Systemic mastocytosis is a less common myeloproliferative variant composed of a heterogeneous disease compilation. In general there is no age or sex predilection.The adult systemic condition has been further stratified numerous ways based on clinical criteria as well as on associated mutations, namely activating c-kit mutations. This discussion is based on the most recent 2008 World Health Organization (WHO) classification. WHO classified four major subtypes of extracutaneous systemic mastocytosis: (1) indolent systemic mastocytosis, (2) systemic mastocytosis with associated clonal hematologic non-mast cell lineage disease (SM-AHNMD), (3) aggressive systemic mastocytosis, and (4) mast cell leukemia.
- Indolent systemic mastocytosis is most frequently seen. Mast cells primarily, yet modestly, infiltrate the bone marrow and may involve other organs, including the spleen, liver, and gastrointestinal tract. Because the disease is typically limited in children and often chronic and stable in adults, prognosis is favorable.
- Systemic mastocytosis with a chronic myeloproliferative neoplasia (SM-AHNMD) has a course and prognosis determined by efficacy of management of the underlying disease.
- Aggressive systemic mastocytosis, in which there is organ destruction from a mast cell infiltrate, is rare and should promote investigation for mast cell leukemia or other hematologic disorders such as myelodysplastic syndromes, myeloproliferative or myelodysplastic disorders, acute myeloid leukemia, and chronic myeloproliferative neoplasia.
- Mast cell leukemia is seen in two-thirds of patients with aggressive systemic mastocytosis and portends rapid progression that could potentially result in multi-organ failure. It is defined as greater than or equal to 20% mast cells in bone marrow smears and by circulating mast cells, often greater than or equal to 10% in peripheral smears. Pancytopenia can occur and may explain instances in which peripheral mast cells comprise less than 10% of the differential.