Diagnosis: Asymmetric periflexural exanthem
Asymmetric periflexural exanthem of childhood, also known as unilateral laterothoracic exanthem, is a skin eruption that typically originates unilaterally on the trunk, often near the axillary region.1 Lesions then extend centrifugally to become bilateral in nearly all patients, but the lesions maintain a predominance on the side where they initially appeared. The lesions consist of discrete papules that are typically 2 to 4 mm in diameter and are usually erythematous. These eruptions can have various morphologies, including morbilliform, scarlatiniform, or eczematous papules that may become confluent.2 Diffuse asymptomatic lymphadenopathy may develop, and pruritus, which is usually mild, occurs in about half of patients.1
An infectious etiology has been assumed for this condition, but no viral or bacterial agent has ever been conclusively linked with the exanthem.2 In a prospective case-control study that investigated possible etiologic agents, investigators found no significant differences in infection rates (bacterial or viral) between patients with and without asymmetric periflexural exanthem.3
The exanthem also appears to have seasonality, with the predominance of cases occurring in late winter and early spring.2 Moreover, the condition is commonly associated with mild fever, upper respiratory symptoms, and/or gastrointestinal symptoms, which suggests a viral trigger.
The most common initial misdiagnosis of this condition is contact dermatitis. The differential diagnosis also includes pityriasis rosea, scarlet fever, miliaria, erythema infectiosum, tinea corporis, and Gianotti-Crosti syndrome.4 Pityriasis rosea may persist for 2 to 3 months, and the symptoms of this condition overlap with those of asymmetric periflexural exanthem. In pityriasis rosea, however, the eruption is symmetric with a predilection for involvement of the lower torso; this eruption is often preceded by a solitary herald patch. Gianotti-Crosti syndrome tends to involve the extensor surfaces of the arms and legs as well as the face and buttocks, and erythema infectiosum is characterized by a lacy reticulated erythema on the extremities and slapped-cheek erythema that waxes and wanes for several months during epidemics (mainly in the winter and spring). The sandpaper rash of scarlet fever followed 2 weeks later by diffuse desquamation is distinctive, and tinea corporis is usually very localized and intensely pruritic. Miliaria rubra is also associated with extremely pruritic erythematous papules; this condition usually resolves within a few days of removal from the offending hot, humid environment.
Within 2 to 3 weeks, the eruption displays its greatest intensity, followed by a late desquamative phase. Treatment consists of supportive care. Antihistamines may be beneficial when significant pruritus is present, and moisturizers can be helpful during desquamation. The response to topical corticosteroids is variable.4 Lesions are self-limited and typically resolve spontaneously within 3 to 6 weeks.2 Occasionally, the eruption may persist for several months.
The eruption in our patient persisted for another 4 weeks and then resolved without treatment. Minimal itching was managed with topical moisturizers and oral antihistamines at bedtime as needed.