Unilateral Laterothoracic Exanthem
Unilateral laterothoracic exanthem (ULE), also reported in the literature as asymmetric periflexural exanthem of childhood (APEC), is a distinctive, self-limiting rash that develops as a reaction to a viral infection elsewhere in the body (a paraviral reactive exanthem). First described by Brunner and colleagues in 1962, ULE stands out in clinical dermatology because of its unusual distribution: it erupts first on one side of the trunk, around the armpit or groin, and although it may later spread mildly to the opposite side, it always remains clearly asymmetric. Most patients are young children aged 1 to 5 years (more than 80% of cases), with peaks in spring and winter; adults are affected less often.
Although a specific cause is not isolated in every patient, epidemiologic, molecular and cellular evidence all point to ULE being a delayed hypersensitivity response of the skin's immune system to a systemic viral infection, most often Epstein-Barr virus, cytomegalovirus, parvovirus B19, human herpesvirus 6/7 or enteroviruses. The lesions do not arise because the virus directly invades and destroys epidermal cells. Instead, CD4+ T cells and Langerhans cells gather around the small vessels of the superficial dermis, producing papillary dermal edema and mild epidermal spongiosis.
At Haute-Derma, we consider accurate recognition of ULE an essential part of pediatric dermatology practice. By carefully analyzing its characteristic one-sided distribution and using modern tools such as dermoscopy, our dermatologists can reliably rule out serious infectious or allergic conditions. This lets us build the most appropriate care plan, avoid unnecessary oral corticosteroids or antibiotics, and give parents real peace of mind while their child's skin is monitored and recovers.
Pathogenesis and Immune Triggers
The Paraviral Hypothesis
- Delayed (type IV) hypersensitivity: One to three weeks after an acute upper respiratory or gastrointestinal infection, circulating viral antigens trigger a T cell–mediated immune response in the skin.
- Viruses implicated:
- Epstein-Barr virus (EBV) and cytomegalovirus (CMV).
- Human herpesvirus 6 (HHV-6) and human herpesvirus 7 (HHV-7).
- Parvovirus B19, adenovirus, enteroviruses and influenza A virus.
Why the Rash Favors One Side
Why the lesions start on, and remain concentrated on, one side of the trunk is still an intriguing biological question. There are two main hypotheses:
- Somatic mosaicism along vascular or nerve pathways: A population of skin or endothelial cells on one side of the trunk carries a mosaic genetic change that makes it more sensitive to viral antigens or increases its expression of adhesion molecules such as ICAM-1 and VCAM-1.
- Lymphatic tropism: The virus, or antigen–antibody complexes, may spread locally through the lymphatic drainage of one side of the body before entering the general circulation.
Clinical Features and Course
Onset and Typical Location:
- Early lesions: Slightly raised red macules and papules 1–3 mm across, or papules with a fine, powdery scale.
- Starting site: The rash always begins in a large skin fold on one side of the body, most often the armpit (more than 50% of cases), followed by the groin, buttock or one side of the lower back.
Progression and Asymmetry:
- Spread: Over the next 1–2 weeks, the papules spread outward across the chest, flank and back on the same side.
- Spread to the other side: In about 50% of cases, lesions extend to the opposite side of the trunk or down onto the thighs. Even so, the side where the rash began stays noticeably denser, redder and more severe than the side involved later.
- Lesion appearance: Red papules, small papulovesicles or mildly psoriasis-like plaques. Lesions are sometimes rimmed by a thin collar of fine scale.
- General symptoms: The child may have a low-grade fever and swollen, slightly tender lymph nodes in the armpit or groin on the same side. Itching ranges from mild to moderate.
Spontaneous Resolution:
- The rash persists for 3–6 weeks (occasionally up to 8 weeks).
- It then fades gradually, sheds a fine scale and clears completely, without scarring or lasting pigment change.
How We Diagnose ULE at Haute-Derma
Clinical Assessment of the One-Sided Pattern: We confirm that the rash began in a skin fold on one side (armpit or groin) and that there is a clear difference between the two sides of the trunk.
Detailed Dermoscopy:
- A pale pink background with evenly distributed dotted vessels.
- Scattered fine scale at the edges of lesions, with no grouped vesicles suggesting a herpesvirus infection and no burrows suggesting scabies.
Tests to Rule Out Other Conditions (When Needed):
- KOH preparation: To exclude tinea corporis (body ringworm) when lesions form scaly patches.
- Complete blood count and CRP: To look for a systemic infection if the child also has a fever.
Management and Care at Haute-Derma
Reassurance and Education for Parents
- We explain clearly that a rash lasting several weeks is simply how ULE behaves; it does not mean the condition is getting worse or that the wrong treatment was used.
- We confirm that, once the rash has appeared, the child is not directly contagious to other children.
Skin Care and Supportive Treatment
- Bathing and moisturizing:
- Bathe in lukewarm water with a soap-free cleanser to avoid irritating the skin.
- Apply a moisturizer containing ceramides, vitamin B5 (panthenol) or colloidal oatmeal 2–3 times a day to soothe the skin and strengthen the epidermal barrier.
- Controlling itch and local inflammation:
- For a very itchy child: a second-generation oral H1 antihistamine syrup, such as levocetirizine or desloratadine.
- Topical treatment: a mild topical corticosteroid such as hydrocortisone butyrate 0.1%, applied once daily for about 5–7 days to red, inflamed, very itchy papules. Avoid prolonged use over large areas.
