Ultrasound Findings of Small Bowel Obstruction
The key ultrasound findings of small bowel obstruction (SBO) include dilated small bowel loops >2.5 cm in diameter proximal to collapsed loops, with decreased or absent peristalsis, which has a sensitivity of 90% and specificity of 96% for diagnosing SBO. 1
Primary Sonographic Findings
- Dilated small bowel loops: Loops measuring >2.5 cm in diameter are the hallmark finding 1
- Collapsed distal bowel: Visualization of the transition point between dilated proximal bowel and collapsed distal bowel 1
- Altered peristalsis: Decreased or absent peristaltic activity in affected bowel segments 1, 2
- Bowel wall thickening: Indicates potential complications such as ischemia 3
- Free intraperitoneal fluid: Particularly important as a marker of severity - large amounts of free fluid between dilated loops suggest worsening mechanical SBO requiring immediate surgery rather than medical management 3
Additional Sonographic Features
- Fluid-filled loops with hyperechoic spots: Visible in the bowel segment proximal to obstruction 3
- To-and-fro or whirling movement of bowel contents in dilated segments 2
- Increased thickness of bowel wall: May indicate complications such as ischemia 2
- Valvulae conniventes: Visible as hyperechoic lines within fluid-filled loops 3
Diagnostic Accuracy
Ultrasound has demonstrated impressive diagnostic accuracy for SBO:
- Sensitivity: 91-100% 2, 4
- Specificity: 81-100% 2, 5
- Overall accuracy: 89-91.7% when bowel gas does not limit examination 6, 4
Point-of-care ultrasound (POCUS) performed by emergency physicians has shown particularly strong results:
- Sensitivity: 94-100% 2
- Specificity: 81-100% 2
- Positive likelihood ratio: 9.55 for bedside scans 5
- Negative likelihood ratio: 0.04 for bedside scans 5
Clinical Application and Limitations
- Ultrasound can help differentiate between functional ileus and mechanical obstruction by demonstrating the presence or absence of peristalsis 3
- The presence of free fluid between dilated loops is particularly important as it suggests higher-grade obstruction requiring surgical intervention 3
- Approximately 11% of patients may have excessive bowel gas that limits ultrasound evaluation (labeled as "gassy" patients) 4
- While CT remains superior for determining the etiology of SBO, ultrasound can correctly identify the cause in approximately 71.4% of mechanical ileus cases 4
Advantages of Ultrasound for SBO
- Non-invasive and radiation-free
- Can be performed at bedside in critically ill patients
- Allows real-time assessment of peristalsis
- Can be repeated to monitor progression
- Particularly useful in pediatric patients and pregnant women 1
Despite these advantages, CT generally provides more comprehensive information about the entire gastrointestinal tract, three-dimensional anatomy, and underlying causes and complications of SBO, making it the preferred imaging modality for surgical planning in adults 1.