Pneumoperitoneum

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.

Pathology

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:

• perforated hollow viscus
  • peptic ulcer disease
  • ischemic bowel
  • bowel obstruction
  • necrotizing enterocolitis
  • appendicitis
  • diverticulitis
  • malignancy
  • inflammatory bowel disease
  • mechanical perforation
    • trauma
    • colonoscopy
    • foreign bodies 
    • iatrogenic
• postoperative free intraperitoneal gas
• peritoneal dialysis
• vaginal "aspiration" ³
  • cunnilingus
  • douching
  • sudden squatting
  • postpartum exercises
  • water-skiing ¹⁰
• mechanical ventilation
• pneumomediastinum
• pneumothorax

Radiographic features

Plain radiograph

Chest radiograph

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.

• subdiaphragmatic free gas
• leaping dolphin sign
• cupola sign (on supine film)
• continuous diaphragm sign

Abdominal radiograph

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:

• bowel-related signs
  • ​​double wall sign (also known as Rigler sign or bas-relief sign)
  • telltale triangle sign (also known as the triangle sign or telltale triangle) 
• peritoneal ligament-related signs
  • football sign
  • falciform ligament sign
  • lateral umbilical ligament sign (also known as inverted "V" sign)
  • urachus sign
• right upper quadrant signs
  • cupola sign
  • fissure for ligamentum teres sign
  • hepatic edge sign
  • lucent liver sign
  • Morison pouch sign (doge cap sign)
  • periportal free gas sign

Ultrasound

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

Recently it has been shown that using contrast-specific software generally used during contrast-enhanced ultrasound (CEUS) can substantially accentuate free abdominal air, thereby increasing the chance of detection ¹³. 

Differential diagnosis

• consider pseudopneumoperitoneum

See also

• abnormal intra-abdominal gas
• tension pneumoperitoneum
• abdominal compartment syndrome