Consider a diagnosis of Langerhans cell histiocytosis (LCH) as a possibility in cases of severe seborrheic dermatitis, especially if atrophy, ulceration, or purpura is present.

Seborrheic dermatitis, commonly known as cradle cap, is frequently seen in infants and children. Typical lesions are scaly, oily, and located on the scalp and sometimes behind the ears. A papulosquamous disorder patterned on the sebum-rich areas of the scalp, face, and trunk, it is commonly aggravated by changes in humidity or seasons, scratching, or emotional stress. It can present in combination with atopic dermatitis. Seborrheic dermatitis is uncommon in children after infancy and before puberty. In this older age group, scalp scaling is likely to be due to other causes such as tinea capitis, atopic dermatitis, or psoriasis. For severe lesions, refractory to the usual therapies, an expanded differential diagnosis must be entertained. This includes disorders with potential for organ involvement, such as LCH.

LCH is a group of idiopathic disorders characterized by the proliferation of specialized bone marrow-derived Langerhans cells (LCs) and mature eosinophils. The Birbeck granule is the histopathological hallmark. The term LCH is generally preferred to the older term, histiocytosis X. The Histio- cyte Society has divided histocytic disorders into three different groups: (a) dendritic cell histiocytosis, (b) erythrophagocytic macrophage disorders, (c) malignant histiocytosis. LCH belongs in group 1. The clinical spectrum ranges from an acute fulminant, disseminated disease (Letterer Siwe) to a few indolent, chronic lesions of bone or other organs called eosinophilic granulomas. Hand-Schüller-Christian disease is an intermediate form that classically presents as the triad of diabetes insipidus, proptosis, and lytic bone
lesions. Disseminated disease appears more commonly in children younger than 2 years of age, whereas the chronic and acute disseminated forms are most common in children ages 2 to 15 years.