What does periappendiceal fat stranding on a computed tomography (CT) scan indicate?

What Does Periappendiceal Fat Stranding on CT Mean?

Periappendiceal fat stranding on CT indicates inflammation of the fat tissue surrounding the appendix and is a key diagnostic finding for acute appendicitis that requires urgent surgical consultation. 1, 2

Clinical Significance

Periappendiceal fat stranding represents inflammatory changes in the mesenteric fat adjacent to the appendix and serves as one of the five major CT criteria for diagnosing appendicitis. 1, 2 However, it is important to understand its diagnostic limitations:

• Fat stranding has relatively low specificity (40%) compared to other CT findings such as extraluminal appendicolith, abscess, extraluminal air, and appendiceal wall enhancement defects (which have 96-100% specificity). 1, 2

• When combined with other findings, diagnostic accuracy improves substantially. When two or more CT signs are present (including fat stranding, appendiceal diameter >6 mm, and absence of intraluminal gas), the odds ratio of appendicitis increases to 6.8. 1

• Fat stranding alone is present in 34.1% of patients with appendicitis versus only 8.9% without appendicitis (P = 0.001). 1

Distinguishing Simple from Complicated Appendicitis

Fat stranding helps differentiate the severity of appendicitis, which directly impacts morbidity and mortality:

• Fat stranding combined with free fluid on CT is significantly associated with complicated appendicitis (perforation, abscess, or peritonitis). 3 In one study, 70% of patients with both fat stranding and free fluid had complicated appendicitis. 3

• The intensity of fat stranding correlates with disease severity. Research shows that relative CT attenuation values of periappendiceal fat can distinguish gangrenous or perforated appendicitis from simple suppurative appendicitis. 4

• Complicated appendicitis carries significantly higher mortality, particularly in elderly patients (11.9-15% versus 1.5-3% for non-perforated appendicitis). 1

Diagnostic Algorithm When Fat Stranding Is Present

Step 1: Confirm Appendicitis Diagnosis

• Verify appendiceal diameter: Optimal cutoff is >8.2 mm outer diameter (sensitivity 88.8%, specificity 93.4%) or >6.6 mm with compression (sensitivity 93.8%, specificity 94.9%). 1, 2

• Look for additional supportive findings: absence of intraluminal gas, periappendiceal fluid, or appendicoliths. 1

Step 2: Assess for Complicated Disease

Search for high-risk features that indicate perforation or abscess formation:

• Extraluminal gas (odds ratio 28.9 for perforation, 100% specificity). 1

• Focal wall enhancement defect (odds ratio 23.40,100% specificity). 1

• Extraluminal appendicolith (96% specificity for perforation). 1, 2

• Circumferential periappendiceal inflammatory changes (odds ratio 5.63). 1

• Appendiceal wall thickness >3 mm (odds ratio 3.2). 1

Step 3: Initiate Management

• Immediate surgical consultation is mandatory when fat stranding is present with confirmed appendicitis, as this indicates active inflammation requiring appendectomy. 2, 5

• For complicated appendicitis with large abscess or phlegmon, percutaneous drainage combined with antibiotics may be first-line, with surgery reserved for drainage failure. 2, 5

Critical Pitfalls to Avoid

• Do not dismiss fat stranding as nonspecific. While it has lower specificity than other findings, in the clinical context of right lower quadrant pain with elevated inflammatory markers, it strongly supports appendicitis and warrants action. 1, 2

• Do not assume fat stranding alone indicates perforation. You must combine it with other high-risk CT features (wall defect, extraluminal air, appendicolith) to accurately stratify perforation risk. 1, 2, 3

• Be aware of mimics. Fat stranding can also occur with epiploic appendagitis (which shows a characteristic hyperattenuated rim and central dot sign) and cecal diverticulitis (which shows an inflamed diverticulum rather than an enlarged appendix). 6, 7, 8

• IV contrast enhances visualization of periappendiceal inflammation and makes fat stranding more apparent, improving diagnostic accuracy from 90% to 96% sensitivity. 2, 5

Special Population Considerations

• In elderly patients (>60-65 years), fat stranding should prompt aggressive management given their significantly higher complication rates with negative appendectomy (25% versus 3% in younger patients) and higher mortality with perforation. 1, 2

• In children, ultrasound is preferred initially, but if CT is performed and shows fat stranding, the same diagnostic principles apply. 5