Axial CT abdomen and pelvis portal venous phase.
We present a case of incidentally detected spontaneous pneumoperitoneum in a clinically well outpatient attending for routine CT abdomen & pelvis, performed to facilitate pre-operative planning for incisional hernia repair. The patient’s history included emergency hiatal repair with Nissen fundoplication for a strangulated, incarcerated paraoesophageal hernia one year previously.
The patient underwent a single portal venous phase CT abdomen & pelvis, approximately two hours following the administration of oral contrast. Scout acquisitions raised concern for pneumoperitoneum. Large volume pneumoperitoneum was confirmed on image review following volumetric acquisition with multiplanar reconstructions. The solid abdominal viscera demonstrated no abnormality of concern. There was no evidence of pneumatosis intestinalis, portal venous gas, ascites, mesenteric fat stranding, abdominopelvic lymphadenopathy or rim-enhancing intra-abdominal collection. Limited visualisation of the lower chest demonstrated a recurrent para-oesophageal hernia, with gastric herniation into the thoracic cavity and resultant lateral displacement of the anterior basal segment of the left lower lobe. Small volume para-gastric locules of air were noted. There was no pneumothorax or pleural effusion.
Pneumoperitoneum describes the presence of free air within the peritoneal cavity. The differential diagnosis is broad. Pneumoperitoneum most commonly arises as a result of perforation of a hollow abdominal viscus.
Perforation may be secondary to causes such as:
Causes of pneumoperitoneum not involving visceral perforation include:
If pneumoperitoneum is suspected, imaging should be considered in a step-wise manner. Depending on the patient’s clinical status, it may be appropriate to proceed directly to cross-sectional imaging.
• Erect chest radiograph
- Subdiaphragmatic free air 
• Plain film of the abdomen
- Bowel related findings, including:
- Rigler sign (double-wall sign). 
- Peritoneal ligament related findings, including:
- Football sign 
- Falciform ligament sign 
- Right upper quadrant signs, including:
- Fissure for ligamentum teres sign 
- Lucent liver sign 
- Can confirm the presence of pneumoperitoneum, help identify a likely aetiology, and assess the degree of contamination.
- Peritoneal stripe sign 
This patient was contacted to attend the Emergency Department for emergent review. Clinical history and physical examination revealed no concerning findings. He was discharged home, with advice to return if clinical condition changed. He remained well at subsequent outpatient clinical follow-up, and no further investigation was arranged.
Pneumoperitoneum, when suspected or detected, should prompt urgent clinical assessment with a high index of suspicion for underlying visceral perforation. When clinical findings are discordant, the clinician should re-assess the patient, considering potential alternative causes. Spontaneous pneumoperitoneum can be a benign finding in a subset of patients, as demonstrated in this case. Clinicians should be cognisant of this, to avoid unnecessary invasive procedures which can carry a significant potential for morbidity and mortality.
Written informed patient consent for publication has been obtained.
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