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DEFINITION
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Mastocytosis is defined by accumulation of clonally expanded mast cells in tissues such as skin, bone marrow, liver, spleen, and gut. The mast cell expansion is generally recognized in skin and/or bone marrow. Mastocytosis occurs at any age and has a slight preponderance in males. Mastocytosis is a rare disorder and its exact prevalence is not known; however, it is estimated to occur in ~1 in 20,000 people. Familial occurrence is rare, and atopy is not increased compared to the general population.

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CLASSIFICATION AND PATHOPHYSIOLOGY
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A consensus classification for mastocytosis recognizes cutaneous mastocytosis with variants, five systemic forms, and rare mast cell sarcoma (Table 347-1).

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TABLE 347-1Classification of Mastocytosis
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Cutaneous mastocytosis is the most common diagnosis of mastocytosis in children and indicates disease limited to skin with absence of pathologic infiltrates in internal organs. It is usually diagnosed within the first year of life with demonstration of fixed, maculopapular, and hyperpigmented lesions (maculopapular cutaneous mastocytosis [MPCM], formerly known as urticaria pigmentosa), mastocytoma(s) or diffuse cutaneous mastocytosis. Systemic mastocytosis (SM) refers to involvement of a non-cutaneous site (usually bone marrow). There are five distinct variants of SM; the form designated as indolent systemic mastocytosis (ISM) accounts for the majority of adult patients. ISM is diagnosed when there is no evidence of an associated hematologic disorder, mast cell leukemia or tissue dysfunction due to mast cell infiltration and is not known to alter life expectancy. Systemic smoldering mastocytosis (formerly considered a subvariant of ISM) is characterized by high mast cell burden as evidenced by a bone marrow infiltration of >30% and a baseline serum tryptase >200 ng/ml (B-findings), but absence of SM-AHNMD or ASM (Table 347-2). In systemic mastocytosis associated with clonal hematologic non–mast cell lineage disease (SM-AHNMD, or SM-AHN for short), the prognosis is determined by the nature of the associated disorder, which can range from dysmyelopoiesis to leukemias usually of myeloid origin. In aggressive systemic mastocytosis (ASM), mast cell infiltration/proliferation in multiple organs such as liver, spleen, gut, bone, and bone marrow resulting in 1 or more C findings and a poor prognosis (Table 347-2). Mast cell leukemia (MCL) is the rarest form of SM and is invariably fatal at present; the peripheral blood contains circulating, metachromatically staining, and atypical mast cells. An aleukemic form of MCL is recognized without circulating ...

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