients is very alarming and warrants urgent surgical intervention to avoid bowel gangrene.

Radiographic Evaluation of Pneumatosis Intestinalis

Image: “Upright AP radiograph showing gas in the wall of the small bowel in the left upper quadrant indicative of pneumatosis intestinalis.” by Jto410. License: CC BY-SA 3.0

Pneumatosis intestinalis might be seen on abdominal x-rays but is better visualized on computed tomography scans. A computed tomography scan is usually indicated to evaluate the extent of bowel obstruction, ischemia or involvement in the most likely causative disease and the finding of pneumatosis intestinalis just adds more confidence to the diagnosis of bowel ischemia or severe infection.

Contrast enhanced computed tomography scans are usually not needed in the initial evaluation of pneumatosis intestinalis. The most specific sign of pneumatosis intestinalis on computed tomography is radiolucency within the bowel wall. The patterns of radiolucency include linear, bubble, or curvilinear gas collections. Computed tomography scans are also helpful in the visualization of free fluid, soft-tissue thickening and bowel wall edema which are signs suggestive of a life-threatening cause behind the pneumatosis.

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