Soft Tissue Infections

STI Classification - Skin Deep Trouble

• Superficial Infections (Epidermis/Dermis):
  • Impetigo: Epidermal infection, honey-colored crusts.
  • Erysipelas: Dermal, superficial lymphatics; well-demarcated, raised, erythematous.
  • Cellulitis: Deeper dermis, subcutaneous tissue; ill-defined borders, spreading.
• Deeper Infections (Subcutaneous/Fascia/Muscle):
  • Cutaneous Abscess: Localized pus in dermis/subcutaneous tissue.
  • Necrotizing Fasciitis: Rapidly progressive, involves fascia; surgical emergency!
  • Myositis/Pyomyositis: Muscle inflammation/abscess.

⭐ Fournier's gangrene is a type of necrotizing fasciitis affecting the perineal, genital, or perianal regions, often polymicrobial.

Microbiology & Risks - The Nasty Crew

• Causative Agents:
  • Staphylococcus aureus (MSSA/MRSA): Most common.
  • Streptococcus pyogenes (GAS): Cellulitis, erysipelas, necrotizing fasciitis.
  • Gram-negatives (Pseudomonas, E. coli): Immunocompromised, burns, diabetic foot infections.
  • Anaerobes (Clostridium, Bacteroides): Gas gangrene, deep abscesses.
  • Polymicrobial infections are frequent.
• Patient Vulnerabilities:
  • Skin barrier disruption: Trauma, surgery, bites, IV catheters.
  • Impaired immunity: Diabetes Mellitus (DM), PVD, steroids, HIV.
  • Systemic factors: Obesity, malnutrition, lymphedema, extremes of age.

⭐ Clostridium perfringens is notorious for causing gas gangrene, characterized by crepitus and rapid tissue destruction.

Diagnosis & Scoring - Red Alert Signs

• Clinical: History, exam (local signs: erythema, warmth, tenderness, fluctuance).
• LRINEC Score (Lab Risk Indicator for Necrotizing Fasciitis):
  • Parameters: CRP >150, WBC >15, Hb <13.5, Na <135, Creatinine >1.6, Glucose >180.
  • Interpretation: <6 (low risk), 6-7 (intermediate), ≥8 (high risk NF).
• Red Alerts (Suspect NF):
  • Pain out of proportion
  • Tense edema, bullae, skin necrosis
  • Crepitus (gas)
  • Systemic toxicity (fever, ↑HR, ↓BP)
  • Wooden-hard induration
  • Rapid progression despite antibiotics
  • Serum lactate >2 mmol/L

⭐ Pain out of proportion to clinical findings is a key early indicator of necrotizing fasciitis.

Treatment Strategies - Slash & Dose

Primary goals: control infection, prevent spread, preserve function.

• Slash (Surgery):
  • I&D: For abscesses.
  • Debridement: Aggressive removal of ALL non-viable tissue; crucial for necrotizing infections. Re-look PRN.
  • Fasciotomy: If compartment syndrome.
• Dose (Antibiotics):
  • Empirical: Broad-spectrum IV (e.g., Vancomycin + Piperacillin-Tazobactam). Add Clindamycin for toxin suppression (📌).
  • Definitive: Tailor to C&S.
• Supportive: Fluids, nutrition, tetanus toxoid. HBO for clostridial myonecrosis/severe NF.

⭐ Early, aggressive surgical debridement is the single most important determinant of outcome in necrotizing soft tissue infections.

Key STIs - Deadly Details

• Necrotizing Fasciitis (NF):
  • "Flesh-eating"; rapid fascial spread.
  • Type I (polymicrobial), Type II (GAS).
  • Severe pain out of proportion, crepitus, bullae. LRINEC >6.
  • Tx: Aggressive surgical debridement, broad-spectrum Abx.
• Gas Gangrene (Clostridial Myonecrosis):
  • C. perfringens (alpha-toxin); muscle necrosis, gas.
  • Bronze skin, foul odor, crepitus.
  • Tx: Debridement, Penicillin + Clindamycin, HBO.
• Fournier's Gangrene:
  • NF of perineum/genitalia; polymicrobial.
  • Rapid onset, pain, swelling, crepitus, necrosis.
  • Tx: Urgent debridement, broad-spectrum Abx.

⭐ > Fournier's gangrene carries high mortality (20-40%) despite aggressive therapy, often linked to diabetes.

High‑Yield Points - ⚡ Biggest Takeaways

• Cellulitis: common S. pyogenes; erysipelas is superficial with sharply demarcated borders.
• Skin abscesses: typically S. aureus (MRSA), require incision and drainage (I&D).
• Necrotizing fasciitis: surgical emergency needing urgent, wide debridement. Fournier's gangrene affects the perineum.
• Gas gangrene (C. perfringens): presents with crepitus, muscle necrosis; requires surgery & antibiotics.
• Diabetic foot infections: often polymicrobial, need aggressive debridement, broad-spectrum antibiotics.
• Bite wounds (Pasteurella, Eikenella) often warrant prophylactic amoxicillin-clavulanate.