The ‘To-and-Fro’ of Gentle (Sound) Waves: POCUS in SBO

Images credit: Natasha Gill MD, Matin Shah, MD

It’s the beginning of your shift, and there are seven patients waiting to be seen. Jane is up first. She is a 46-year-old female who comes in with acute, progressive abdominal pain, bloating, and bilious emesis for several days.

She tells you that her abdominal pain is very severe, diffuse, and constant. She has a history of MDD, seizure disorder, cholecystectomy, and a chronic ventral hernia.

On presentation vitals are stable (Temp 36.1°C | Pulse 74 | Resp 19 | SpO2 98% | BP 136/94). Her abdominal exam is distended and diffusely tender in all quadrants with hypoactive bowel sounds.

Back at your station, what tests do you order?

You might say basic labs, X-ray and a CT because you are concerned for a small bowel obstruction (SBO). Did you consider a bedside ultrasound?

Did you know that a meta-analysis published in Academic Emergency Medicine in 2013 showed that a bedside ultrasound had a pooled sensitivity of 97% and specificity of 90% compared to CT scan of 87% and 81%, respectively, for diagnosing SBO?

How did you interpret these clips and images below?

Figure 1:  “To-and-fro” movement of the bowel contents.

Figure 1: “To-and-fro” movement of the bowel contents.

Figure 2: A.  Dilated bowel loops measuring 3.38 cm.  B.  Keyboard sign

Figure 2: A. Dilated bowel loops measuring 3.38 cm. B. Keyboard sign

What is the keyboard sign?

  • The inner wall of the small intestine is lined with numerous folds of mucus membrane called plica circularis (or valves of Kerckring). These are especially prominent in the distal duodenum and jejunum, and rare in the ileum. In small bowel obstruction, these finger-like projections become more prominent and look like a keyboard.

What are some ultrasound findings of SBO, and which one is the most sensitive and specific?

  • Dilated fluid-filled small bowel loops greater than 2.5 cm is the most sensitive and specific

  • “To-and-fro” movement of the bowel contents due to increased peristalsis or increased intestinal contents

  • Keyboard sign

If you saw no peristalsis, bowel wall thickening (more than 3mm), and/or fluid-filled loops with extra luminal free fluid, would it change your management?

These ultrasound findings are concerning for bowel ischemia which is associated with a higher mortality rate and larger portion of bowel resection, therefore requiring prompt surgical evaluation.

Granted there are some limitations to using ultrasound, recent literature shows that it is a promising adjunct for the evaluation of SBO.

A prospective study in the ED by Unluer et al showed that four relatively inexperienced EM residents had a sensitivity of 97.7% and a specificity of 92.7% to detect SBO using ultrasound.

Pros and Cons of Ultrasound to Detect SBO


  • Lower cost than CT

  • Can be performed at bedside

  • Can be performed rapidly and with high accuracy (even in less experienced operators)

  • No side effects from radiation or possible contrast reactions


  • Operator dependent

  • May be difficult to identify:

    • Location of obstruction

    • Partial SBO

    • Specific cause of obstruction

What are some extrinsic, intrinsic, and intraluminal causes of SBO?


  • Crohn’s disease*

  • Neoplasia

  • Intussusception

  • Hematoma


  • Adhesions*+

  • Neoplasms

  • Hernia (umbilical, inguinal)

  • Endometriosis

Intraluminal (least common)

  • Foreign Body

*Most common cause within categories; +Most common cause overall

Ultrasound Tips to Detect SBO

  • Curvilinear probe set on abdominal mode (may also use phased-array)

  • Scan all four quadrants

  • Scan in two planes

Case Conclusion

Prior to transfer to our institution, Jane had a CT performed at an outside hospital and was diagnosed with SBO. These images, however, were initially not available to view. Instead of repeating the CT scan, a bedside ultrasound was completed and confirmed the diagnosis. The scan took less than 5 minutes. The surgery team was promptly consulted, and CT images were obtained from the other institution. Not only did the ultrasound save time and resources, the patient was also very satisfied with the care she received in the ED. She ultimately underwent a ventral hernia repair with reduction of bowel with underlay mesh repair.


  • Bedside ultrasound can be a quick, inexpensive, and useful tool in detecting SBO

  • Look for dilated fluid-filled small bowel loops greater than 2.5 cm, “to-and-fro” movement (increased peristalsis), and the keyboard sign

  • If there is no peristalsis, bowel wall thickening (more than 3mm), and/or fluid-filled loops with extra luminal free fluid, obtain prompt surgical evaluation


  1. Lim JH. Intestinal obstruction. In: Maconi G, Porro GB, eds. Ultrasound of the gastrointestinal tract. Berlin, Germany: Springer-Verlag, 2007; 27–34.

  2. Meiser G, Meissner K. Sonographic differential diagnosis of intestinal obstruction: results of a prospective study of 48 patients [in German]. Ultraschall Med1985; 6(1): 39–45.

  3. Taylor MR, Lalani N. Adult small bowel obstruction. AcadEmerg Med 2013; 14. Jun;20(6):528-44.

  4. Suri S, Gupta S, Sudhakar PJ, et al. Comparative evaluation of plain films, ultrasound and CT in the diagnosis of intestinal obstruction. Acta Radiol1999; 40(4): 422-8.

  5. 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.

  6. Wilson SR. The gastrointestinal tract. In: Rumack CM, Wilson SR, Charboneau JW, eds. Diagnostic ultrasound. 3rd ed. St Louis, Mo: Mosby, 2005; 269–320.

Ultrasound Case of the Month

Case: Submitted by Dr. Sam Goldman

This is an 83-year old woman with a history of prior abdominal surgeries presenting to the ED as a transfer from her SNF with increasing abdominal distention. Patient has not had a bowel movement in four days although endorses passing occasional flatus. She denies emesis though endorses nausea, hiccupping, and burping. She denies any abdominal pain, fevers, chills, dysuria or urinary frequency. 


Small Bowel Obstruction

Image was acquired with the curvilinear probe, but any high penetration probe (eg curvilinear of phased array probe) can also be used.  Multiple regions of the abdomen should be interrogated when evaluating for SBO.

What are we looking for with abdominal US for SBO?

When evaluating for an SBO, we are looking for fluid filled small bowel loops >2.5-3cm in width. You maybe more likely to see an increase in intestinal contents (fluid and echogenic materials) and you may see to-and-fro or whirling of the intestinal contents. In more severe cases, you may see bowel wall thickening (greater than 3mm) and free fluid which is extraluminal. pSBO may be more difficult to evaluate with the US machine.   

What do we see in this video?

  • Dilated loops of bowel > 2.5cm measured outer wall to outer wall (most sensitive and specific finding). 
  • Bidirectional flow of bowel contents (to and fro or whirling)
  • Visualization of plicae circularis (“keyboard sign”)

How good is U/S for Detecting SBO?

Ultrasound is superior to abdominal plain films and approaches the sensitivity and specificity of CT scan in many cases. 

Sensitvity Specificity
Abdominal Films 66-77% 50-57%
CT 92% 93%
Ultrasound 88% 96%

Faculty Reviewer: Dr. Kristin Dwyer


(Mallo RD, et al. Computed tomography diagnosis of ischemia and complete obstruction in small bowel obstruction: a systematic review. Journal Gastrointestinal Surgery. 2005. May-Jun;9(5):690-4.)

(Ogtata M, et al. Prospective Evaluation of Abdominal Sonography for the Diagnosis of Small Bowel Obstruction. Annals of Surgery. 1996. 23(3):237-241.) 

Additional resources:

Podcast on US of SBO from Episode 36 - Small Bowel Obstruction - Ultrasound Podcast

Great view of Plicae Circularis (“Keyboard Sign”) from Emory University