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- Histiocytic disorders are derived from mononuclear phagocytic cells and dendritic cells. They are divided into three groups.
- Dendritic cell disorders (e.g., Langerhans cell histiocytosis [LCH], juvenile xanthogranuloma [JXG], Erdheim-Chester disease)
- Macrophage-related disorders (e.g., hemophagocytic lymphohistiocytosis [HLH], macrophage activation syndrome [MAS], Rosai-Dorfman disease)
- Malignant histiocytosis (lymphoma subtype)
- LCH (the focus of this chapter) results from the clonal proliferation of immature myeloid dendritic cells that share similar morphologic expression to skin LCH cells.
- Other names include the following (Langerhans cell histiocytosis is the preferred terminology):
- Histiocytosis X, Hand-Schüller-Christian syndrome, Letterer-Siwe disease, eosinophilic granuloma
- Hashimoto-Pritzker syndrome: infant dermatologic involvement, often self-limited
- 2 to 10 cases per million children
- LCH may occur at any age. Median age is 30 months.
- Single-system disease in 55% of patients; multisystem disease is more common in children 1 to 3 years of age.
- Male-to-female ratio approximately 1
- May be more common in whites of northern European descent than African Americans
- No evidence that relatives of LCH patients are at increased risk of disease.
- Very rare reports of recurrence within families
- Specific HLA alleles associated with disease phenotype in some case series
- Single-system LCH: limited to one organ system; most commonly bone, followed by skin
- Multisystem LCH involves two or more organs/systems with or without risk organs.
- Risk organs include hematopoietic system, liver, and/or spleen and portend a worse prognosis. Lungs are no longer considered a risk organ.
- CNS risk lesions: Lesions in the facial or anterior or middle cranial fossa bones are associated with 3 times increased risk of CNS involvement.
- CNS lesions can include mass lesions, pituitary stalk involvement, or neurodegenerative disease.
LCH etiology is incompletely understood. BRAFV600E mutations are found in ~60% of cases but are not prognostic. Activation of the RAS/MAPK/MEK/ERK pathway is common regardless of BRAF mutation status. Inflammatory infiltrates and abnormal cytokine production in lesions are common, although role in disease remains unclear.