US - SBO: Ultrasound of the Month
/The Case
Male patient in his 30s with a past medical history of Crohn's disease who presents with diffuse abdominal pain, nausea, and vomiting. The pain woke him from sleep and was followed by two episodes of non-bloody, non-bilious emesis. He reports it is diffuse, although worse in the epigastrium. Of note, he was admitted for an intra-abdominal abscess that required IR drainage and a long course of PO antibiotics approximately one month prior to this presentation. He denies any fevers, chills, melena, hematochezia, or constipation and had his last bowel movement on the morning of presentation. His temperature was 36.6 C, pulse 67, blood pressure 121/90, respirations 16 per minute, and oxygen saturation was 100% on room air. His exam demonstrated a man who was uncomfortable appearing, with a moderately diffusely tender abdomen that had no rigidity or rebound. Otherwise the exam was unremarkable. Ultrasound imaging of his abdomen was obtained, revealing the following findings:
+ What do you see on Ultrasound?
A large area of contained hypo and isoechoic fluid, which is flowing back and forth. The still shows a loop of bowel that is dilated to 2.90 cm, suggestive of small bowel obstruction.
Small bowel obstruction (SBO) represents a prominent pathology in the realm of acute abdominal pain presenting to the emergency department (ED). Published data suggest that up to 2% of all abdominal pain patients presenting to an ED suffer from bowel obstuction (1). Most often these occur as a result of fibrous scars forming between loops of bowel after surgical instrumentation of the abdomen, although inflammatory pathologies (Crohn's, malignancy) or anatomic variation (hernias) can lead to obstruction as well. The mechanical blockage of forward flow classically produces abdominal distention and pain followed by nausea and vomiting. Complications are common: 15% of SBO may produce bowel necrosis, possibly progressing to perforation, peritonitis, and frank shock (2).
As this disease is prevalent, and potentially deadly, it represents a common concern for the emergency physician. Historically, plain film abdominal x-rays have been utilized, followed by the much more sensitive computed tomography (CT) scan. However, ultrasound has been described in the diagnosis of SBO as early as 1979 (3). With the recent flourishing of ultrasound in the ED, SBO has been an object of renewed interest. In 2011, Jang et al. compared pain film to ultrasound in patients presenting to an academic ED with symptoms and signs concerning for possible SBO (4). When compared to the diagnostic gold standard, which was a CT read as SBO by a board-certified radiologist, ultrasound outperformed plain films with 91% sensitivity and 84% specificity, compared to 46% and 67%, respectively. Further, 36% of abdominal films were read as indeterminate, adding to the lack of utility in this archaic imaging choice. Of note, this study population did have a high prevalence of SBO at 43%. Additionally, patients needed to be deemed appropriate for CT imaging by a physician prior to enrollment, possible biasing towards sicker patients overall.
Other projects have added to the growing credibility of ultrasound as an adjunct diagnostic modality in SBO. Unluer et al. found that when using surgical diagnosis as the gold standard for a true positive study and a negative CT as the standard for a true negative, ultrasound demonstrated excellent test characteristics, see table below (5). In a 2017 meta-analysis published in The American Journal of Emergency Medicine, collective data suggest that ultrasound is indeed a valuable tool in the investigation of SBO in adults, with a pooled positive liklihood ratio of 27.5 and a negative likelihood ratio of 0.08 (6). Not all of the 11 studies included were performed in an ED. however, there appeared to be no significant difference in the diagnostic characteristics of the ultrasound between settings.
Interestingly, physicians in Jang's study performing the ultrasound imaging underwent a simple 10 minute training session and were only required to complete 10 scans prior to the study. Physicians in the Unluer study received a 6 hour training regimen. Both of these investigators performed quite well in their analyses, suggesting this could be implemented on a larger scale relatively quickly in any particular ED. Logistically, the scan is simple to perform. A curvilinear or phased array probe is used to survey each quadrant of the abdomen in turn. A positive scan has been defined variably in the literature, however, a commonly employed cutoff is a bowel lumen exceeding 20-25mm in diameter in the jejunum or 15mm in the ileum, accompanied by normal bowel distally or an absence of peristalsis (6). It is important to note that the absence of peristalsis is a late finding, so it is less sensitive. Another helpful finding is bidirectional peristalsis (seen in the video), which also is suggestive of obstruction.
To conclude our case...the patient received a follow up CT scan of the abdomen that confirmed SBO, with a transition point in the mid abdomen. He was admitted to the gastroenterology service. The acute care surgery service was consulted and recommended conservative measures rather than operative management. He declined nasagastric decompression, was treated symptomatically.
+ Learning Points
- Plain films have a low sensitivity and specificity in diagnosing small bowel obstruction
- Published data suggest that ultrasound is a useful imaging adjunct if there is clinical suspicion for obstruction
- The exam itself is simple to learn and easy to perform at bedside
- A positive exam is defined variably, but consistently refers to findings of bowel >25mm in diameter accompanied by normal bowel distally or the absence of peristalsis
