Focus on abnormal air: diagnostic ultrasonography for the acute abdomen

Emergency ultrasonography is a frequently used imaging tool in the bedside diagnosis of the acute abdomen. Classic indications include imaging for acute abdominal aneurysm, acute cholecystitis, hydronephrosis, and free intra-abdominal fluid in patients with trauma or suspected vascular or ectopic pregnancy rupture. Point-of-care sonographic imaging often emphasizes the diagnostic utility of fluid and edema, both as a significant finding and as a desirable adjunct for improved imaging. Conversely, the finding of sonographic intra-abdominal air is commonly 'tolerated' as a necessary evil that can foil image acquisition. This is in stark contrast to the accepted diagnostic utility of air in other imaging modalities for the acute abdomen, such as computed tomography and conventional radiography. Countering the bias against air as a deterrent for diagnostic ultrasound's accuracy are several published studies suggesting that abnormal air patterns can be used with high precision to diagnose pneumoperitoneum. These studies advocate that sonographic findings of abnormal air can be straightforward and can become crucial for increasing the diagnostic yield of bedside ultrasound of the acute abdomen. They suggest that practitioners should familiarize themselves with the findings and techniques to gain the experience required to make the diagnosis with confidence. This article will discuss four groups of abnormal air patterns found in the abdomen and the retroperitoneum and the respective scanning techniques, with a focus on the use of ultrasound for diagnosing pneumoperitoneum and a suggested scanning approach in the emergency setting.