Why is an ultrasound of the abdomen (USg abdomen) necessary in cases of constipation?

Ultrasound Abdomen in Constipation: Role and Indications

Abdominal ultrasound is not routinely necessary in simple constipation cases but is indicated when complications or underlying pathologies are suspected, particularly to rule out bowel obstruction, inflammatory conditions, or other structural abnormalities. 1, 2

When Ultrasound is Indicated in Constipation

Suspected Complications

• Ultrasound should be considered when there are signs of potential complications such as bowel obstruction, which can be detected with 90% sensitivity and 96% specificity 1
• Point-of-care ultrasound can identify dilated bowel loops (>2.5 cm) with decreased or absent peristalsis, which are characteristic findings in paralytic ileus 3
• In cases with severe symptoms, ultrasound can help detect free fluid, abscesses, or intestinal distension that may indicate complications beyond simple constipation 1

Alarm Features Requiring Imaging

• Presence of alarm symptoms/signs such as:
  • Abdominal pain disproportionate to clinical findings 4
  • Palpable abdominal mass 5
  • Significant weight loss 5
  • New-onset constipation, especially in older adults 5
  • Suspected inflammatory bowel disease 1

Special Populations

• Ultrasound is particularly valuable in:
  • Pregnant patients where radiation exposure should be avoided 1
  • Patients with renal failure who cannot receive contrast for CT or MRI 1
  • Pediatric patients where rectal diameter measurement by ultrasound has been shown to correlate with clinical constipation (mean difference 12 mm compared to controls) 6

Advantages of Ultrasound in Constipation Evaluation

• Non-invasive and radiation-free compared to CT enterography 1
• Can be performed at the bedside as point-of-care ultrasound (POCUS) 7
• Allows real-time assessment of bowel peristalsis and pliability 3
• Can help differentiate between functional and mechanical causes of constipation 4
• More cost-effective compared to CT, MRI, or endoscopic procedures 1

Limitations of Ultrasound in Constipation

• Operator-dependent with variable sensitivity based on technician experience 1
• Limited visualization in obese patients or those with excessive bowel gas 3
• Cannot provide the detailed mucosal assessment that endoscopy offers 1
• May miss small lesions that could be detected by other imaging modalities 1

What Ultrasound Can Detect in Constipation Cases

• Bowel wall thickness, which can indicate inflammation or malignancy 1
• Dilated bowel loops suggesting obstruction 1
• Rectal diameter, which correlates with clinical constipation 6
• Free fluid or abscesses that may indicate complications 1
• Reduced or absent peristalsis in cases of ileus 3

When to Choose Other Imaging Modalities

• CT enterography is preferred when:

  • Complications such as perforation or abscess are strongly suspected 1
  • Detailed assessment of extraluminal complications is needed 1
  • Ultrasound findings are inconclusive but clinical suspicion remains high 1

• MR enterography should be considered when:

  • Detailed assessment of small bowel disease is required, particularly for Crohn's disease 1
  • Repeated imaging is anticipated, to minimize radiation exposure 1

Conclusion

While abdominal ultrasound is not routinely necessary for all patients with constipation 2, it serves as a valuable diagnostic tool when complications are suspected or in patients with alarm features. The 2020 study by the American Journal of Emergency Medicine found that plain abdominal radiography had low value in patients with simple constipation, suggesting that more targeted imaging like ultrasound may be more appropriate when imaging is indicated 2. Ultrasound offers the advantages of being non-invasive, radiation-free, and capable of real-time assessment, making it particularly suitable for initial evaluation before proceeding to more invasive or radiation-exposing modalities.