Unilateral laterothoracic exanthem (ULE), also known as asymmetric periflexural exanthem of childhood, is a self-limited, likely reactive process to a viral infection commonly seen in infancy and early childhood.

The clinical presentation includes a mildly pruritic eczematous or morbilliform rash that typically presents in a unilateral, localized distribution on the trunk and/or extremities. The eruption can spread to involve the contralateral side in 65%-70% cases over 2-4 weeks, but the rash usually remains more prominent on the initial affected side. The eruption can develop a more eczematous quality before resolving with fine, superficial desquamation. Associated symptoms include fever and pruritus. The mean time for resolution is 5 weeks, and recurrence is unusual.

ULE affects children who are 2-3 years of age (although it has been described in children 8 months to 10 years of age) with a 2:1 female-to-male predominance. It is usually seen in the winter and spring seasons. There may be a preceding history of fever or other symptoms of upper respiratory or gastrointestinal illness.

There are reports of family members concomitantly affected with a similar rash, which is suggestive of an infectious etiology. However, a study investigating a large panel of bacterial and viral agents in patients with ULE was unable to identify a single etiologic agent.

There is no association between ULE and immunocompromised states or malignancy. There is one reported case of ULE in a child with acute lymphoblastic leukemia. The exanthem followed the typical clinical course, although it lasted for 4 months, and did not require either delay or cessation of oncologic therapy.