Bowel Decompression During MRI
Primary Mechanism
Bowel decompression during MRI occurs when a proximal fistula or sinus tract provides an alternative drainage pathway, preventing the expected upstream bowel dilatation that would normally accompany a stricture. 1
Pathophysiology
In penetrating Crohn's disease, fistulae typically develop in high-pressure regions at the mid- or proximal portions of strictures with transmural inflammation. 1
When a fistula forms proximal to a stricture, it creates a decompression pathway that allows bowel contents to bypass the obstruction, resulting in absent or minimal prestenotic dilatation despite the presence of a hemodynamically significant stricture. 1
This decompression effect should prompt careful evaluation for penetrating complications, as "prestenotic dilatation may be absent due to decompression through a proximal fistula and 'a stricture is likely to be present' may be reported." 1
Clinical Implications
The absence of expected bowel dilatation upstream from a stricture is a critical imaging finding that should trigger systematic evaluation for fistulous tracts, as this represents penetrating disease that may alter surgical management. 1
Bowel segments demonstrating imaging findings of inflammation, stenosis, or both should be carefully evaluated for the presence of penetrating disease, particularly when expected upstream dilatation is absent. 1
Additional Considerations
Hypoperistaltic medications (glucagon or hyoscine butylbromide) administered during MR enterography may reduce peristaltic motion artifact but can cause nausea in some patients, though this represents pharmacologic bowel relaxation rather than true decompression. 1
Adequate bowel distension from oral contrast (volumes exceeding 1,000 mL) is essential for MR enterography diagnostic performance, as insufficient distension significantly reduces diagnostic accuracy for detecting strictures and penetrating complications. 2