How can I differentiate impetigo from an acid chemical burn in a skin lesion?

Differentiating Impetigo from Acid Chemical Burns

Impetigo presents with honey-colored crusts on erythematous bases that evolve from vesicles/pustules over 4-6 days, while acid burns show immediate tissue destruction with well-demarcated necrosis, coagulation, and eschar formation at the contact site without the characteristic progression through vesicular stages.

Key Clinical Features That Distinguish These Conditions

Impetigo Characteristics

• Lesion evolution: Begins as erythematous papules that rapidly progress to vesicles, then pustules, and finally form thick honey-colored crusts over 4-6 days 1
• Distribution pattern: Typically occurs on exposed areas, most frequently the face and extremities, often at sites of minor trauma or insect bites 1
• Bullous variant: When caused by toxin-producing S. aureus, presents with fragile, thin-roofed vesicopustules that rupture easily, leaving denuded areas with collarette of scale 1
• Associated findings: Regional lymphadenitis may be present, but systemic symptoms (fever, malaise) are usually absent 1
• Contagious nature: Highly transmissible through direct contact, with multiple lesions often appearing in clusters 2

Acid Chemical Burn Characteristics

• Immediate onset: Tissue damage occurs instantly upon contact with the caustic agent, not over days 3
• Coagulation necrosis: Acid burns cause protein denaturation with well-demarcated borders and eschar formation 3
• Geographic pattern: Lesion shape corresponds precisely to the area of chemical contact, often with splash or drip patterns 3
• Depth of injury: Burns extend through dermal layers with visible tissue destruction, not superficial epidermal involvement 3
• Pain characteristics: Immediate severe burning pain at time of exposure, whereas impetigo is typically painless or mildly tender 3

Diagnostic Approach

History Is Critical

• Temporal progression: Impetigo develops gradually over several days with characteristic stage progression, while chemical burns are acute with immediate symptom onset 1, 3
• Exposure history: Direct questioning about chemical contact, occupational exposures, or assault is essential for burn diagnosis 3
• Preceding trauma: Impetigo often follows minor skin breaks, insect bites, or occurs secondary to eczema or herpes lesions 2

Physical Examination Findings

• Crust appearance: The honey-colored (golden) crust is pathognomonic for impetigo and will not be present in acute chemical burns 1
• Lesion borders: Impetigo has irregular, spreading borders with satellite lesions, while acid burns have sharp, well-demarcated edges corresponding to contact area 1, 3
• Surrounding tissue: Impetigo shows mild erythema around lesions; chemical burns show more extensive surrounding inflammation and edema 1, 3

When to Obtain Tissue Diagnosis

• Biopsy indications: If clinical presentation is atypical or diagnosis remains uncertain after initial assessment, biopsy of the lesion margin should be performed for both histological and microbiological evaluation 3
• Culture specimens: For suspected impetigo with treatment failure or atypical features, obtain cultures from vesicle fluid, pus, or erosions to confirm bacterial etiology and guide antibiotic selection 1
• Histopathology differences: Impetigo shows subcorneal pustules with neutrophils and gram-positive cocci, while chemical burns demonstrate coagulation necrosis through dermal layers 3

Common Pitfalls to Avoid

• Don't rely on surface swabs alone: For burn wounds, surface swabs reflect colonizing flora rather than true pathogens; tissue biopsy is superior for accurate diagnosis 3
• Consider secondary infection: Chemical burns can become secondarily infected with S. aureus or Streptococcus, potentially mimicking impetigo if the burn history is not elicited 3
• Bullous impetigo vs. burn blisters: Bullous impetigo has thin-roofed, flaccid bullae that rupture easily leaving minimal raw surface, while burn blisters are tense with underlying tissue destruction 1, 3
• Ecthyma confusion: Ecthyma (deeper impetigo variant) presents with circular erythematous ulcers with adherent crusts, but these evolve from vesicles over time rather than appearing acutely 1

Algorithmic Approach

1. Establish timeline: If lesions appeared immediately after known/suspected chemical exposure → consider acid burn; if evolved over 4-6 days through papule-vesicle-pustule-crust stages → impetigo 1, 3

2. Assess lesion morphology: Honey-colored crusts on erythematous base → impetigo; coagulated necrotic tissue with eschar → chemical burn 1, 3

3. Evaluate distribution: Multiple scattered lesions on exposed areas with satellite lesions → impetigo; single or few lesions with geometric pattern matching chemical contact → burn 1, 3

4. If diagnosis uncertain: Obtain tissue biopsy for histopathology and culture to definitively distinguish between infectious and chemical etiology 3