Most Likely Diagnosis and Recommended Work-Up
The most likely diagnosis is Crohn's disease affecting the mid-to-distal small bowel, and you should proceed with MR enterography (preferred over CT to avoid radiation) followed by ileocolonoscopy with biopsies from at least five distinct sites including the terminal ileum. 1, 2
Primary Differential Diagnosis
The CT findings of mid-to-distal small bowel wall thickening with mesenteric edema but sparing the terminal ileum create an atypical pattern for Crohn's disease, which most commonly affects the terminal ileum in roughly one-third of cases. 2 However, Crohn's can involve any segment of the gastrointestinal tract with skip lesions, making this presentation still consistent with the diagnosis. 2
Key Diagnostic Considerations:
Crohn's disease remains the leading diagnosis when segmental mural hyperenhancement and wall thickening coexist, particularly if asymmetric involvement is present. 3
Infectious enteritis must be excluded, as it can produce identical imaging findings of segmental hyperenhancement, wall thickening, and mesenteric edema. 3 The absence of terminal ileum involvement makes infectious causes more plausible than typical Crohn's patterns.
NSAID enteropathy, ischemia, vasculitis, and radiation enteritis are additional causes of segmental small bowel wall thickening that can mimic inflammatory bowel disease. 3
Critical Imaging Features to Assess
Features Supporting Crohn's Disease:
Asymmetric wall thickening, particularly affecting the mesenteric border more than the antimesenteric border, is highly specific for Crohn's disease. 3, 1, 2
Wall thickness ≥10 mm indicates severe inflammation, while 5-9 mm represents moderate inflammation. 1, 4
Stratified (bi- or tri-laminar) enhancement pattern with submucosal edema indicates active Crohn's inflammation. 3, 1
Small ulcerations appearing as focal breaks in the bowel wall surface correlate with severe endoscopic inflammation. 3, 1, 2
Perienteric fat stranding disproportionately more severe than the degree of wall thickening suggests inflammatory conditions over malignancy. 3, 2, 5
Features Suggesting Alternative Diagnoses:
Smooth, symmetric, circumferential thickening is more likely associated with benign processes including edema, ischemia, or infectious enteritis rather than Crohn's disease. 4, 5
Homogeneous, symmetric transmural hyperenhancement can indicate edema, ischemia, or shock bowel. 3
Recommended Diagnostic Algorithm
Step 1: Advanced Cross-Sectional Imaging
Obtain MR enterography with oral and IV contrast as the preferred next step. 1, 4 MRE has 66-95% sensitivity and 64-97% specificity for detecting Crohn's disease-related small bowel inflammation and avoids radiation exposure. 1
MRE is superior to non-contrast MRI, which has only 50% sensitivity for small bowel wall thickening. 1
If IV contrast cannot be administered, non-contrast MRE with T2-weighted and diffusion-weighted imaging is an acceptable alternative. 1
Look specifically for: intramural edema on T2-weighted sequences, restricted diffusion on high b-value DWI, asymmetric involvement, ulcerations, and perienteric inflammatory changes. 3, 1
Step 2: Endoscopic Evaluation
Perform complete ileocolonoscopy with biopsies from at least five distinct sites (minimum two from the terminal ileum, four colonic segments, and the rectum). 2 This is essential even though the terminal ileum appears uninvolved on CT, because:
Cross-sectional imaging may detect small bowel inflammation not seen at ileocolonoscopy. 3
Endoscopy provides histologic confirmation and can identify skip lesions, deep ulcerations, and the cobblestone appearance characteristic of Crohn's disease. 2
Non-caseating granulomas, though identified in only a minority of biopsies, are pathognomonic for Crohn's disease. 2
Step 3: Laboratory and Stool Studies
Obtain stool cultures and serologies if infectious etiology is suspected. 1, 4 This is particularly important given the atypical distribution sparing the terminal ileum.
Test for bacterial pathogens (Salmonella, Shigella, Campylobacter, Yersinia), parasites (Giardia), and consider viral etiologies.
Inflammatory markers (CRP, fecal calprotectin) help quantify disease activity but are nonspecific. 6
Additional Imaging Findings to Address
Focal Fatty Infiltration of Falciform Ligament:
This is typically an incidental benign finding unrelated to the small bowel pathology and requires no specific work-up unless there are concerning features suggesting malignancy. 5
Subcutaneous Fluid Collection Over Right Hip:
The description as "benign-appearing" suggests a simple sebaceous cyst or other benign fluid collection. No immediate intervention is needed unless symptomatic, but document size and characteristics for comparison if future imaging is performed. 5
Common Pitfalls to Avoid
Do not dismiss Crohn's disease solely because the terminal ileum is spared. Crohn's can affect any segment of the GI tract with skip lesions, and approximately one-third of patients have isolated small bowel disease proximal to the terminal ileum. 2
Do not rely on CT enterography alone without endoscopic correlation. CTE with only oral contrast will not detect or stage colonic inflammation as well as ileocolonoscopy. 3
Do not assume symmetric thickening excludes Crohn's disease. While asymmetry is specific, some Crohn's lesions can appear symmetric, particularly in early disease. 3
Residual mural thickness or fatty deposition may persist after mucosal healing, potentially causing false-positive findings of active disease on imaging. 3 Correlation with clinical symptoms and inflammatory markers is essential.
Treatment Implications Based on Severity
If Crohn's disease is confirmed and imaging demonstrates severe inflammation (wall thickness ≥10 mm, ulcerations, marked mesenteric edema):