Smoldering systemic mastocytosis

Smoldering systemic mastocytosis (SSM) is a subtype of systemic mastocytosis, a rare clonal mast cell disorder characterized by the abnormal accumulation and proliferation of mast cells in one or more organ systems, most commonly the bone marrow, skin, liver, spleen, and gastrointestinal tract. SSM is classified as an intermediate form between indolent systemic mastocytosis and advanced systemic mastocytosis, carrying a higher risk of progression to more aggressive disease. It is defined by the presence of two or more B-findings (indicators of high mast cell burden), such as bone marrow biopsy showing greater than 30% infiltration by mast cells, serum tryptase levels exceeding 200 ng/mL, hepatomegaly or splenomegaly without organ impairment, or dysplasia in myeloid lineages, but without evidence of organ damage (C-findings). Patients with smoldering systemic mastocytosis may experience symptoms related to mast cell mediator release, including flushing, pruritus (itching), abdominal pain, diarrhea, nausea, and episodes of anaphylaxis or near-anaphylaxis. Skin involvement, often presenting as urticaria pigmentosa (maculopapular cutaneous mastocytosis), is common. Bone pain and osteoporosis may also occur. Despite the high mast cell burden, patients do not have overt organ dysfunction at diagnosis, which distinguishes SSM from aggressive systemic mastocytosis. The majority of cases harbor the KIT D816V somatic mutation, which drives mast cell proliferation. Treatment is primarily focused on symptom management using antihistamines (H1 and H2 blockers), mast cell stabilizers, leukotriene antagonists, and epinephrine for anaphylactic episodes. Patients require close monitoring for disease progression to aggressive systemic mastocytosis or mast cell leukemia. Emerging targeted therapies, including KIT inhibitors such as avapritinib and midostaurin, have shown promise and may be considered in cases with significant symptom burden or evidence of progression. Regular assessment of serum tryptase levels, bone marrow evaluation, and organ function monitoring are essential components of long-term management.

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