Last revised by Rohit Sharma on 5 Apr 2024

Pneumoperitoneum (aeroperitoneum is a rare synonym 12) describes gas within the peritoneal cavity, often due to critical illness. There are numerous causes and several mimics.

The most common cause of pneumoperitoneum is the disruption of the wall of a hollow viscus. In children, the causes are different from the adult population and are considered in the neonatal pneumoperitoneum article.

The causes and, hence, the corresponding severity of accompanying illness, are variable:

An erect chest x-ray is probably the most sensitive plain radiograph for the detection of free intraperitoneal gas. If a large volume pneumoperitoneum is present, it may be superimposed over a normally aerated lung with normal lung markings.

Free gas within the peritoneal cavity can be detected on an abdominal radiograph. The signs created by the free intraperitoneal air can be further divided by anatomical compartments in relation to the pneumoperitoneum:

Maybe useful in the appropriate clinical setting. A linear-array transducer (10-12 MHz) is considered more sensitive than a standard curvilinear abdominal transducer (2-5 MHz).

Recognized direct features include:

  • enhancement of the peritoneal stripe (peritoneal stripe sign) 7

    • either alone or associated with repeating, horizontal long-path reverberation artifacts which extend into the far field

  • discrete, hyperechoic foci representing gas bubbles

    • may adopt a linear arrangement, often associated with short path reverberation artifacts which appear as comet tails, or if tapering, ring down artifacts

    • may also cluster in a manner that results in the attenuation and diffuse reflection of reflected ultrasound waves, which results in an underlying inhomogenous (or dirty) acoustic shadow

Ultrasonography may also be combined with dynamic maneuvers

  • free intraperitoneal air is expected to demonstrate movement with patient position

  • it may also be displaced with caudal pressure on the probe above the collection, reappearing with cessation of the pressure

  • unlike air within the lung, does not demonstrate respirophasic changes

Using contrast-specific software generally used during contrast-enhanced ultrasound (CEUS) can substantially accentuate free abdominal air, thereby increasing the chance of detection 13

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