Free Intraperitoneal Air
Pneumoperitoneum

• Etiology
  • Disruption of wall of hollow viscus
    • Blunt or penetrating trauma
    • Perforating foreign body (eg, thermometer injury to rectum)
    • Iatrogenic perforation
      • Laparoscopy / laparotomy (58%)
      • Absorbed in 1-24 days depending on initial amount of air introduced and body habitus (80% in asthenic, 25% in obese patients)
      • Leaking surgical anastomosis
      • Endoscopic perforation
      • Enema tip injury
      • Diagnostic pneumoperitoneum
    • Diseases of GI tract
      • Perforated gastric / duodenal ulcer
      • Perforated appendix
      • Ingested foreign-body perforation
      • Diverticulitis (ruptured Meckel diverticulum / sigmoid diverticulum, jejunal diverticulosis)
      • Necrotizing enterocolitis with perforation
      • Inflammatory bowel disease (eg, toxic megacolon)
      • Obstruction* (gas traversing intact mucosa): neoplasm, imperforate anus, Hirschsprung disease, meconium ileus
      • Ruptured pneumatosis cystoides intestinalis
      • Idiopathic gastric perforation = spontaneous perforation in premature infants (congenital gastric muscular wall defect)
  • Through peritoneal surface
    • Transperitoneal manipulation
    • Abdominal needle biopsy / catheter placement
    • Mistaken thoracentesis / chest tube placement
    • Endoscopic biopsy
  • Extension from chest
    • Dissection from pneumomediastinum (positive pressure breathing, rupture of bulla / bleb, chest surgery)
    • Bronchopleural fistula
    • Rupture of urinary bladder
    • Penetrating abdominal injury
  • Through female genital tract
    • Iatrogenic
    • Perforation of uterus / vagina
    • Culdocentesis
    • Rubin test = tubal patency test
    • Pelvic examination
    • Spontaneous
    • Intercourse, orogenital insufflation
      • Douching
    • Knee-chest exercise, water skiing, horseback riding
  • Intraperitoneal
    • Gas forming peritonitis
    • Rupture of abscess
    • Air in lesser peritoneal sac gas in scrotum (through open processus vaginalis)
• Imaging findings
  • Large collection of gas
  • Abdominal distension, no gastric air-fluid level
  • "Football sign" = large pneumoperitoneum outlining entire abdominal cavity
  • "Double wall sign" = "Rigler sign" = air on both sides of bowel as intraluminal gas and free air outside (usually requires >1,000 mL of free intraperitoneal gas + intraperitoneal fluid)
  • "Telltale triangle sign" = triangular air pocket between 3 loops of bowel
  • Depiction of diaphragmatic muscle slips = two or three 6-13 cm long and 8-10 mm wide arcuate soft-tissue bands directed vertically inferiorly + arching parallel to diaphragmatic dome superiorly outline of ligaments of anterior inferior abdominal wall:
  • "Inverted V sign" = outline of both lateral umbilical ligaments (containing inferior epigastric vessels)
  • Outline of medial umbilical ligaments (obliterated umbilical arteries)
  • "Urachus sign" = outline of middle umbilical ligament
     

Falciform Ligament Sign. Blue arrows point to falciform ligament, made visible by a large amount of free air in the peritoneal cavity. The red arrows demonstrate both sides of the wall of the stomach (Rigler's sign), a sign of free air. The yellow arrow points to a skin fold.

Falciform Ligament Sign (Free Air). White arrows point to falciform ligament, made visible by a large amount of free air in the peritoneal cavity. The green arrow demonstrate both sides of the wall of the bowel wall (Rigler's sign), a sign of free air. The red arrow points to increased lucency over the liver from a large amount of free air.

• RUQ gas (best place to look for small collections)
  • Single large area of hyperlucency over the liver
  • Oblique linear area of hyperlucency outlining the posteroinferior margin of liver
  • Doge's cap sign = triangular collection of gas in Morrison pouch (posterior hepatorenal space)
  • Outline of falciform ligament = long vertical line to the right of midline extending from ligamentum teres notch to umbilicus; most common structure outlined
  • Lligamentum teres notch = inverted V-shaped area of hyperlucency along undersurface of liver
  • Ligamentum teres sign = air outlining fissure of ligamentum teres hepatis (= posterior free edge of falciform ligament) seen as vertically oriented sharply defined slitlike / oval area of hyperlucency between 10th and 12th rib within 2.5-4.0 cm of right vertebral border 2-7 mm wide and 6-20 mm long
  • "Saddlebag / mustache / cupola sign" = gas trapped below central tendon of diaphragm
  • Parahepatic air = gas bubble lateral to right edge of liver

Pneumoperitoneum. There is a very large pneumoperitoneum which renders the entire abdomen more lucent that normal (white arrows). Both sides of the bowel wall are visible (red arrows). The new born also has severe hyaline membrane disease in the chest.