What is the appropriate management for a patient presenting with a subcutaneous nodule, erythema, and leg weakness?

Management of Subcutaneous Nodule with Erythema and Leg Weakness

This patient requires urgent biopsy of the subcutaneous nodule with tissue sent for both histopathology and comprehensive microbiological cultures (bacterial, fungal, and mycobacterial), combined with immediate empiric broad-spectrum antimicrobial therapy covering resistant gram-positive, gram-negative, and fungal pathogens if immunocompromised. 1

Immediate Diagnostic Priorities

Assess Immune Status First

• Determine if the patient is immunocompromised (chemotherapy, transplant recipient, immunosuppressive medications, diabetes, neutropenia) as this dramatically alters the differential diagnosis and urgency of intervention 1, 2
• Immunocompromised patients with subcutaneous nodules and erythema may have disseminated fungal infection (Fusarium, Aspergillus, Mucor, Candida), bacterial infection with angioinvasion, or Nocardia 1
• The combination of leg weakness with a subcutaneous nodule raises concern for deep tissue involvement including myositis, fasciitis, or septic arthritis 1

Obtain Tissue Diagnosis Immediately

• Perform biopsy or aspiration of the subcutaneous nodule within hours to obtain material for histological examination and microbiological cultures (aerobic, anaerobic, fungal, and mycobacterial) 1, 2
• Skin biopsy should include deep tissue to assess for vasculitis, angioinvasion, or deeper structure involvement 1
• Blood cultures should be obtained if disseminated infection is suspected, particularly for Fusarium or Candida species 1, 2

Critical Clinical Assessment

Evaluate for Deep Tissue Infection

• Probe the wound or nodule to assess depth - if bone is palpable or structures deeper than subcutaneous tissue are involved, this indicates moderate-to-severe infection requiring aggressive management 1
• Measure the extent of erythema from the nodule margin - erythema extending >2 cm indicates at least moderate infection 1
• Assess for systemic inflammatory response syndrome (SIRS): temperature >38°C or <36°C, heart rate >90 bpm, respiratory rate >20 breaths/min, WBC >12,000 or <4,000 cells/μL 1
• The asymmetric leg weakness and larger thigh circumference suggest deep venous thrombosis, compartment syndrome, or deep soft tissue infection requiring urgent imaging 3

Obtain Laboratory Studies

• Complete blood count with differential, erythrocyte sedimentation rate, C-reactive protein 1
• Markedly elevated ESR is suggestive of deeper infection including osteomyelitis 1
• Blood cultures if fever or systemic symptoms present 1
• Serum glucose and metabolic panel to assess for diabetes and metabolic stability 1

Imaging Studies

• Obtain MRI of the affected thigh to evaluate for deep abscess, myositis, fasciitis, osteomyelitis, or deep venous thrombosis 1, 3
• Plain radiographs should be obtained to assess for gas in soft tissues or underlying bone involvement 1
• Consider CT if MRI unavailable or contraindicated 1, 3

Empiric Antimicrobial Therapy

If Immunocompetent

• Start vancomycin plus piperacillin-tazobactam or ceftriaxone to cover MRSA, streptococci, and gram-negative organisms including resistant Enterobacteriaceae 1, 3, 4
• Vancomycin dosing: 15-20 mg/kg IV every 8-12 hours (target trough 15-20 mcg/mL for serious infections) 1
• Piperacillin-tazobactam: 3.375-4.5 g IV every 6-8 hours 3, 4
• Alternative: Ceftriaxone 1-2 g IV every 12-24 hours if less severe 5, 6, 7

If Immunocompromised or Neutropenic

• Initiate vancomycin plus an antipseudomonal beta-lactam (piperacillin-tazobactam, cefepime, or meropenem) PLUS empiric antifungal coverage with voriconazole or liposomal amphotericin B 1
• Voriconazole is preferred for suspected Aspergillus, Scedosporium, or Fusarium species 1
• Liposomal amphotericin B for suspected Mucor/Rhizopus species 1
• Consider adding trimethoprim-sulfamethoxazole if Nocardia is in the differential (painless "cold" subcutaneous abscess) 1

Surgical Consultation

Indications for Urgent Surgery

• Obtain surgical consultation within hours if necrotizing soft tissue infection, Fournier's gangrene, or deep abscess is suspected 1, 3
• Surgical debridement is indicated for necrotic tissue, large subcutaneous abscesses, or suspected necrotizing fasciitis 1, 3
• Pain out of proportion to physical findings is an early sign of necrotizing infection requiring emergency surgical exploration 3
• The combination of ascending leg pain with a perineal/buttock lesion raises concern for Fournier's gangrene spreading along fascial planes 3

Hospitalization Decision

Admit if Any of the Following Present:

• Systemic signs of infection (fever, tachycardia, hypotension, altered mental status) 1
• Erythema extending >2 cm from the nodule 1
• Deep tissue involvement (muscle, tendon, bone, joint) 1
• Immunocompromised state 1
• Critical limb ischemia or concern for compartment syndrome 1
• Inability to care for self or inadequate home support 1

Common Pitfalls to Avoid

• Do not delay biopsy or surgical consultation while awaiting imaging results - tissue diagnosis is essential and imaging should not delay definitive management 1, 3
• Do not rely on soft tissue or sinus tract cultures to guide therapy for deep infections - these do not accurately reflect deeper tissue pathogens 1
• Do not assume absence of fever rules out serious infection - 50% of patients with limb-threatening diabetic foot infections lack systemic symptoms 1
• Do not use antibiotics for clinically uninfected wounds - presence of bacteria alone without inflammation does not warrant treatment 1
• In immunocompromised patients, do not assume bacterial etiology - fungal and atypical pathogens (Nocardia, Fusarium, Aspergillus, Mucor) are common and require specific therapy 1

De-escalation Strategy

• Narrow antimicrobial therapy based on culture results and clinical response within 48-72 hours 4, 8
• Prolonged broad-spectrum therapy encourages antimicrobial resistance 4, 8
• Duration of therapy depends on depth of infection: 7 days for superficial, 10-14 days for deep soft tissue, and 6-24 months for Nocardia with prolonged immunosuppression 1, 9