Pneumoperitoneum is pneumatosis (abnormal presence of air or other gas) in the peritoneal cavity, a potential space within the abdominal cavity. The most common cause is a perforated abdominal organ, generally from a perforated peptic ulcer, although any part of the bowel may perforate from a benign ulcer, tumor or abdominal trauma. A perforated appendix seldom causes a pneumoperitoneum.

Spontaneous pneumoperitoneum is a rare case that is not caused by an abdominal organ rupture. This is also called an idiopathic spontaneous pneumoperitoneum when the cause is not known.

In the mid-twentieth century, an "artificial" pneumoperitoneum was sometimes intentionally administered as a treatment for a hiatal hernia.  This was achieved by insufflating the abdomen with carbon dioxide.  The practice is currently used by surgical teams in order to aid in performing laparoscopic surgery.

Causes
 Perforated duodenal ulcer – The most common cause of rupture in the abdomen. Especially of the anterior aspect of the first part of the duodenum.
 Perforated peptic ulcer
 Bowel obstruction
 Ruptured diverticulum
 Penetrating trauma
 Ruptured inflammatory bowel disease (e.g., megacolon)
 Necrotising enterocolitis/pneumatosis coli
 Bowel cancer
 Ischemic bowel
 Steroids
 After laparotomy
 After laparoscopy
 Breakdown of a surgical anastomosis
 Bowel injury after endoscopy
 Peritoneal dialysis (PD), although the prevalence of pneumoperitoneum is estimated to be less than 4% among people with PD in a more recent study in the United Kingdom.
 Vaginal insufflation (air enters via the fallopian tubes; e.g., water-skiing, oral sex)
 Colonic or peritoneal infection
 From chest (e.g., bronchopleural fistula)
 Non-invasive PAP (positive airway pressure) can force air down duodenum as well as down trachea.

Spontaneous pneumoperitoneum
A spontaneous pneumoperitoneum is a rare case that is not caused by an abdominal organ rupture. This is also called an idiopathic spontaneous pneumoperitoneum when the cause is not known. Causes of a spontaneous pneumoperitoneum, with no peritonitis include a barotrauma due to mechanical ventilation,  and a tracheal  rupture following an emergency intubation. In the ventilation case, air had passed from the chest into the abdominal cavity through the  diaphragm. In the tracheal rupture air had passed along the great vessels.

Diagnosis

When present, pneumoperitoneum can often be seen on projectional radiography, but small amounts are often missed, and CT scan 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.

Signs that can be seen on projectional radiography are shown below:

The double wall sign marks the presence of air on both sides of the intestine. However, a false double wall sign can result from two loops of bowel being in contact with one another. The sign is named after Leo George Rigler. It is not the same as Rigler's triad.

The football sign is when the abdomen appears as a large oval radiolucency reminiscent of an American football on a supine projectional radiograph. The football sign is most frequently seen in infants with spontaneous or iatrogenic gastric perforation causing pneumoperitoneum. It is also seen in bowel obstruction with secondary perforation, as in Hirschprung disease, midgut volvulus, meconium ileus and intestinal atresia. Iatrogenic causes like endoscopic perforation may also give football sign.

The Cupola sign is seen when air is accumulated under the central tendon of the diaphragm.

Differential diagnosis
As differential diagnoses, a 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.

Treatment
Treatment depends on the cause of the condition.

Terminology
Pneumoperitoneum can be described as peritoneal emphysema, just as pneumomediastinum can be called mediastinal emphysema, but pneumoperitoneum is the usual name.

See also 
 Cupola sign
 Football sign
 Pneumoretroperitoneum
 Rigler's sign

References

External links 

Disorders of fascia
Peritoneum disorders