What is the diagnosis for a patient presenting with sudden onset of leg pain, fever, and skin changes, including extensive mottled purpura and necrotic patches, with hypotension, tachycardia, and elevated lactate levels?

Diagnosis: Necrotizing Fasciitis with Septic Shock

This patient has necrotizing fasciitis complicated by septic shock, a surgical emergency requiring immediate operative debridement and broad-spectrum antibiotics. The combination of sudden onset leg pain, extensive mottled purpura with necrotic patches, cool extremity, fever, hypotension (BP 85/52), tachycardia (HR 122), and elevated lactate (5.8 mmol/L) are pathognomonic for this life-threatening soft tissue infection 1.

Key Diagnostic Features Present

Clinical hallmarks that confirm necrotizing fasciitis:

• Pain disproportionate to physical findings - a critical early sign that distinguishes this from simple cellulitis 1
• Violaceous/mottled purpura and necrotic patches - indicating vascular thrombosis and tissue necrosis 1
• Cool extremity - suggests obliterative endarteritis with thrombosis of surrounding vessels 1
• Systemic toxicity with hypotension - meeting criteria for septic shock (systolic BP <90 mmHg despite fluid resuscitation) 1
• Elevated lactate (5.8 mmol/L) - indicating tissue hypoperfusion and severe sepsis 1

Pathophysiology

The infection spreads along fascial planes with obliterative endarteritis causing vessel thrombosis and profound tissue ischemia 1. This promotes anaerobic bacterial proliferation and fascial necrosis 1. The "wooden-hard" feel of subcutaneous tissues is pathognomonic - unlike cellulitis where tissues remain yielding, in necrotizing fasciitis the fascial planes cannot be discerned by palpation 1.

Likely Microbiology

Given the presentation, this could be either:

• Monomicrobial (Type II): Most likely Streptococcus pyogenes if there was antecedent minor trauma, with mortality approaching 50-70% when accompanied by hypotension and organ failure 1, 2
• Polymicrobial (Type I): Involving bowel flora (coliforms and anaerobes) if associated with perianal source, injection drug use, or penetrating trauma 1, 2
• Less common pathogens: Vibrio vulnificus (water exposure), Aeromonas hydrophila (freshwater exposure), or Klebsiella pneumoniae (diabetic patients) 2, 3, 4

Immediate Management Algorithm

1. Emergent surgical consultation - this is the most critical intervention 1

• Direct inspection and exploration of fascial planes is both diagnostic and therapeutic 1
• Surgical debridement must not be delayed for imaging studies 1

2. Aggressive fluid resuscitation - at least 20 mL/kg crystalloid bolus immediately 1

• Target: ≥10% increase in systolic BP, ≥10% reduction in heart rate, improved mental status and peripheral perfusion 1
• Adult septic patients may require several liters in first 24 hours 1

3. Broad-spectrum empiric antibiotics - start immediately after blood cultures 1

• Vancomycin (for MRSA coverage) PLUS
• Antipseudomonal beta-lactam (piperacillin-tazobactam, cefepime, or carbapenem) 1
• This covers both monomicrobial streptococcal/staphylococcal and polymicrobial infections 1

4. Obtain blood cultures and laboratory parameters 1

• Blood cultures, complete blood count with differential
• Creatinine, bicarbonate, creatine phosphokinase, C-reactive protein
• These help assess severity and guide prognosis 1

Critical Pitfalls to Avoid

• Do NOT delay surgery for imaging - CT or MRI may show fascial plane edema but requesting these studies delays definitive treatment 1
• Do NOT rely on surface wound cultures - they represent colonizing organisms, not the causative pathogen; deep tissue specimens obtained during surgery are required 2
• Do NOT underestimate fluid requirements - but avoid overaggressive resuscitation which can cause pulmonary edema 1
• Clinical judgment trumps scoring systems - LRINEC score has poor sensitivity (43-80%) and should not rule out necrotizing fasciitis when clinical suspicion is high 1

Prognostic Indicators

Poor prognostic factors present in this patient:

• Hypotension (BP 85/52) 1, 5
• Elevated lactate (5.8 mmol/L) 1
• Tachycardia (HR 122) 1
• Rapid progression with extensive skin changes 1, 5

The mortality rate for necrotizing fasciitis with hypotension and organ dysfunction ranges from 30-70%, making this a true surgical emergency 1, 2.