Small Bowel Obstruction & PoCUS – Medical Student Pearl

Patrick Rogers, Clinical Clerk (CC3)

Memorial University of Medicine Class of 2021

Reviewed by Dr. Kavish Chandra

Small bowel obstructions (SBO) are a common cause of acute abdominal pain in emergency departments across Canada. Diagnostic imaging plays a key role in the diagnosis and management of SBO as the history, clinical examination and laboratory investigations lack the sensitivity and specificity needed. Furthermore, diagnostic imaging may help differentiate SBO from other causes of abdominal pain (hernias, malignancies, intussusception, etc).

Historically, plain film abdominal radiography (AXR) has been an initial investigation in emergency departments when an SBO is suspected.  However, the current literature suggests that abdominal radiography is a relatively poor test for the diagnosis or exclusion of SBO when compared to other available modalities like US, CT, or MRI. In fact, multiple studies argue for the reduction of abdominal x-rays, especially when patients come in presenting with general abdominal tenderness. ¹ Fortunately, there exists a compelling alternative: point of care ultrasound (PoCUS), and is being increasingly used as a first line investigation for SBO. ²

There are several reasons why physicians may start to choose PoCUS over traditional diagnostic modalities:

• PoCUS avoids the radiation exposure that patients receive from cumulative plain films and abdominal CT’s. ³
• PoCUS has been shown to reduce time to diagnosis and treatment in comparison to abdominal plain films. ³
• PoCUS is more sensitive/specific modality when compared to abdominal plain film. ⁴
• PoCUS allows for serial examination in the ED. ⁵
• PoCUS may be rapidly available to centers with limited access to CT scanner. ⁶

The current evidence is highly favorable for the diagnostic efficacy of PoCUS in SBO. Here are the findings of peer-reviewed studies on the subject (published between 2013-2020):

• PoCUS has high diagnostic accuracy and may also decrease time to diagnosis of SBO in comparison to other imaging modalities like CT and plain film.²
• PoCUS has been found to have superior diagnostic accuracy for SBO in comparison to plain abdominal radiography. ⁴
• PoCUS has been shown to be an accurate tool in the diagnosis of SBO with a consistently high sensitivity of 94-100% and specificity of 81-100%. ⁵
• Current evidence suggests PoCUS is comparable in sensitivity and specificity to a CT scan when diagnosing SBO. ⁶
• Ultrasound was found to be equivalent to CT in terms of diagnostic accuracy with a sensitivity of 92.31% (95% CI, 74.87% to 99.05%) and a specificity of 94.12% (95% CI, 71.31% to 99.85%) in the diagnosis of SBO. ⁷
• In a study comparing XR, US, CT, and MRI, the abdominal x-ray was shown to be to be the least accurate imaging modality for the diagnosis of SBO. AXR’s were found to have a positive likelihood ratio of 1.64 (95% CI 1.07 to 2.52). In contrast, CT and MRI were both quite accurate in diagnosing SBO with positive likelihood ratios of 3.6 (95% CI = 2.3 to 5.4) and 6.77 (95% CI = 2.13 to 21.55). The use of ultrasound was found to have a positive likelihood ratio of 9.55 (95% CI = 2.16 to 42.21) and a negative likelihood ratio of 0.04 (95% CI = 0.01 to 0.13) for beside scans. ⁴

There are two major barriers identified in the literature that may prevent the effective use of PoCUS in the diagnosis of SBO. First, not every emergency physician has been trained on the use of PoCUS. Fortunately, two recent studies show that even minimally trained ED physicians can use it accurately. ⁸ Secondly, some surgeons have argued that PoCUS does not show the location of the obstruction accurately. This becomes a concern when the care team elects for surgical management of the patient’s SBO. However, recent evidence suggests that PoCUS may lead to quicker time to diagnosis and enteric tube insertion in conservative management. ⁸

Finally, how can learners use this technology? ⁵ Here are some specific sonographic findings to look for when evaluating a patient for SBO with US:

• Dilatation of small bowel loops > 25 mm ^*
• Altered intestinal peristalsis ^*
• Increased thickness of the bowel wall
• Intraperitoneal fluid accumulation

Figure 1. Dilatation of small bowel loops. Image courtesy Dr. Kavish Chandra

Figure 2. Altered intestinal peristalsis*. Image courtesy Dr. Kavish Chandra

Figure 3. – abnormal peristalsis “to and fro”⁹

References

1. Denham G, Smith T, Daphne J, Sharmaine M, Evans T. 2020. Exploring the evidence-practice gap in the use of plain radiography for acute abdominal pain and intestinal obstruction: a systematic review and meta-analysis. International Journal of Evidence Based Healthcare. DOI: 10.1097/XEB.0000000000000218
2. Guttman J, Stone M, Kimberly H, Rempell J. 2015. Point of care ultrasonography for the diagnosis of small bowel obstruction in the emergency department. CJEM. DOI: 10.2310/8000.2014.141382
3. Flemming H, Lewis D. 2016. SBO- A New Focus for PoCUS. Saint John Regional Hospital Department of Emergency Medicine
4. Taylor M, Lalani N. 2013. Adult small bowel obstruction. Academic Emergency Medicine. DOI: 10.1111/acem.12150
5. Pourman A, Dimbil U, Shokoohi H. 2018. The accuracy of point of care ultrasound in detecting small bowel obstruction in emergency department. Emergency Medicine International. DOI: 10.1155/2018/3684081
6. Gottlieb M, Peska, G, Pandurangadu A, Nakitende D, Takhar S, Seethala R. 2018. Utilization of ultrasound for the evaluation of small bowel obstruction: A systematic review and meta-analysis. The American Journal of Emergency Medicine. DOI: 10.1016/j.ajem.2017.07.085
7. Tamburrini S, etal. 2019. Diagnostic accuracy of ultrasound in the diagnosis of small bowel obstruction. Diagnostics. DOI: 10.3390/diagnostics9030088
8. Carpenter C. 2013. The end of X-Rays for suspected small bowel obstruction? Using evidence-based diagnostics to inform best practices in emergency medicine. Academic Emergency Medicine. https://doi.org/10.1111/acem.12143
9. The PoCUS Atlas. https://www.thepocusatlas.com/bowel-gi

Copyedited by Dr. Mandy Peach