Skin and soft tissue infections

Overview

• Skin and soft tissue infections (SSTIs) range from superficial bacterial infections to life-threatening deep tissue necrosis.
• Key Step 2 CK objective: Differentiate superficial vs. deep infections and determine when urgent surgical intervention is required.

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Bacterial Skin Infections

Impetigo

• Etiology: S. aureus (MCC) or S. pyogenes (GAS).
• Clinical Features:
  • Non-bullous: “Honey-colored” crusts on erythematous base, painful/pruritic, face/extremities.
  • Bullous: Flaccid bullae w/ clear/yellow fluid, ruptures leaving thin brown crust (mediated by S. aureus exfoliative toxin).
• Tx:
  • Localized: Topical mupirocin.
  • Widespread: PO Abx (cephalexin, dicloxacillin).
• Complication: Poststreptococcal glomerulonephritis (PSGN). Note: Tx of impetigo prevents PSGN transmission but does not prevent PSGN development in the affected pt (unlike rheumatic fever in strep pharyngitis).

Erysipelas vs. Cellulitis

• Erysipelas:
  • Pathology: Upper dermis and superficial lymphatics. MCC is Streptococcus pyogenes. c
  • Risk factors:
    • Lymphatic obstruction (e.g., post-mastectomy, chronic lymphedema, venous insufficiency).
    • Local skin barrier breakdown (tinea pedis, venous stasis ulcers, insect bites, trauma).
    • Prior history of erysipelas.
    • Obesity.
  • PE: Well-demarcated, raised, bright red border, rapid onset, systemic symptoms (fever, chills) early. Often on face or lower extremity.
  • Tx: IV/PO penicillin, cephazolin, or cephalexin.
• Cellulitis:
  • Pathology: Deep dermis and subcutaneous tissue. MCC is GAS or S. aureus (including MRSA).
  • PE: Ill-defined, flat, erythematous borders, warm, tender, fluctuance suggests abscess.
  • Tx:
    • Non-purulent (GAS): Cephalexin.
    • Purulent/Suspected MRSA: Clindamycin, TMP-SMX, doxycycline.
    • Severe/Systemic: IV vancomycin, linezolid, or daptomycin.

Abscess, Furuncles, & Carbuncles

• Pathology: Wall of infection deep in dermis/hair follicles. MCC is S. aureus (often MRSA).
• PE: fluctuant, painful nodule. Carbuncle = coalescence of several furuncles.
• Tx:
  • Small (< 5 cm) without systemic symptoms: Incision & Drainage (I&D) alone.
  • Large (> 5 cm), systemic symptoms, or immunosuppressed: I&D + PO Abx (TMP-SMX or doxycycline).

Necrotizing Fasciitis

• Pathology: Deep subcutaneous tissue and fascia necrosis. Polymicrobial (Type I) or Monomicrobial (Type II, GAS).
• PE: Pain out of proportion to exam, rapid progression, crepitus, purple/blue skin discoloration, bullae, anesthesia of affected skin.
• Dx: Clinical presentation is paramount. X-ray/CT showing gas in soft tissue is highly specific but non-sensitive. Do not delay Tx for imaging.
• Tx: Urgent surgical debridement (emergent consult) + empiric broad-spectrum IV Abx (e.g., vancomycin + piperacillin-tazobactam or meropenem + clindamycin to inhibit toxin production).