Asymmetric periflexural exanthem of childhood

Morbilliform, eczematous or scarlatiniform - fine pink papules on a faintly erythematous background, sometimes with a sandpaper feel. The skin beside the axilla looks dusky pink and slightly scaly, with a definable edge towards the midline.

The morbilliform and scarlatiniform erythema that defines the eruption in pale skin may be nearly invisible, and the fine background redness is the first thing lost. The asymmetry is the sign to rely on, and it is tone-independent - compare the two axillae and the two flanks directly rather than trying to judge whether an area is red. Side-lighting helps, because the papules are palpable and raise a visible texture when light crosses them obliquely. Post-inflammatory hyperpigmentation may outlast the eruption by months. No skin-of-colour series specific to this exanthem was found, and the two defining series describe European and Canadian cohorts without stating skin type.

• Onset beside one axilla, and a persistent unilateral predominance even after it spreads
• A well child - this is not an illness, it is a rash
• Itch is common but variable; it is not a defining feature
• Duration measured in weeks, not days: the original series resolved within four weeks, a later series averaged five
• Often follows a reported infection, but no single organism has ever been identified

• Atopic eczema and infected eczema — The eruption is genuinely eczematous and often treated as eczema; eczema is symmetrical and flexural on both sides, and has a history behind it
• Contact dermatitis - allergic and irritant — Also asymmetrical and eczematous, but shaped by the contact - a sharp geometric edge, not a centrifugal spread from one axilla
• Scarlet fever — Also scarlatiniform and sandpapery, but symmetrical, febrile, with a sore throat and a strawberry tongue
• Tinea corporis, cruris and pedis — Also asymmetrical and scaly with a defined edge, but annular with central clearing and far more slowly progressive
• Drug eruptions - morbilliform and fixed — Morbilliform and often blamed on a recent antibiotic; a drug eruption is symmetrical and does not begin at one axilla

1. Undress the child fully and compare the two sides. Asymmetry that persists is the whole diagnosis, and it cannot be seen through a vest
2. Name it, and say plainly that it is not an allergy and not a reaction to the antibiotic the child was given - this family has usually already been told it was one or the other
3. Do not investigate. No infectious agent has been identified in any series and serology answers nothing
4. Offer a topical corticosteroid only for itch, and say honestly that it may or may not shorten anything - the prospective series found exactly that

Discharge with the diagnosis and the expected duration. No follow-up is needed for the rash itself.

• Bodemer C, de Prost Y. Unilateral laterothoracic exanthem in children: a new disease? J Am Acad Dermatol 1992;27(5 Pt 1):693-6 - the original 18-patient series
• McCuaig CC, Russo P, Powell J, Pedneault L, Lebel P, Marcoux D. Unilateral laterothoracic exanthem. A clinicopathologic study of forty-eight patients. J Am Acad Dermatol 1996;34(6):979-84
• Chuh A, Zawar V, Sciallis GF, Kempf W, Lee A. Pityriasis rosea, Gianotti-Crosti syndrome, asymmetric periflexural exanthem, papular-purpuric gloves and socks syndrome, eruptive pseudoangiomatosis, and eruptive hypomelanosis: do their epidemiological data substantiate infectious etiologies? Infect Dis Rep 2016;8(1):6418
• NICE NG240 - rashes may be harder to detect on brown and black skin