A systemic disorder defined by a clonal expansion of mast cells that in most instances is indolent and nonmalignant. Generally involves bone marrow, skin, gastrointestinal (GI) mucosa, liver, and spleen. Classified as cutaneous mastocytosis with variants, five systemic forms (indolent, smoldering, associated with concomitant hematologic non-mast cell lineage disease, aggressive, mast cell leukemia), and rare mast cell sarcoma.
Pathophysiology and Clinical Manifestations 
The clinical manifestations of systemic mastocytosis are due to tissue occupancy by the mast cell mass, the tissue response to that mass (fibrosis), and the release of bioactive substances acting both locally (urticaria pigmentosa, crampy abdominal pain, gastritis, peptic ulcer) and at distal sites (headache, pruritus, flushing, vascular collapse). Clinical manifestations may be aggravated by alcohol, temperature changes, stress, narcotics, and NSAIDs.
Although the diagnosis of mastocytosis may be suspected on the basis of clinical and laboratory findings, it can be established only by tissue biopsy (usually bone marrow biopsy). The diagnostic criteria for systemic mastocytosis are shown in Table 159-2 Diagnostic Criteria for Systemic Mastocytosisa . Serum tryptase level is elevated in >50%. Other studies are directed by organ-specific presentations. Other flushing disorders (e.g., carcinoid syndrome, pheochromocytoma) should be excluded.
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Systemic Mastocystosis
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