John Hauber, MS4

Article: Hoffmann B, Nürnberg D, Westergaard MC. Focus on abnormal air: diagnostic ultrasonography for the acute abdomen. Eur J Emerg Med. 2012 Oct;19(5):284-91. doi: 10.1097/MEJ.0b013e3283543cd3.

Background: Classical teaching in ultrasonography usually relegates intraabdominal air to no more than an impediment to clear imaging; however, several studies show that ultrasonography may be a useful modality for the diagnosis of pathological intraabdominal air. Most practitioners lack an understanding of the sonographic features of pathologic free air and may benefit from increased awareness of and experience in searching for these features.

Review highlights: Air in sonography impedes and reflects ultrasound waves. It’s appearance is therefore a solid, white, highly echogenic focus with distal shadowing and/or comet tail artifact. Smaller pockets of air may appear as small, punctate, highly-echogenic foci without any distal artifact.

Four main subdivisions of pathologic intraabdominal air patterns are described: extraluminal (pneumoperitoneum and pneumoretroperitoneum), intraluminal, intraparenchymal, and intramural.

Extraluminal:

Enhanced peritoneal stripe sign (EPSS) was coined by Muradali et al. to describe the seemingly thickened, hyperechoic peritoneal stripe with posterior shadowing and/or reverberation artifact that is seen in areas containing free air.
Semilateral left decubitus position with slight thorax elevation is optimal for visualizing free air over the ventral liver.
Shifting phenomenon - free air will shift to a higher point in the peritoneal cavity with pressure from the ultrasound probe, but should not shift with respirations.
Two studies of 4000 and 487 patients demonstrated sensitivities of 85-90% and specificity of 100% for pneumoperitoneum.
Practitioners must be aware of mimics of pneumoperitoneum, including Chilaiditi syndrome and rib shadowing.
Pneumoretroperitoneum is visualized as free air surrounding retroperitoneal structures such as the kidney or great vessels.

Intraluminal:

Hyperechoic air collections can be visualized in the biliary, arterial, and venous systems secondary to iatrogenic, infectious, and traumatic causes (e.g. pneumobilia secondary to sphincterotomy).

Intraparenchymal:

Hyperechoic air collections can be visualized in parenchymal tissue but must be distinguished from mimics such as calcifications or cholesterol deposits.
Some examples include gas formation in abscesses, introduction of air during biopsy, or fistulas/erosions into solid organ tissue.

Intramural air:

Air collections secondary to infection, ischemia and trauma may be visualized in the walls of the intestine, gallbladder, bladder or stomach.
Pneumatosis is a difficult diagnosis to make via ultrasound, as practitioners must reliably differentiate normal intraluminal air and pseudopneumatosis (artifact seen at the non-dependent wall) from pathologic intramural air.

Takeaway: Ultrasonography in emergency medicine could be enhanced by training practitioners to recognize the patterns of pathologic intraabdominal air. Depending on the clinical scenario, a screening protocol for pathologic intraabdominal air could expedite patient care and clinical decision making. Further studies should focus on the reliability and sensitivity of ultrasonography for this application in the point of care setting.