USG Abdomen Findings Suggesting Diarrhea

1. Fluid-Filled, Dilated Bowel Loops

• Normally, small bowel contains air which blocks ultrasound visualization.
• In diarrheal states, the bowel fills with fluid, making loops clearly visible on USG.
• Finding: Multiple fluid-filled loops of bowel, often with visible plicae circulares (valvulae conniventes) along the periphery confirming small bowel origin.
• Combined with hyperactive, to-and-fro peristalsis (unlike the absent/decreased peristalsis of obstruction), this points to a secretory/inflammatory cause rather than obstruction.
• Harrison's Principles of Internal Medicine 22E, POCUS section

2. Bowel Wall Thickening

• Normal small bowel wall thickness is ≤3 mm; colon wall is ≤4 mm.
• Thickening beyond this is pathological.
• Causes to consider based on the pattern:
  • Infectious enteritis/enterocolitis (Campylobacter, Salmonella, Yersinia): diffuse mucosal and submucosal thickening of the terminal ileum and cecum with intact wall layers - muscularis and serosa are NOT affected. The appearance is called infectious ileocecitis.
  • Crohn's disease: multiple non-contiguous segments of thickened bowel wall (up to 7 mm), loss of normal wall stratification, increased vascularity on Doppler (hyperemia), creeping fat sign (mesenteric fat wrapping around bowel loops).
  • Clostridium difficile colitis: colon wall thickening present in ~91% of toxin-positive patients - a highly sensitive finding for CDI.
  • CMV colitis: pancolitis pattern, especially in immunosuppressed patients.
• Radiology Assistant - Ultrasound in Acute Abdomen; MDPI Medicina review on GI ultrasound in infections

3. Increased/Hyperactive Peristalsis

• On real-time USG, you can watch bowel movement.
• Increased, rapid, disordered peristalsis (hyperperistalsis) with fluid rushing back and forth in loops = secretory/inflammatory diarrhea pattern.
• This contrasts with absent peristalsis (paralytic ileus) or obstructive patterns.
• In SIBO (Small Intestinal Bacterial Overgrowth) causing diarrhea: increased peristalsis + increased intraluminal fluid + thickened jejunal folds seen in >84% of patients.

4. Mesenteric Lymphadenopathy

• Enlarged mesenteric lymph nodes (short axis >10 mm, or clusters of smaller nodes) suggest infectious ileocecitis, mesenteric adenitis, Crohn's disease, or lymphoma.
• Campylobacter, Yersinia, and Salmonella infections classically show enlarged nodes in the root of the mesentery alongside ileocecal thickening.
• Nodes in infectious cases remain oval, well-defined, and hypoechoic - they do NOT lose their fatty hilum (which would suggest malignancy).

5. Free Fluid / Ascites

• Small amounts of free peritoneal fluid can be seen between bowel loops.
• Seen in severe infectious enterocolitis, IBD flares, and ischemic bowel causing diarrhea.

6. Specific Pattern - Infectious Ileocecitis (Key Exam Point)

• Diffuse mucosal + submucosal thickening of terminal ileum and cecum
• Normal appendix (rules out appendicitis - this is why it matters clinically)
• Wall layers are intact (muscularis and serosa spared)
• No omental/mesenteric involvement, no abscess, no fistula (helps distinguish from Crohn's)
• The ascending colon may appear contracted with prominent haustration
• Stool cultures will typically reveal Campylobacter, Salmonella, or Yersinia

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