What are the next steps for a patient with imaging showing a poorly defined area of subcutaneous (under the skin) fat layer that may be swollen or have extra fluid?

Next Steps for Imaging Showing Poorly Defined Subcutaneous Fat with Swelling or Fluid

The most critical next step is to determine whether this represents necrotizing soft tissue infection (NSTI), which requires emergency surgical consultation and debridement within 12 hours, versus a benign process that can be managed conservatively. 1

Immediate Clinical Assessment

Evaluate for features suggesting necrotizing fasciitis, which is a surgical emergency:

• Severe pain disproportionate to clinical findings 1
• Failure to respond to initial antibiotic therapy 1
• Hard, wooden feel of subcutaneous tissue extending beyond visible skin involvement 1
• Systemic toxicity with altered mental status 1
• Edema or tenderness extending beyond cutaneous erythema 1
• Crepitus indicating gas in tissues 1
• Bullous lesions or skin necrosis/ecchymoses 1

Advanced Imaging Decision Algorithm

If Clinical Suspicion for NSTI is High:

MRI without and with IV contrast is the most sensitive imaging modality for necrotizing fasciitis, though it should not delay surgical exploration if clinical suspicion is high. 1

• MRI findings of NSTI include: deep fascial/intermuscular edema, soft tissue gas, fluid collections along fascial planes, and focal or diffuse non-enhancing fascia (indicating necrosis) 1
• Noncontrast MRI alone has the highest sensitivity for detecting fascial edema/fluid signal and can identify all key NSTI findings 1
• CT with IV contrast is an acceptable alternative if MRI is unavailable or contraindicated, showing fat stranding, fluid and gas collections dissecting along fascial planes, fascial thickening, and non-enhancing fascia 1
• CT has 100% sensitivity and 81% specificity for NSTI, with 100% negative predictive value 1

If Clinical Suspicion for NSTI is Low:

Ultrasound is the appropriate next imaging study for poorly defined subcutaneous fat abnormalities when infection is not suspected. 1

• Ultrasound can differentiate localized mass from diffuse edema and solid from cystic lesions 1
• For superficial subcutaneous lesions, ultrasound has 94.1% sensitivity and 99.7% specificity 1, 2
• Ultrasound findings in NSTI include: subcutaneous fat edema (increased echogenicity), irregular thickening and fluid >4mm along fascial planes, and soft tissue gas 1

Referral Pathways

Emergency Surgical Referral (Within Hours):

Refer immediately to general surgery or surgical oncology if any of the following are present:

• Clinical features suggesting NSTI as listed above 1
• Imaging showing gas in soft tissues, non-enhancing fascia, or extensive fascial fluid 1
• Profound toxicity, fever, hypotension, or advancement during antibiotic therapy 1

Surgical debridement must occur within 12 hours of admission for suspected NSTI to reduce mortality. 1

Routine Referral Pathways:

For benign-appearing subcutaneous masses without infectious features:

• Refer to general surgery or plastic surgery for symptomatic lesions requiring excision 2
• Refer to dermatology for multiple painful subcutaneous nodules suggesting Dercum's disease (adiposis dolorosa) 3, 4, 5
• Refer to orthopedic surgery or musculoskeletal oncology if the mass is deep-seated, >5cm, or has concerning features on imaging 2

Common Pitfalls to Avoid

Do not delay surgical exploration for imaging if clinical suspicion for NSTI is high—clinical judgment is paramount and imaging should not delay definitive diagnosis and treatment. 1

Do not rely on ultrasound alone to exclude NSTI, as sensitivity varies from 42-88% and it has limited visualization of deep fascial planes in larger patients. 1

Do not assume poorly defined subcutaneous fat is always benign—failure to respond to antibiotics after a reasonable trial mandates surgical exploration. 1

Recognize that fascial enhancement patterns on MRI/CT are variable and can show enhancement, non-enhancement, or mixed patterns depending on stage of necrosis. 1