Bullous eruption on the posterior thigh
Diagnosis: Bullous impetigo, caused by methicillin resistant S aureus
Impetigo is a highly contagious superficial skin infection, with peak incidence among children aged 2 to 6 years.1,2 Nonbullous impetigo (70% of cases) is caused by Staphylococcus aureus or betahemolytic Streptococcus.3 Bullous impetigo is almost always caused by S aureus. Epidermolytic toxins produced by phage group II strains cause loss of cell adhesion in the stratum granulosum due to proteolytic attack of desmoglein 1, resulting in bullae.4
Bullous impetigo may occur after minor skin injury, such as an insect bite, abrasion, or dermatitis. Lesions generally start as small vesicles on the face, buttocks, extremities, or perineum, and may progress to a coalescence of thin-roofed bullae. The flaccid bullae rupture easily, draining serous or purulent fluid.
Lesions are usually painless, and systemic findings are rare. Lymphadenopathy is rare in bullous impetigo but common in nonbullous impetigo. The disease is generally self-limited and complications are uncommon. However, ecthyma (ulcerative impetigo) may result from an untreated impetigo infection.5
Differential diagnosis
The differential diagnosis for bullous impetigo is broad, and may include allergic contact dermatitis, herpes simplex, herpes zoster, pemphigus foliaceus, bullous pemphigoid, pemphigus vulgaris, and (in this case specifically) erythema migrans.
Allergic contact dermatitis is a delayed hypersensitivity reaction, usually caused by skin contact with an allergen. Lesions can be vesicular, edematous, erythematous, and pruritic. In this case, the patient did not have allergen exposure or a pruritic lesion.
Herpes zoster is a reactivation of the varicella zoster virus, characterized by stabbing, neuritic pain in a dermatomal distribution. Clear vesicles on an erythematous, edematous base distributed along a dermatome constitutes the classic appearance. This was not the case with this patient.
