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Small bowel obstruction: Diagnosis by ultrasonography

SBOuprightA 64 year old man with an extensive history of abdominal surgeries presents to the emergency department with abdominal pain and vomiting. Because you suspect a bowel obstruction, you bring an ultrasound machine to the bedside prior to the completion of any laboratory testing or other imaging. A curvilinear probe in the abdominal mode setting was used to scan in all four quadrants of the abdomen looking in both the sagittal and transverse planes.

Ultrasound imaging

With a curvilinear probe on the patient’s abdomen (Figure 1), the following ultrasound (US) image (Figure 2) and video (Figure 3) were obtained. The images showed dilated, fluid-filled bowel loops with thickened bowel walls, as well as minimal peristalsis. A CT scan confirmed a diagnosis of a distal small bowel obstruction (SBO).

Curvilinear-probe-on-abdomen 550

Figure 1. Position of the curvilinear prone on the patient’s abdomen


Figure 2. Curvilinear probe used to demonstrate multiple small bowel loops and abdominal free fluid

Figure 3. Ultrasound video showing multiple small bowel loops and abdominal free fluid

SBO: Background

Annually, in the United States, approximately 10% of all visits to the emergency department, or 13 million patients, present with the chief complaint of abdominal pain1. Of those patients, approximately 2% are diagnosed with an SBO.2 While not a very common cause of abdominal pain, it is associated with high rates of severe complications,3 including strangulation and bowel necrosis.4,5 Bickell et al found that a delay in the diagnosis and management of an SBO was associated with a higher risk of bowel resection. Only 4% of patients appropriately managed less than 24 hours after symptom onset required resection compared with 10-14% of patients managed more than 24 hours after symptom onset.6 Emergency medicine (EM) physicians have a unique role to lower the likelihood of poor outcome in individuals with SBO as the vast majority of patients subsequently diagnosed with an SBO initially present to the ED.7

SBO: History and Physical

When a patient presents with abdominal pain, the history and physical exam can help differentiate SBO from other causes of pain. Components of the history and physical exam more commonly associated with an SBO include a history of previous surgeries, constipation, abnormal bowel sounds, and abdominal distention.3 In one study, a history of previous abdominal surgery with adhesions was seen in 75% of patients with SBO.8 Constipation and abdominal distention can also point towards the diagnosis of an SBO, but have been found to have poor sensitivities.9,10

SBO: Imaging

While the physical exam plays an important part in the initial evaluation of a suspected SBO, imaging plays a critical role in its definitive diagnosis. Multiple imaging modalities have been described to diagnose SBO, including CT, MRI, X-ray, and US.3

  • CT has been found to be highly accurate for the diagnosis of SBO. Two studies using a 64 slice multidetector CT demonstrated sensitivities of 93%-96% and specificities of 93%-100% for the diagnosis of SBO.11,12 With its high accuracy, the CT scan is considered to be the gold standard for diagnosis of SBO.13
  • MRI can be a useful alternative to a CT scan and has been reported to have similar accuracies when compared to a 64-slice CT scanner.14,15
  • Abdominal X-Ray (AXR) have been found to have disappointing accuracy. The only publication that used CT as the sole gold standard for SBO found AXR to have a sensitivity of 46.2% and a specificity of 66.7%.16

Although highly accurate, both the CT and the MRI have the distinct disadvantages of not being able to performed at the bedside, as well as being time consuming, more costly, and in the case of CT, carrying the side effects of radiation and possible contrast reactions. Ultrasound is a bedside testing modality that has recently arisen as a viable alternative.

SBO: What about ultrasonography?

There has been a recent explosion in research and expansion of bedside US as an imaging tool in the ED. Subsequently, US has emerged as a possible adjunct in the accurate and timely diagnosis of SBO. Limited research to date has been performed using US as a diagnostic modality for an SBO. A recent meta analysis and systemic review identified six prospective US studies, only two of which were done in the ED.3

  1. Unluer et al performed a prospective study that enrolled 174 patients in the ED, 90 of which eventually were given the diagnosis of an SBO.13 This study used four relatively inexperienced EM residents as operators and found their ultrasounds to have a sensitivity of 97.7% and a specificity of 92.7%.
  2. Jang et al enrolled 76 patients, 33 of whom were eventually diagnosed with an SBO by CT scan. They found that dilated bowel on US had a sensitivity of 91% and a specificity of 84%, while decreased peristalsis had a specificity of 98% and sensitivity of 27%.16

Ultrasound criteria for diagnosing SBO

Specific criteria used in the sonographic diagnosis of an SBO vary slightly in the medical literature, but the publications reviewed considered a fluid-filled small bowel lumen >2.5 cm to be consistent with the diagnosis of SBO.13,16–18 Fluid seen outside of the dilated loops of bowel are thought to confer a worse prognosis.19

Advantages of using ultrasound for SBO

Ultrasound is a promising adjunct to the evaluation of a patient with a suspected SBO. It can be performed rapidly and with high accuracy, even in the hands of providers with minimal training. In the study by Jang et al, EM residents with 10 minutes of didactic time and previous experience with only 5 SBO ultrasounds performed with high accuracy.16 Ultrasound is a non-carcinogenic, bedside imaging modality that has the potential to decrease costs, and may be preferred in patients with relative contraindications to CT scans. Even in patients without contraindications to CT scans, US may be used to safely and quickly identify and risk-stratify those who require further imaging versus those who can be safely discharged home. In addition, patients with recurrent episodes of SBO could potentially be managed with US as the sole imaging modality to avoid multiple and repeated dosages of ionizing radiation in the form of a CT scan.

Further research on a larger scale is needed to continue to explore the utility of bedside US as a rapid, accurate and potentially life-saving option for imaging in patients with potential small bowel obstructions, and to specifically address if a patient with US as the sole imaging modality can be managed based on lack or presence of findings suspicious for SBO.

Emergency Department Visits. Centers for Disease Control.
Hastings R, Powers R. Abdominal pain in the ED: a 35 year retrospective. Am J Emerg Med. 2011;29(7):711-716. [PubMed]
Taylor M, Lalani N. Adult small bowel obstruction. Acad Emerg Med. 2013;20(6):528-544. [PubMed]
Cheadle W, Garr E, Richardson J. The importance of early diagnosis of small bowel obstruction. Am Surg. 1988;54(9):565-569. [PubMed]
Fevang B, Fevang J, Stangeland L, Soreide O, Svanes K, Viste A. Complications and death after surgical treatment of small bowel obstruction: A 35-year institutional experience. Ann Surg. 2000;231(4):529-537. [PubMed]
Bickell N, Federman A, Aufses A. Influence of time on risk of bowel resection in complete small bowel obstruction. J Am Coll Surg. 2005;201(6):847-854. [PubMed]
Foster N, McGory M, Zingmond D, Ko C. Small bowel obstruction: a population-based appraisal. J Am Coll Surg. 2006;203(2):170-176. [PubMed]
Menzies D, Ellis H. Intestinal obstruction from adhesions–how big is the problem? Ann R Coll Surg Engl. 1990;72(1):60-63. [PubMed]
Böhner H, Yang Q, Franke C, Verreet P, Ohmann C. Simple data from history and physical examination help to exclude bowel obstruction and to avoid radiographic studies in patients with acute abdominal pain. Eur J Surg. 1998;164(10):777-784. [PubMed]
Eskelinen M, Ikonen J, Lipponen P. Contributions of history-taking, physical examination, and computer assistance to diagnosis of acute small-bowel obstruction. A prospective study of 1333 patients with acute abdominal pain. Scand J Gastroenterol. 1994;29(8):715-721. [PubMed]
Shakil O, Zafar S, Zia-ur-Rehman, Saleem S, Khan R, Pal K. The role of computed tomography for identifying mechanical bowel obstruction in a Pakistani population. J Pak Med Assoc. 2011;61(9):871-874. [PubMed]
Pongpornsup S, Tarachat K, Srisajjakul S. Accuracy of 64 sliced multi-detector computed tomography in diagnosis of small bowel obstruction. J Med Assoc Thai. 2009;92(12):1651-1661. [PubMed]
Unlüer E, Yavaşi O, Eroğlu O, Yilmaz C, Akarca F. Ultrasonography by emergency medicine and radiology residents for the diagnosis of small bowel obstruction. Eur J Emerg Med. 2010;17(5):260-264. [PubMed]
Regan F, Beall D, Bohlman M, Khazan R, Sufi A, Schaefer D. Fast MR imaging and the detection of small-bowel obstruction. AJR Am J Roentgenol. 1998;170(6):1465-1469. [PubMed]
Beall D, Fortman B, Lawler B, Regan F. Imaging bowel obstruction: a comparison between fast magnetic resonance imaging and helical computed tomography. Clin Radiol. 2002;57(8):719-724. [PubMed]
Jang T, Schindler D, Kaji A. Bedside ultrasonography for the detection of small bowel obstruction in the emergency department. Emerg Med J. 2011;28(8):676-678. [PubMed]
Musoke F, Kawooya M, Kiguli-Malwadde E. Comparison between sonographic and plain radiography in the diagnosis of small bowel obstruction at Mulago Hospital, Uganda. East Afr Med J. 2003;80(10):540-545. [PubMed]
Patel T. Contemporary health scene–its relevance to our country (Dr. B.C. Dasgupta Memorial Oration). Indian J Public Health. 1975;19(4):155-169. [PubMed]
Grassi R, Romano S, D’Amario F, et al. The relevance of free fluid between intestinal loops detected by sonography in the clinical assessment of small bowel obstruction in adults. Eur J Radiol. 2004;50(1):5-14. [PubMed]

ALiEM Copyedit

Hello, and thanks for being the first to submit using our new process. This is a great topic -- ultrasound for SBO. We should have been more clear, but this is different from a case-based clinical review that is typically seen in journals. We\'re going for less of a book-report and more of a clinical relevance spin. Also in the age of short-attention spans, people looking for the answer won\'t have the patience to read the entire case report. I would try to get to your core content as quickly as possible.

So some specific notes:

1. I would delete the first two paragraphs (grayed out currently).
2. The first paragraph should be a \"teaser\" for the post, since this will be the only part displayed on the home page before the \"read more...\" button. So I rephrased it for you. Hope it reads ok. Feel free to continue editing.
3. As we described in the Instructions, readers tend to be F-style readers, so I would avoid the traditional long paragraphs seen in journals. I used bolded headers <h3 style=\"\"> to help break up the flow and help readers find the answer they want quickly. Of note, readers have often reached ALiEM pages looking for a quick answer to a bedside clinical question. Let\'s help them find what they are looking for.
4. If at all possible, think of when you can convert things to tables or lists. So, I converted the content for SBO: Imaging section. Let me know what you think.
5. One question still stands out to me. Is there no consistent definition of SBO on US? Sounds like it\'s still open to debate? If it\'s still a moving target, I\'d be more explicit about that in the text. Also can you describe what we should be seeing figure 2 more? I don\'t see the back and forth spot echo movements. Are we just seeing dilated loops of bowel?

Thanks again. Looking forward to getting this through the peer review system quickly to publish soon.

Michelle Lin, MD
ALiEM Editor in Chief; Associate Professor of Emergency Medicine, UC San Francisco

Expert Peer Review

My comments are more nit picky.

  • \"Abdominal X-Ray (AXR), while traditionally the initial choice in bedside imaging, has been found to have sensitivities as low as 46% [16], and specificities as low as 50% [17].\" - this kinda reads like a straw man argument. Do you have a range / average or pooled sens/spec?
  • \"Alternative to a CT scan in patients that have contraindications to contrast\" - neither IV or PO contrast are absolutely necessary to diagnose SBO with CT.
  • \"Ultrasound is a bedside test\" - change to \"Ultrasound is a testing modality.\"
  • \"Defined as back and forth movements of spot echoes within the lumen the absence of forward peristalsis [16]\" -something is missing here. I don\'t understand this.
  • \"It can be done rapidly\" - change to \"it can be performed rapidly\"...
  • \"with only an US as the imaging modality\" - change to \"with US as the sole imaging modality\"
Ben C. Smith, MD
Emergency Ultrasound Fellowship Director of the University of Tennessee, Department of Emergency Medicine, Chattanooga, TN

Expert Peer Review

Great post! Looks like I\'m a little late to the discussion and you all have fleshed it out quite a bit.

Here are a couple things:

1. Sens and spec of AXR has a huge range there, which makes it hard to know what to do with it. Therefore, I\'d mention in 1 sentence when describing the Jang article what they found it to be. They were specifically comparing the two, so it\'s a good number to be able to use when comparing US and AXR.

2. Listed for \"Criteria for Diagnosing SBO\" you have the dilated loops and the flecks mentioned. I think this could be a bit confusing. Do they need both? Just one? I\'d personally remove the flecks one and just have \"Dilated loops of bowel >2.5cm or Dilated ileum >1.5cm\". In both Jang article and Unluer article dilated loops was found to perform the best. I like that simplicity. You could get more subtle and mention the great specificity of decreased peristalsis, but honestly, keeping it simple and focusing on the dilated loops is probably the best.

3. Lastly, I know this is a pretty simple scan of just rubbing the probe over the belly and not a lot of subtlety to get the image, but one picture of it or 3 second clip of hand position and someone doing it I think would be very helpful. If you don\'t have one I happen to have an iphone camera, abdomen, and ultrasound probe with me most of the time. You probably have the same.

Great work!

Matt Dawson, MD
Director of Point of Care Ultrasound at the University of Kentucky. Co-creator of Ultrasound Podcast, Introduction to Bedside Ultrasound Volumes 1 and 2 digital textbooks, One Minute Ultrasound smartphone app, &
Jacob Avila, MD RDMS

Jacob Avila, MD RDMS

Clinical instructor and Ultrasound fellow
Department of Emergency Medicine
University of Kentucky
Jessica Whittle, MD, PhD, FACEP

Jessica Whittle, MD, PhD, FACEP

Director of Research and Assistant Professor
Department of Emergency Medicine
University of Tennessee, Chattanooga College of Medicine
Attending Physician, Erlanger Hospital
Jessica Whittle, MD, PhD, FACEP

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