A previously healthy 20-month-old girl presented with a 1-week history of erythematous pruritic papules that had originated over the right axilla and upper chest and had extended to involve the right arm, leg, back, and the contralateral body.
On examination, there were well-demarcated, thin erythematous plaques in the inguinal and axillary regions. The skin surface was smooth, atrophic, non-scaly, and slightly shiny. He also had pitting of the fingernails. The rest of the physical examination was unremarkable.
Also known as superimposed lateralized exanthem or asymmetric periflexural exanthem of childhood, unilateral laterothoracic exanthem is a distinctive skin eruption characterized by its unilateral and abrupt onset and asymmetric location, and that it begins close to the axilla/groin and later progresses to the lateral trunk and extremities and, in the later stages, to the contralateral side.1,2
The condition was first described by Brunner and colleagues, who reported on 75 young children in Chicago with “a new papular erythema of childhood” that had erupted unilaterally near the axilla and subsequently had extended to the trunk and arm.3 Some authors prefer the term asymmetric periflexural exanthem of childhood, because the exanthem is not always unilateral and is not always over the laterothoracic area.4 However, the condition also occurs in adults.1,5 Thus, the term superimposed lateralized exanthem is more preferable, because the exanthem does not always remain unilateral, is not restricted to the laterothoracic or periflexural area, and is not exclusive to children.1,6,7 In spite of this, the term unilateral laterothoracic exanthema, coined in 1992 by Bodemer and de Prost,8 remains more commonly used.
The exact incidence is not known, since the literature on this condition is very limited. We suspect that the condition is more common than is generally appreciated. The disorder is more prevalent in the white population than in dark-skinned individuals.5,9,10 Unilateral laterothoracic exanthem occurs most commonly between 6 months and 4 years of age, with a peak age of onset between 2 and 3 years.9,11,12 Occasionally, the condition has been reported in adults.1,5,6,13-15 The female to male ratio is approximately 2 to 1.2,10 The condition occurs year-round, with a peak in the spring and winter months.9,10,12,16
The exact etiology of unilateral laterothoracic exanthem is not known. Because of the seasonal occurrence in winter and spring, close temporal relation to upper respiratory tract infections, occurrences of small epidemics, intrafamilial cases, young age of patients (typically 1 to 5 years), clinical appearance and course similar to viral exanthems, regional lymphadenopathy, nonspecific histology, lack of response to topical corticosteroids, lack of response to systemic antibiotics, and spontaneous resolution, a viral etiology (Epstein-Barr virus, parvovirus B19, adenovirus, parainfluenza virus 2 and 3, human herpesvirus 6 and 7) is suspected.4,6,7,12,13,16-19
The condition is also more common in individuals with immunodeficiencies.16,20 So far, a causal relationship between a viral or bacterial agent and the condition has not been substantiated.21
Some authors suggest that a postzygotic mutation at an early stage of embryogenesis may have changed the cutaneous epitopes, rendering the keratinocytes of one side of the body more responsive to infective agents.7 Because the keratinocytes of the other side of the body do not carry the postzygotic mutation, they are less reactive to the infective agents.7 This may explain that although the eruption may become bilateral, unilateral predominance is generally maintained.2
Histopathologic findings are nonspecific and include a perivascular and periappendageal lymphocytic infiltrate in the dermis and mild mononuclear cell exocytosis and spongiosis in the epidermis.7,9,16
Unilateral laterothoracic exanthem usually presents as an acute eruption in an asymptomatic healthy child. A prodrome consisting of low-grade fever, runny nose, cough, and diarrhea is common.10,15,19 Typically, the eruption starts unilaterally in or around the axilla and spreads down the side of the trunk and medial surface of the corresponding proximal arm, or it may start on one side of the trunk and extend toward the axilla.2,3 Less often, the eruption starts in the groin and spreads centrifugally.2,12,22 Involvement of the face, palms, soles, and mucous membranes is rare.18
The lesions consist of discrete erythematous macules and papules, 1 to 4 mm in diameter, with some tendency to coalesce.2,18,23 Pale halos may be seen around some of the lesions.2,12 At times, the eruption can be scarlatiniform, morbilliform, and eczematous.11,15 There is no left or right dominance.18 Progression to the contralateral side of the body is common, but a pronounced asymmetry is maintained; thus, the asymmetric nature of the rash is conserved.7,18
Regional lymphadenopathy confined to the areas of initial eruption is found in approximately 50% of patients.4,9,17 Pruritus is usually mild and present in approximately 50% of patients.4,10,17 Excoriations, oozing, crusting, and lichenification are usually not noted.10 Systemic symptoms are absent. The exanthem usually resolves in 4 to 6 weeks with fine desquamation and without scars nor a tendency to relapse.1,2,7,9-15
The diagnosis of unilateral laterothoracic exanthem is mainly clinical. A spot diagnosis can usually be made by a clinician who is familiar with the condition. Laboratory investigations and skin biopsy are generally not warranted.
Differential diagnosis includes a nonspecific viral exanthem, contact dermatitis, drug eruption, atopic dermatitis, scarlet fever, miliaria, tinea corporis, lichen striatus, erythema multiforme, guttate psoriasis, scabies, Gianotti-Crosti syndrome, and atypical pityriasis rosea.1,10,15
Occasionally, residual dryness, fine desquamation, and minimal postinflammatory hyperpigmentation may occur but are usually transient.9,12 The general health of the patient is not affected. No systemic illness is associated with this condition.
Given the benign nature of the condition with spontaneous resolution, treatment is usually unnecessary apart from reassurance. Pruritus, if present, can be treated with bland emollients, calamine lotion, and/or oral antihistamines.12 The use of topical or systemic corticosteroids is unwarranted.9,12
Alexander K. C. Leung, MD, is a clinical professor of pediatrics at the University of Calgary and a pediatric consultant at the Alberta Children’s Hospital in Calgary, Alberta, Canada.
Benjamin Barankin, MD, is a dermatologist and the medical director and founder of the Toronto Dermatology Centre in Toronto, Ontario, Canada.
Joseph M. Lam, MD, is a clinical associate professor at the University of British Columbia and an associate member at the Department of Dermatology and Skin Sciences at the University of British Columbia, Vancouver, British Columbia, Canada.