Written by Christopher Thom
Spoon Feed
The diameter threshold with the highest accuracy for detection of small bowel obstruction on ultrasound was 2.75 cm. See the clip, still image, and pro tips below.
How to spot SBO on ultrasound
This was an analysis of four prior prospective studies on POCUS for SBO across five academic EDs. The authors attempted to identify the most accurate diameter threshold for SBO identification on POCUS, using CT or surgical reports as the gold standard. Prior investigational studies on POCUS for SBO have used a cut-off of 2.5 cm as the criteria for pathologic bowel dilation. This lines up with the commonly referenced CT bowel dilation diameter, but there is not universal consensus on this measurement (1). There may also be differences in measurements on POCUS versus CT.
The authors identified 2.75 cm as the cut-off with the highest overall accuracy in the cohort of 367 patients. This yielded a sensitivity of 72% (95%CI 63.7%–79.1%) and specificity of 67.1% (95%CI 60.6%–73.2%). The traditional threshold of 2.5 cm was more sensitive at 85% but less specific at 59%. A threshold of 1.7 cm was 100% sensitive but only 22% specific. There was a slight variance between genders, with females having an optimal threshold of 2.75 cm and males 2.95 cm.
How will this change my practice?
POCUS for small bowel obstruction continues to gain traction. In many cases, it can make the diagnosis from the outset and can rapidly reduce our cognitive workload. The diagnosis is clinched from the start, and dead-end pursuits, like biliary colic, pancreatitis, gastroenteritis, renal colic, or cannabis hyperemesis, are not inadvertently pursued. The present study supports what many of us have anecdotally noted, which is that the 2.5 cm diameter may lack specificity. 2.75 cm appears to provide the best trade-off of sensitivity and specificity. Recall also that a diameter less than 1.7 cm was 100% sensitive in ruling out the diagnosis.
Pro-tips
Identifying dilated loops of small bowel can be achieved through a graded compression ultrasound technique. This involves serial compressions with the ultrasound transducer in a “lawn mower” approach to cover the abdomen. Compression can help displace bowel gas and improve bowel visualization. The low frequency curvilinear probe is generally used for adults, though high frequency transducers can be used for thin patients and children. At times, a focused evaluation at the location of maximal pain can be helpful in identifying signs of obstruction. Another tip is to move the transducer laterally when encountering a high degree of bowel gas from the anterior views. And keep an eye out for hernias, as these can be a common cause of SBO. Ultrasound can determine if they are fat containing or bowel containing in nature.
Source
Optimal bowel diameter thresholds for diagnosing small bowel obstruction and surgical intervention with point-of-care ultrasound. Am J Emerg Med. 2024 Oct;84:1-6. doi: 10.1016/j.ajem.2024.07.019. Epub 2024 Jul 14. PMID: 39043061
Works Cited
- Review of small-bowel obstruction: the diagnosis and when to worry. Radiology. 2015;275(2):332-342.
- Diagnostic Accuracy and Time-Saving Effects of Point-of-Care Ultrasonography in Patients With Small Bowel Obstruction: A Prospective Study. Ann Emerg Med. 2020;75(2):246-256.
