Anaerobic and Necrotizing Infections

Description

• Necrotizing soft tissue infections (NSTI) are rare and rapidly progressing infections involving any layer of soft tissue, including skin, subcutaneous fat, fascia, and/or muscle (1).
• NSTI can affect any part of the body but are more commonly seen in extremities and abdominal wall (1).
• NSTI are associated with extensive tissue destruction, systemic toxicity, limb loss and are potentially fatal.
• NSTI represent a medical emergency. Early diagnosis, prompt surgical consultation, and initiation of broad-spectrum antibiotics are essential in improving outcomes (2).
• Associated terms: necrotizing fasciitis, progressive bacterial synergistic gangrene, synergistic necrotizing cellulitis, gas gangrene, nonclostridial anaerobic cellulitis, Fournier gangrene, Ludwig angina, “flesh-eating” infections

Epidemiology

• Predominant age: any age
• Predominant sex: male = female

Incidence

• 4 per 100,000
• Increased incidence in patients >50 years

Etiology and Pathophysiology

• Type I NSTI—most common
  • Polymicrobial, typically a combination of aerobic and anaerobic species (1)
  • More common in patients with specific risk factors (see below), although no specific inciting event is identified in 20–50% of patients (3)
• Type II NSTI
  • Monomicrobial infection
  • Tends to occur in otherwise healthy individuals (3)
  • Most commonly caused by group A β-hemolytic streptococci (Streptococcus pyogenes), followed by methicillin-resistant Staphylococcus aureus (MRSA) (2)
• Type III NSTI
  • Vibrio vulnificus—associated with exposure to marine environments, especially in patients with severe liver disease
  • Aeromonas hydrophila—associated with exposure to freshwater environments
  • Clostridium perfringens
• Bacteria enter the soft tissue after trauma causing skin or mucosal breach. Bacterial invasion may also occur without definite site of entry in a host with transient bacteremia after deep tissue trauma (2).
• Bacterial toxins and surface proteins cause a local inflammatory response. Vascular occlusion follows leading to deep tissue ischemia and necrosis (2).
• Bacterial toxins may trigger a systemic inflammatory response, leading to multiorgan failure or sepsis (2).

Risk Factors

• Can occur in young, healthy persons without risk factors
• Predisposing risk factors (3)
  • Advanced age
  • Obesity
  • Diabetes, cirrhosis, end-stage renal failure
  • Malnutrition
  • Immune suppression (HIV, malignancies, alcoholism, corticosteroid use)
  • History of cellulitis or impaired venous and lymphatic drainage (4)
  • Peripheral vascular disease
• Precipitating risk factors
  • IV drug abuse
  • Trauma, burns
  • Skin ulceration
  • Herpes zoster
  • Human, animal, or insect bites (4)
• Surgical risk factors
  • Prior operations
  • Hypoalbuminemia

General Prevention

• Management of predisposing risk factors (4)
• Diabetics should have regular foot examinations (4).
• Appropriate wound care after trauma involving irrigation, foreign body removal, and débridement of devitalized tissue (4)
• Sterile surgical procedures and skin closure (2)