What could be causing progressive skin tenderness that started on the buttock and has now spread to the inner thigh?

Progressive Skin Tenderness from Buttock to Inner Thigh

You must urgently evaluate this patient for necrotizing fasciitis, a life-threatening deep soft tissue infection that requires immediate surgical intervention and can be fatal if diagnosis is delayed. 1

Immediate Red Flag Assessment

The progressive spread of tenderness from buttock to inner thigh is a critical warning sign that demands immediate evaluation for necrotizing soft tissue infection. The key distinguishing features you must assess NOW include: 1

• Pain severity: Severe pain disproportionate to visible skin findings is the hallmark of necrotizing fasciitis 1
• "Wooden-hard" feel: Palpate the subcutaneous tissue—if it feels firm and indurated (rather than yielding as in simple cellulitis), this strongly suggests deeper fascial involvement 1
• Systemic toxicity: Check for fever, hypotension, tachycardia, altered mental status, or signs of septic shock 1
• Skin changes: Look for edema extending beyond any erythema, bullae, skin necrosis, ecchymoses, or crepitus (gas in tissues) 1
• Rapid progression: Failure to respond to antibiotics or advancement of symptoms despite treatment 1

Critical Differential Diagnosis Algorithm

Life-Threatening Conditions (Rule Out First)

Necrotizing fasciitis is your primary concern given the anatomic location (buttock/perineum) and progressive spread. This infection tracks along fascial planes and extends well beyond superficial signs. 1 The buttock-to-thigh progression suggests potential Fournier's gangrene (perineal necrotizing fasciitis), which has mortality rates up to 88% if untreated. 1

Key clinical decision point: If the patient has diabetes, obesity, immunosuppression, or recent perineal trauma/infection, the risk of Fournier's gangrene increases dramatically. 1

Moderate-Risk Infections

Cellulitis/erysipelas can cause tenderness and may develop petechiae or ecchymoses, but the subcutaneous tissue remains palpable and yielding (not wooden-hard). 1 However, if there is progression despite appropriate antibiotics, you must reconsider necrotizing infection. 1

Intertrigo with secondary infection occurs in skin folds (buttocks, inner thighs) and presents with erythema, moisture, and peripheral scaling. 2, 3 Secondary bacterial (Streptococcus, Corynebacterium) or fungal (Candida) superinfection is common. 3 This typically causes discomfort rather than severe pain and lacks systemic toxicity. 2, 3

Lower-Risk Conditions

Alpha-1 antitrypsin deficiency-associated panniculitis presents with painful, tender nodules on thighs/buttocks that ulcerate and drain oily fluid. 1 This is rare and typically occurs in younger adults with known AAT deficiency. 1

Pressure injury/decubitus can occur on buttocks, particularly with prolonged immobility or altered consciousness. 4 This would not typically "spread" to the inner thigh unless there is secondary infection. 4

Immediate Diagnostic Workup

Do not delay surgical consultation for imaging if necrotizing fasciitis is suspected. 1 Clinical judgment is paramount—imaging may delay definitive treatment. 1

Bedside Assessment

• Probe any open areas with a blunt instrument—easy dissection along fascial planes confirms necrotizing infection 1
• Check for crepitus by palpation 1
• Assess hemodynamic stability and end-organ perfusion 1

Laboratory Studies

• Complete blood count with differential (leukocytosis common) 1
• Serum creatinine, electrolytes, inflammatory markers (CRP, procalcitonin) 1
• Blood glucose and HbA1c (undiagnosed diabetes is a major risk factor) 1
• Blood cultures if febrile or systemically ill 1
• Consider LRINEC score (Laboratory Risk Indicator for Necrotizing Fasciitis) for early diagnosis 1

Imaging (Only if Diagnosis Uncertain and Patient Stable)

• CT or MRI may show fascial plane edema and gas, but sensitivity/specificity are poorly defined 1
• Do not delay surgery for imaging if clinical suspicion is high 1

Treatment Algorithm

If Necrotizing Fasciitis Suspected or Confirmed

Immediate surgical debridement is the primary treatment—antibiotics alone are insufficient. 1 The patient should return to the operating room every 24-36 hours until no further debridement is needed. 1

Empiric broad-spectrum antibiotics (start immediately, do not wait for cultures): 1

• Vancomycin 15 mg/kg IV every 6 hours (for MRSA coverage) PLUS
• Piperacillin-tazobactam or a carbapenem (for polymicrobial coverage including anaerobes) PLUS
• Clindamycin 600-900 mg IV every 8 hours (decreases toxin production) 1

For suspected Fournier's gangrene originating from bowel/genitourinary flora, ensure coverage of coliforms and anaerobes. 1

If Cellulitis Without Deep Tissue Involvement

• Oral antibiotics: cephalexin, dicloxacillin, or clindamycin for 5 days if improving 1
• Add MRSA coverage (e.g., trimethoprim-sulfamethoxazole, doxycycline) if there are risk factors: injection drug use, known MRSA colonization, or purulent drainage 1
• Reassess in 24-48 hours—if worsening or not improving, reconsider necrotizing infection 1

If Intertrigo with Secondary Infection

• Minimize moisture and friction with barrier creams or absorptive powders 2, 3
• Topical antifungals (nystatin, clotrimazole) for Candida 3
• Topical mupirocin or oral penicillin for streptococcal superinfection 3
• Oral erythromycin for Corynebacterium 3

Critical Pitfalls to Avoid

Never dismiss progressive tenderness as simple cellulitis without thoroughly assessing for deep tissue involvement. 1 The initial presentation of necrotizing fasciitis often resembles cellulitis, and the diagnosis may not be apparent on first evaluation. 1

Do not wait for fever or dramatic skin changes to act. 1 Severe pain with minimal skin findings is the classic early presentation of necrotizing fasciitis. 1

Avoid the trap of assuming antibiotics will work if this is a deeper infection. 1 Failure to respond to initial antibiotic therapy within 24-48 hours mandates surgical exploration. 1

In the buttock/perineal region, always consider Fournier's gangrene, especially in diabetic, obese, or immunocompromised patients. 1 This has extremely high mortality without aggressive surgical debridement. 1