Langerhans cell histiocytosis (LCH), sometimes referred to as histiocytosis X, is a disease of abnormal clonal proliferation of a unique type of cell in the monocyte-macrophage cell line known as the Langerhans cell. It is named after the medical student Paul Langerhans, the first scientist to describe the cell (1868). Patients with the disease were described as early as the late 1800's. The original cases ranged from solitary bone involvement to multiple lesions of bone, liver, spleen, lung, and renal pelves. These original cases, and their considerable variation in presentation and degree of involvement, reflect the wide variability of the disease process referred to today as LCH. LCH may infiltrate many tissues, including bone, bone marrow, lung, liver, spleen, brain, and kidney. Patients with solitary or limited foci of disease tend to do quite well, whereas those patients with multi-organ involvement, especially of the marrow, liver, and spleen, tend to fare much worse, and may succumb to their disease.

Traditionally, LCH has been divided into 3 forms, based on the organs involved, age of onset, and clinical course. The localized form is the mildest and most common form, representing 70-75% of LCH; it consists of bone and/or lung involvement. This limited form may be referred to as eosinophilic granuloma. The term esosinophilic granuloma (EG) derives from the considerable number of eosinophils typically seen in the bone lesions, although Langerhans cells are nonetheless the dominant cellular constituent. EG is usually monostotic when occurring in bone, presents between 5-15 years of age, and is associated with minimal morbidity. While children with localized LCH more often have bone lesion disease, adults have a greater propensity to lung involvement, with or without bone disease.

There are 2 forms of disseminated LCH: the chronic form and the fulminant form. The chronic form, also known as Hand-Schuller-Christian Disease (HSCD), represents about 20% of LCH cases, and typically involves bone, liver, spleen, lymph nodes, and skin. Patients usually present before the age of 5 years. The classic triad of diabetes insipidus (from basilar skull disease or direct infiltration of the posterior pituitary), exophthalmos (from mass effect of orbital bone disease), and destructive bone lesions (often of the calvarium) is seen in 10-15% of cases. HSCD is fatal in approximately 15% of patients.

Finally, in the fulminant form (about 10% of LCH, sometimes referred to as Letterer-Siwe disease), there is disseminated involvement of many organs, especially liver, spleen, bone marrow, lymph nodes, and skin. Bone involvement is less common in this form of LCH. It usually presents in children under the age of 2 years. The course is rapidly progressive and usually fatal.

Pathologically, LCH is characterized by tissue infiltration with Langerhans cells, with a mixture of mononuclear and inflammatory cells, especially eosinophils. The appearance of these abnormal aggregates varies with the tissue in which they occur, as well as with the activity of the infiltrate. As the infiltrates become less active, they leave behind fibrous connective tissue and other forms of scar.

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