Impetigo

Impetigo Overview - Skin's Sticky Situation

• Definition: Highly contagious, superficial bacterial skin infection, primarily affecting the epidermis.
• Etiology:
  • Staphylococcus aureus (most common, ~80%), often MRSA.
  • Streptococcus pyogenes (Group A Strep, GAS).
  • Mixed infections are also common.
• Epidemiology:
  • Predominantly affects children (peak 2-5 yrs).
  • More prevalent in summer/fall; favors warm, humid climates.
  • Risk factors: Poor hygiene, pre-existing skin trauma (e.g., insect bites, eczema, abrasions).
  • Transmission: Highly contagious via direct contact.
• Pathophysiology: Bacteria invade compromised skin → proliferate within epidermis → produce toxins (e.g., exfoliative toxins by S. aureus causing bullae in bullous impetigo).

⭐ Impetigo is the most common bacterial skin infection in children worldwide.

Clinical Presentation - Crusts, Bullae, Ulcers

• Ecthyma:
  • Deeper, ulcerative form extending into the dermis.
  • Presents as "punched-out" ulcers covered by thick, adherent crusts.
  • Heals with scarring.
  • Etiology: Often S. pyogenes; S. aureus can be involved.
  • Common on lower extremities, sites of neglect/poor hygiene.

Diagnosis & DDx - Spotting the Signs

Diagnosis: Primarily clinical, based on characteristic lesions.

Diagnostic Pathway:

Investigations (if atypical, widespread, recurrent, MRSA suspected, or Rx failure):

• Gram stain: Gram-positive cocci (clusters/chains). From moist lesion base or bulla fluid.
• Culture & Sensitivity: Identifies organism & guides Rx; crucial for MRSA.

Differential Diagnosis (DDx):

• Non-bullous Impetigo:
  • Herpes simplex (grouped vesicles, often painful)
  • Tinea corporis (annular, central clearing, active scaly border)
  • Atopic dermatitis (eczematous plaques, intense pruritus)
  • Scabies (burrows, intense nocturnal pruritus)
• Bullous Impetigo:
  • Bullous insect bites
  • Bullous pemphigoid (tense bullae, elderly)
  • Epidermolysis bullosa (inherited, friction-induced)
  • Burns
• Ecthyma:
  • Cutaneous leishmaniasis
  • Vasculitic ulcers
  • Pyoderma gangrenosum

⭐ While diagnosis is usually clinical, bacterial culture is crucial for identifying MRSA and guiding antibiotic therapy in complicated cases.

Management & Complications - Healing & Hazards

• General Measures: Gentle cleansing, crust removal. Good personal hygiene, hand washing. Avoid scratching. School/daycare exclusion until lesions dry or 24h post-antibiotics.
• Topical Therapy (for limited, localized non-bullous or few bullous lesions):
  • Mupirocin 2% ointment/cream (TID for 5-7 days).
  • Retapamulin 1% ointment (BID for 5 days).
  • Fusidic acid 2% cream (TID for 5-7 days).
• Systemic Antibiotics (for extensive disease, >5 lesions, ecthyma, bullous impetigo, oral cavity involvement, systemic symptoms, or outbreaks; typically 7 days):
  • Standard: Dicloxacillin, Cephalexin.
  • MRSA suspected/confirmed: Clindamycin, Doxycycline (contraindicated <8 yrs), TMP-SMX. 📌 Mnemonic: "Try Drugs for Clean MRSA" (TMP-SMX, Doxycycline/Minocycline, Clindamycin, Linezolid).

• Complications:
  • Post-Streptococcal Glomerulonephritis (PSGN): After S. pyogenes impetigo; latent period 1-3 weeks. Antibiotics may not prevent.
  • Cellulitis, lymphangitis.
  • Staphylococcal Scalded Skin Syndrome (SSSS) (with toxin-producing S. aureus).
  • Scarlet fever (with GAS).
  • Rheumatic fever: NOT a complication of cutaneous streptococcal infections.

⭐ Unlike streptococcal pharyngitis, antibiotic treatment of streptococcal impetigo does not reliably prevent the development of post-streptococcal glomerulonephritis (PSGN).

High-Yield Points - ⚡ Biggest Takeaways

• Impetigo is a highly contagious, superficial bacterial skin infection, common in children.
• Predominantly caused by Staphylococcus aureus; also Streptococcus pyogenes.
• Non-bullous impetigo (most common) shows honey-colored, stuck-on crusts.
• Bullous impetigo is caused by S. aureus exfoliative toxin A, forming flaccid bullae.
• Typically affects the face (perioral, perinasal) and extremities.
• Post-streptococcal glomerulonephritis (PSGN) can follow streptococcal impetigo; rheumatic fever does not.
• Management includes topical mupirocin for localized cases or systemic antibiotics for extensive disease.