Air-Fluid Levels at Different Heights in the Same Loop of Bowel

Upright abdominal radiograph of the same patient shows multiple air-fluid levels in the small bowel. Note 'different heights of air-fluid levels in the same loop' (arrows). SB = small bowel, C = colon.

Differential Air-Fluid Levels

• Two distinct air-fluid interfaces on an upright abdominal radiograph that are at different heights but within the same loop of bowel.
• Once believed to be strongly suggestive of mechanical small bowel obstruction (due to ongoing peristalsis against mechanical blockage BUT it can be found in both mechanical obstruction and ileus (in one series, 50% of mechanical obstruction and 29% of ileus)
  
• Helpful to suggest mechanical small bowel obstruction if a differential air-fluid level is 2 cm or greater (however, not the other way around!) with a reasonable positive predictive value (PPV). As height increases, PPV and specificity increases.
• Differential height of 2.5 cm or greater is suggestive of high-grade small bowel obstruction.

This case: partial, low-grade small bowel obstruction, likely due to adhesion (history of multiple previous abdominal surgery).

Reference:
1. Harlow CL, et al. Diagnosis of bowel obstruction on plain abdominal radiographs: significance of air-fluid levels at different heights in the same loop of bowel. AJR Am J Roentgenol 1993;161:291.
2. Lappas JC, et al. Abdominal radiography findings in small bowel obstruction: relevance to triage for additional diagnostic imaging. AJR Am J Roentgenol 2001;176:167.