Why is an abdominal erect X-ray (abdominal erect X-ray) preferred over a chest erect X-ray (chest erect X-ray) for suspected perforated viscus?

Abdominal Erect X-ray vs Chest Erect X-ray for Suspected Perforated Viscus

Both abdominal erect X-ray and chest erect X-ray can detect pneumoperitoneum, but chest erect X-ray is preferred for initial evaluation of suspected perforated viscus because it is more sensitive in detecting small amounts of free air under the diaphragm. 1, 2

Radiographic Detection of Pneumoperitoneum

Chest X-ray Advantages

• Chest erect X-ray is more sensitive than abdominal erect X-ray for detecting pneumoperitoneum because:
  • The dome of the diaphragm is better visualized on chest radiographs
  • Even small amounts of free air (1-2 mL) can collect under the diaphragm and be visible on chest X-ray
  • The upright lateral chest radiograph shows pneumoperitoneum in 98% of cases compared to only 80% with upright posteroanterior chest radiograph 2

Diagnostic Approach for Suspected Perforation

1. First-line imaging: CT scan is the preferred first-line imaging for suspected perforation 1, 3
2. If CT unavailable: Chest/abdominal X-ray should be performed as initial routine diagnostic assessment 1
3. If pneumoperitoneum not detected but clinical suspicion remains high: Consider water-soluble contrast studies either orally or via nasogastric tube 1

Clinical Considerations

Sensitivity and Specificity

• Plain radiography has an overall positivity rate of 89.2% for detecting pneumoperitoneum in perforated viscus 4
• Sensitivity varies by perforation site:
  • Highest for stomach and duodenal perforation (94.19%)
  • Lowest for appendicular perforation (7.69%) 4

Pitfalls to Avoid

• Pseudopneumoperitoneum: Conditions like Chilaiditi syndrome (interposition of bowel between liver and diaphragm) can mimic pneumoperitoneum on plain radiographs 5
• Patient positioning: Proper upright positioning (at least 10-15 minutes) is essential for free air to rise to the subdiaphragmatic space
• False negatives: Up to 12% of patients with perforations may have normal imaging studies 1

Alternative Diagnostic Methods

• Ultrasound can detect pneumoperitoneum through:
  • Enhanced peritoneal stripe sign (EPSS)
  • Peritoneal stripe reverberations
  • Focal air collections with dirty shadowing 6
• However, CT scan remains superior with higher sensitivity and specificity for detecting free air 1

Current Guideline Recommendations

The World Journal of Emergency Surgery guidelines recommend:

1. CT scan as first-line imaging for suspected perforated peptic ulcer 1
2. Chest/abdominal X-ray as initial assessment if CT is not promptly available 1
3. Additional imaging with water-soluble contrast when free air is not seen but clinical suspicion remains high 1

In resource-limited settings where CT may not be readily available, plain radiography (particularly chest erect X-ray) remains a valuable screening tool with high positivity rates for detecting pneumoperitoneum 4.