To the Editor: Rashes in young children are common. The underlying diagnoses are often straightforward, but unusual cases or rashes that do not respond to conventional treatment should prompt scrutiny. We report a case of a four-month-old with a persistent scalp rash and a milk protein allergy who was admitted to the hospital for failure to thrive and suspected pneumonia.

The patient had 10 discrete, skin-colored papules on the scalp, several of which were erythematous, hemorrhagic, or had yellow crust (Figure 1). Macerated skin and several ulcerative lesions were noted in the neck folds and submental skin. The anterior chest had seven skin-colored, umbilicated papules. There were no lesions in the diaper area or on the extremities.

A biopsy confirmed a diagnosis of Langerhans cell histiocytosis, which is a rare, inflammatory, neoplastic disorder of myeloid dendritic cells that results in the infiltration of organs of the body. Bone is affected most frequently (80%), and the skull is the most commonly involved bone in all ages.¹ Other systemic manifestations include fever, lymphadenopathy, hepatomegaly, splenomegaly, and pulmonary lesions.¹ The oncogenic mutation BRAF V600E is found in approximately 64% of these cases.² The cells responsible for this condition are from the myeloid progenitor cells of the bone marrow, not from the Langerhans cells of the skin.

Cutaneous disease is the most common manifestation of Langerhans cell histiocytosis in patients younger than two years; however, Langerhans cell histiocytosis is rarely limited to the skin. Systemic involvement occurs in 87% to 93% of patients with cutaneous findings, and children younger than three years typically have acute disseminated multisystem disease.¹ Involvement of the liver, bone marrow, or spleen signals a worse prognosis, with five-year survival rates of less than 77%.¹ Children older than two years are more likely to have disease involving a single organ, with survival rates nearing 100%.¹

In children, especially infants, Langerhans cell histiocytosis classically presents with scaly, red-brown, seborrheic dermatitis-like papules or an eczematous, erythematous, scaly eruption on the scalp and flexural folds. Erosions in the flexural folds are common. Langerhans cell histiocytosis has many potential cutaneous morphologies, and petechiae, purpura, or hemorrhagic crusts should raise suspicion. Langerhans cell histiocytosis affects four to five children per 1 million.³ Despite the rarity, it should be considered in children presenting with atypical rashes or those who are recalcitrant to treatment.³ A high clinical suspicion for systemic disease is essential in children younger than three years to prevent a delay in diagnosis and appropriate management.