Fat Involving the Colon Wall: Diagnosis and Management
The finding of fat involving the colon wall on CT imaging most commonly represents pericolonic fat stranding, which indicates an acute inflammatory or infectious process requiring urgent evaluation to differentiate between diverticulitis (most common), epiploic appendagitis, omental infarction, appendicitis, or less commonly, colon cancer with inflammatory changes. 1, 2
Primary Diagnostic Considerations
Most Likely Diagnoses Based on Fat Stranding Pattern
Disproportionate fat stranding (fat stranding more severe than the degree of bowel wall thickening) narrows the differential diagnosis significantly and suggests mesentery-centered pathology rather than primary bowel disease 2:
- Diverticulitis - Most common cause, characterized by mild smooth bowel wall thickening with pericolonic fat stranding, scattered diverticula, and absence of lymphadenopathy 1, 2
- Epiploic appendagitis - Shows central areas of high attenuation with hyperattenuated rim, characteristically located adjacent to the colon 2
- Omental infarction - Always centered in the omentum rather than adjacent to bowel 2
- Appendicitis - Most specific finding is dilated, fluid-filled appendix with pericolonic fat stranding 2
Fat Halo Sign (Intramural Fat)
True intramural fat (fat halo sign within the bowel wall itself) has different implications 3:
- In 21% of patients without intestinal disease, this represents a normal variant strongly associated with obesity (76% of cases occur in patients >200 lbs) 3
- When associated with inflammatory bowel disease, indicates chronicity and is unrelated to whether active inflammation is present 1
- Fat density measurements range from -18 to -64 Hounsfield units (mean -41 HU) 3
Imaging Approach
CT is the Definitive Imaging Modality
Contrast-enhanced CT of the abdomen and pelvis is rated 8/9 (usually appropriate) by the American College of Radiology for evaluating left lower-quadrant pain with suspected diverticulitis 1:
- Sensitivity and specificity approach 100% for diagnosing sigmoid diverticulitis 1
- Determines extent and severity of disease 1
- Identifies complications including perforation, abscess formation, and fistulas 1
Key CT Findings to Document
Most common findings in acute diverticulitis 1:
- Pericolonic fat stranding (most sensitive finding)
- Bowel wall thickening
- Presence of diverticula
- Free fluid or free air (if perforated)
- Fascial thickening
- Inflamed diverticulum or "arrowhead sign"
Two distinct CT patterns of colitis with fat stranding 1:
- Diffuse colitis with mesenteric vessel engorgement
- Segmental colitis with moderate wall thickening and pericolonic fat stranding in areas of pre-existing diverticulosis 1
Clinical Management Algorithm
Step 1: Assess Clinical Severity
Mild symptoms with known diverticulitis history may not require imaging 1, but acute severe left lower-quadrant pain with fever mandates CT imaging 1
Step 2: Interpret CT Findings
If disproportionate fat stranding is present 2:
- Mild smooth wall thickening + no lymphadenopathy = Diverticulitis (treat medically)
- Central high attenuation + hyperattenuated rim adjacent to colon = Epiploic appendagitis (self-limited, conservative management)
- Omental location = Omental infarction (conservative management)
- Dilated fluid-filled appendix = Appendicitis (surgical consultation)
If fat stranding is proportionate to wall thickening, consider 4, 5:
- Focal irregular asymmetric thickening = Malignancy (requires colonoscopy with biopsy)
- Segmental symmetric thickening = Inflammatory bowel disease, ischemia, or infection (requires colonoscopy)
Step 3: Rule Out Malignancy
Colonoscopy is mandatory for all patients with colonic wall abnormalities on CT when cancer cannot be excluded radiologically 5:
- Focal wall thickening >1.5 cm with extension into adjacent mesentery suggests tumor 1
- Pericolic fat stranding occurs in 36.8% of colon cancers, more frequently with circumferential tumors ≥75% and longer longitudinal length 6
- Complete colonoscopy should be performed within 3-6 months if not done preoperatively 1
Step 4: Exclude Infectious Causes
Before initiating immunosuppression for presumed inflammatory conditions 1:
- Stool culture, Clostridium difficile testing, CMV PCR
- Fecal calprotectin and fecal occult blood testing
- Consider colonoscopy with biopsy if grade ≥2 symptoms 1
Critical Pitfalls to Avoid
Do not assume fat halo sign always indicates inflammatory bowel disease - it may be a normal variant in obese patients without any gastrointestinal pathology 3
Do not delay colonoscopy in patients with focal colonic wall thickening - 24% of such cases represent colon cancer, and 36% represent inflammatory bowel disease requiring specific treatment 5
Do not rely solely on imaging for diagnosis - while CT has excellent sensitivity for diverticulitis, histologic confirmation via colonoscopy is essential when malignancy cannot be excluded radiologically 1, 5
Recognize that immune checkpoint inhibitor-associated colitis can present with segmental wall thickening and pericolonic fat stranding in areas of diverticulosis, mimicking infectious diverticulitis 1