Integrating intestinal ultrasound into inflammatory bowel disease training and practice in the United States

What is IUS, and when is it performed?

IUS is a sonographic exam performed by a gastroenterology-trained professional who scans the abdominal wall (and perineum when the rectum and perineal disease is evaluated), using both a convex low-frequency probe and linear high-frequency probe to evaluate the small intestine, colon, and rectum. The bowel is composed of five layers with alternating hyperechoic and hypoechoic layers: the mucosal-lumen interface (not a true part of the bowel wall), deep mucosa, submucosa, muscularis propria, and serosa. (Figure)

^{Dr. Noa Krugliak Cleveland}

Intestinal ultrasound (IUS) image showing a longitudinal view of the sigmoid colon. Outlined depiction of labeled bowel wall layers with transabdominal IUS exam: lumen, mucosa, submucosa, muscular propria, serosa. Gray double-headed arrow shows the borders of the measured bowel wall from the lumen-mucosal interface to the muscularis propria-serosal interface (also shown by yellow caliper measurement).

The most sensitive parameter for assessment of IBD activity is bowel wall thickness (≤ 3 mm in the small bowel and colon and ≤ 4 mm in the rectum are considered normal in adults).^8,10 The second key parameter is the assessment of vascularization, in which presence of hyperemia suggests active disease.¹¹ There are a number of indices to quantify hyperemia, with the most widely used being the Limberg score.¹² Additional parameters include assessment of loss of the delineation of the bowel wall layers (loss of stratification signifies active inflammation), increased thickness of the submucosa,¹³ increased mesenteric fatty proliferation (with increased inflammation, mesenteric fat proliferation will appear as a hyperechoic area surrounding the bowel), lymphadenopathy, bowel strictures, and extramural complications such as fistulae and abscess. Shear wave elastography may be an effective way to differentiate severe fibrotic strictures, but this is an area that requires more investigation.¹⁴

IUS has been shown to be an excellent tool in not only assessing disease activity and disease complication (with higher sensitivity than the Harvey-Bradshaw Index, serum C-reactive protein),¹⁵ but, unique to IUS, can provide early prediction of response in moderate to severe active UC.^6,7 This has also been shown with transperineal ultrasound in patients with UC, with the ability to predict response to therapy as early as 1 week from induction therapy.¹⁶ Furthermore, it can be used to assess transmural healing, which has been shown to be associated with improved outcomes in Crohn’s patient, such as lower rates of hospitalizations, surgery, medication escalation, and need for corticosteroids.¹⁷ IUS is associated with great patient satisfaction and greater understanding of disease-related symptoms when the patient sees the inflammation of the bowel. (Table)