What is the indication for an erect abdominal radiograph (X-ray) in a patient with peritonism?

Indications for Erect Abdominal Radiograph in Patients with Peritonism

In patients with peritonism, erect abdominal radiographs are primarily indicated to detect free intraperitoneal air (pneumoperitoneum) as evidence of gastrointestinal perforation, though CT scan is more sensitive and should be used when available and patient condition allows. 1

Diagnostic Value and Limitations

Primary Indications

• Detection of pneumoperitoneum (free intraperitoneal air) in suspected perforation
• Upright or decubitus abdominal radiographs can detect small amounts of free peritoneal air, with a positive predictive value of 92% for iatrogenic colonoscopic perforations 1
• Higher diagnostic value in perforations from diagnostic procedures (PPV 100%) compared to therapeutic procedures (PPV 45%) 1

Limitations

• Detects free air in only 60-80% of perforation cases 2
• Insensitive to the presence of fluid 1
• Limited sensitivity compared to CT scan for detecting small amounts of free air 1
• May miss perforations in certain anatomical locations, particularly extra-peritoneal perforations 1

Clinical Decision Algorithm

When to Order Erect Abdominal Radiographs:

1. First-line imaging in patients with peritonism when:

   • CT is not immediately available
   • Patient is hemodynamically stable
   • Clinical suspicion of perforation exists

2. Specific clinical scenarios:

   • Abdominal pain with guarding/rebound tenderness
   • Abdominal distension
   • Tachycardia
   • Fever
   • Recent colonoscopy or other invasive gastrointestinal procedure 1

When to Proceed Directly to CT:

1. Patient with diffuse peritonitis and hemodynamic instability requiring immediate surgical intervention 1
2. Normal plain radiograph but persistent clinical suspicion of perforation 1
3. Need to determine the exact site and cause of perforation 1
4. Suspected complications beyond simple perforation (e.g., abscess formation) 1

Interpretation and Next Steps

Positive Findings:

• Presence of subdiaphragmatic free air indicates likely perforation
• The height of the air column under the diaphragm correlates with:
  • Size of perforation
  • Amount of peritoneal soiling
  • Potential mortality risk 3

Negative Findings:

• A negative erect abdominal radiograph does NOT rule out perforation
• If clinical suspicion persists, proceed to CT scan with contrast enhancement 1
• CT can detect small amounts of both free intra-peritoneal air and fluids, with foci of gas sometimes congregating near the perforation site 1

Alternative Imaging Considerations

• CT scan: More sensitive than standard abdominal radiographs for detecting free air (Recommendation Grade 1C) 1
• Ultrasound: May be useful when radiation exposure should be limited (children, pregnant women) but should not be considered definitive in excluding pneumoperitoneum 1
• Double contrast CT: Useful in cases of localized peritoneal signs to confirm feasibility of non-operative management 1

Clinical Pearls and Pitfalls

• Peritonism can occur without perforation (e.g., transmural thermal injury after polypectomy with serosal irritation) 1
• All three peritonism tests (inspiration, expiration, and cough) being positive is significantly associated with hospital admission and may indicate more serious pathology 4
• The presence of peritonism should prompt imaging studies even if the clinical picture is suggestive of perforation, as biochemical and imaging studies are always indicated when perforation is suspected 1
• CT scan should not delay appropriate treatment in patients with clear signs of diffuse peritonitis 1

Remember that while erect abdominal radiographs remain a useful initial screening tool for pneumoperitoneum in patients with peritonism, CT scanning offers superior sensitivity and specificity and provides more comprehensive information about the underlying pathology.