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
     

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.

• 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