What are the typical appearance, diagnosis, and treatment of impetigo on the arm?

Impetigo on the Arm: Clinical Appearance, Diagnosis, and Treatment

Clinical Appearance

Impetigo on the arm typically presents as erythematous papules that rapidly evolve into vesicles and pustules, eventually forming characteristic thick honey-colored crusts. 1

Nonbullous Impetigo (70% of cases)

• Begins as small red papules that quickly progress to vesicles containing cloudy fluid 1
• Vesicles rupture within 4-6 days, leaving the pathognomonic honey-colored or golden-yellow crusts 1
• Lesions commonly appear on exposed areas including the extremities 1
• Regional lymphadenitis may be present, though systemic symptoms are typically absent 1

Bullous Impetigo (30% of cases)

• Characterized by fragile, thin-roofed vesicopustules that rapidly enlarge into flaccid bullae (>5mm diameter) 1
• Caused exclusively by toxin-producing Staphylococcus aureus strains 1
• More likely to affect intertriginous areas but can occur on arms 2

Diagnosis

The diagnosis is primarily clinical based on the characteristic appearance of honey-colored crusts or bullae. 1

When to Obtain Cultures

• Treatment failure after 48-72 hours of appropriate therapy 1, 3
• Suspected methicillin-resistant S. aureus (MRSA) infection 1, 3
• Recurrent infections 1, 3
• Culture vesicle fluid, pus, erosions, or ulcers to establish the causative organism 1

Differential Diagnosis to Consider

• Folliculitis (more superficial, inflammation confined to epidermis) 1
• Impetiginized eczema (secondary infection on underlying atopic dermatitis) 1
• Ecthyma (deeper infection with circular erythematous ulcers) 1

Treatment Algorithm

For Localized Impetigo (Few Lesions)

Topical mupirocin 2% ointment applied three times daily for 5-7 days is the first-line treatment. 1, 3

• Topical antibiotics are superior to placebo (RR 2.24,95% CI 1.61-3.13) and equal or superior to oral antibiotics for localized disease 1
• Alternative: Retapamulin 1% ointment applied twice daily for 5 days 3, 4
• Mupirocin and fusidic acid show equivalent efficacy 1, 5

For Extensive Impetigo (Multiple Sites or Numerous Lesions)

Oral antibiotics should be used when impetigo involves multiple sites, is extensive, topical therapy is impractical, or topical treatment has failed. 1, 3

For Methicillin-Susceptible S. aureus (MSSA):

• Dicloxacillin 250 mg four times daily for adults (7-10 days) 3, 4
• Alternative: Cephalexin 250-500 mg four times daily for adults (7-10 days) 3, 4
• Pediatric dosing: Dicloxacillin 12 mg/kg/day divided into 4 doses 3

For Suspected or Confirmed MRSA:

• Clindamycin 300-450 mg three times daily for adults 3, 4
• Alternative: Trimethoprim-sulfamethoxazole (TMP-SMX) 1, 4
• Alternative: Doxycycline (avoid in children <8 years and pregnant women) 3, 4

Common Pitfalls to Avoid

• Never use penicillin alone - it lacks adequate coverage against S. aureus and is inferior to other antibiotics 4, 5
• Topical disinfectants are inferior to antibiotics and should not be used 1, 5
• Tetracyclines (doxycycline, minocycline) must not be used in children under 8 years of age 1, 3

Adjunctive Measures

• Keep lesions covered with clean, dry bandages to prevent spread 1, 3
• Maintain good personal hygiene with regular handwashing 3
• Debride crusts gently to improve topical antibiotic penetration 6

Monitoring and Follow-Up

• Re-evaluate if no improvement after 48-72 hours of therapy 1, 3
• If treatment fails, obtain cultures and adjust therapy based on susceptibility results 1, 3
• Most cases resolve within 2-3 weeks without scarring 2