PEDIATRIC RASH

here is the discussion,

Unilateral laterothoracic exanthem (ULE) is a term coined initially by Bodemer and de Prost in 1992, when they reported observing 18 children with a predominantly laterothoracic exanthem.1 The characteristic features in their patients included initial unilaterality and localization of a scarlatiniform or eczematous eruption. Pruritis was reported by more than half of the patients, and the clinical course of the eruption consisted of either extension to a hemicorporeal distribution or evolution into a bilateral, generalized exanthem with unilateral predominance. All patients experience spontaneous resolution of the eruption within four weeks.
Multiple cases of similar eruptions have been reported in the literature, and in 1994 Gelmetti et al2 suggested that the term "asymmetric periflexural exanthem of childhood" better describes the entity, because the exanthem is not always unilateral (although it usually starts on one side of the body) and distribution at sites outside the laterothoracic region, such as the lower extremities, is frequently seen.
The disorder seems to predominate in females and appears at a mean age of approximately two years,3 with a reported age range between ten months4 and ten years.5 It often begins in a unilateral fashion, usually close to the axilla, and spreads to become bilateral, although it frequently retains a unilateral predominance. Palms, soles and mucous membranes are spared. The patients are usually otherwise well, with minimal pruritis and, occasionally, report a history of a preceding upper respiratory infection. The eruption usually lasts four to six weeks, followed by spontaneous resolution; it responds minimally to topical corticosteroids and disappears without sequelae or post-inflammatory hyperpigmentation. Consequently, reassuring parents of the exanthem as benign, self-limited nature and treatment with moisturizers are the mainstays of therapy.
The cause of ULE remains unclear. An infectious etiology has been repeatedly hypothesized, although serologic tests for hepatitis, borreliosis, Mycoplasma, EBV, CMV, toxoplasmosis, parvovirus, rickettsiae, HIV and coxsackie virus were negative in one large series. 1 While occasional patients have tested positive for respiratory pathogens such as adeno- and parainfluenza virus,3,5 causality remains questionable, and past reports suggesting group XIII Spiroplasma as a cause have not been confirmed. (Serologic testing for this agent has been negative in five out of five patients tested in one series.1
The differential diagnosis of ULE includes a non-specific viral exanthem, contact dermatitis, miliria, pityriasis rosea, Gianotti-Crosti syndrome, scabies, tina corporis and, occasionally, scarlet fever. Clinical features favoring unilateral laterothoracic exanthem are an acute erythematous macular and papular eruption initially localized in an asymmetric unilateral fashion over the upper trunk, axilla and proximal upper extremity, evolving frequently into a generalized distribution. The patients are usually otherwise well, have minimal pruritis and occasionally report a history of a preceding upper respiratory infection. The eruption usually spontaneously disappears within four to six weeks with no sequelae or mild post-inflammatory hyperpigmentation; it responds minimally to topical steroids. Reassurance of the benign, self-limited nature of the exanthem and treatment with moisturizers are the mainstays of therapy.