MEDizzy - A 5-year-old girl is brought to your office by her mother,

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Hematology

A 5-year-old girl is brought to your office by her mother, who states that the girl has been drinking a lot of water lately. Physical examination reveals a young girl whose eyes protrude slightly. Further workup reveals the presence of multiple lytic bone lesions involving her calvarium and the base of her skull. Which of the following is the most likely diagnosis?

ExplanationLangerhans cell histiocytosis, previously known as histiocytosis X, refers to a spectrum of clinical diseases that are associated with the proliferation of Langerhans cells. These cells, not to be confused with the Langerhans-type giant cells found in caseating granulomas of tuberculosis, have Fc receptors and HLA-D/DR antigens and react with CD1 antibodies. These cells contain distinctive granules, seen by electron microscopy, that are rod-shaped organelles resembling tennis rackets. They are called LC (Langerhans cell) granules, pentilaminar bodies, or Birbeck granules. There are three general clinical forms of Langerhans histiocytosis. Acute disseminated LC histiocytosis (Letterer-Siwe disease) affects children before the age of 3 years. These children have cutaneous lesions that resemble seborrhea, hepatosplenomegaly, and lymphadenopathy. The LCs infiltrate the marrow, which leads to anemia, thrombocytopenia, and recurrent infections. The clinical course is usually rapidly fatal; however, with intensive chemotherapy 50% of patients may survive 5 years. Multifocal LC histiocytosis (Hand-Sch¸ller-Christian disease) usually begins between the second and sixth years of life. The characteristic triad consists of bone lesions, particularly in the calvarium and the base of the skull; diabetes insipidus; and exophthalmos. These lesions are the result of proliferations of LCs. Lesions around the hypothalamus lead to decreased ADH production and signs of diabetes insipidus. Unifocal LC histiocytosis (eosinophilic granuloma), seen in older patients, is usually a unifocal disease, most often affecting the skeletal system. The lesions are granulomas that contain a mixture of lipid-laden Langerhans cells, macrophages, lymphocytes, and eosinophils.