Pneumoperitoneum

Overview
Pneumoperitoneum is air or gas in the abdominal (peritoneal) cavity, often seen on x-ray, but small amounts are often missed and CT is nowadays regarded as a criterion standard in the assessment of a pneumoperitoneum. , CT can visualize quantities as small as 5 cm³ of air or gas. The most common cause is a perforated abdominal viscus, generally a perforated ulcer, although any part of the bowel may perforate from a benign ulcer, tumor or trauma. A perforated appendix seldom causes a pneumoperitoneum.

A pneumoperitoneum is deliberately created by the surgical team in order to perform laparoscopic surgery. This is achieved by insufflating the abdomen with carbon dioxide.

Causes

 * Perforated peptic ulcer
 * Bowel obstruction
 * Ruptured diverticulum
 * Penetrating trauma
 * Ruptured inflammatory bowel disease (e.g. megacolon)
 * Necrotising enterocolitis/Pneumatosis coli
 * Ischemic bowel
 * Steroids
 * After laparotomy
 * After laparoscopy
 * Break down of a surgical anastomosis
 * Bowel injury after endoscopy
 * Peritoneal dialysis
 * Vaginal insufflation (air enters via the fallopian tubes, e.g. water-skiing, oral sex)
 * Colonic or peritoneal infection
 * From chest (e.g. bronchopleural fistula)

Subphrenic abscess, bowel interposed between diaphragm and liver (Chilaiditi syndrome), and linear atelectasis at the base of the lungs can simulate free air under the diaphragm on a chest x-ray.