Systemic Mastocytosis Diagnosis and Classification

Introduction to Systemic Mastocytosis Subtypes

• The National Comprehensive Cancer Network recommends that Indolent Systemic Mastocytosis (ISM) is characterized by low mast cell burden, absence of organ damage, younger age at presentation, and excellent prognosis with median survival of 301 months 1
• The National Comprehensive Cancer Network suggests that ISM has a low mast cell burden with no evidence of C-findings or associated hematologic neoplasm 1, 2
• Skin involvement is common in ISM, with 85% of patients having skin lesions and 78% experiencing cutaneous symptoms, according to the National Comprehensive Cancer Network 1, 3
• The National Comprehensive Cancer Network states that constitutional symptoms are uncommon in ISM, with only 15% of patients experiencing systemic symptoms 1
• The National Comprehensive Cancer Network recommends that Bone Marrow Mastocytosis (BMM) is a specific subvariant of ISM where mast cell infiltration is strictly confined to the bone marrow without skin lesions or multiorgan visceral involvement 1, 2
• BMM has a higher incidence of mediator-related symptoms, with 86% of patients experiencing symptoms from mast cell mediator release, compared to 67% in ISM and 50% in smoldering SM, according to the National Comprehensive Cancer Network 1, 2
• The National Comprehensive Cancer Network suggests that Aggressive Systemic Mastocytosis (ASM) is defined by the presence of one or more C-findings without meeting criteria for mast cell leukemia, and carries a significantly worse prognosis with median survival of only 41 months 1, 2
• The National Comprehensive Cancer Network recommends that ASM requires the presence of at least one C-finding, which includes cytopenias, hepatic dysfunction, skeletal involvement, splenic dysfunction, or gastrointestinal malabsorption 1, 2, 4
• The National Comprehensive Cancer Network states that the distinction between ASM and SM-AHN is crucial, as SM-AHN has different prognostic implications and treatment approaches 1, 2, 4

Prognostic Implications of Systemic Mastocytosis Subtypes

• The National Comprehensive Cancer Network recommends that ISM and BMM have a near-normal life expectancy, with a median survival of over 300 months and a less than 1% risk of transformation to AML or ASM 1, 2
• The National Comprehensive Cancer Network suggests that ASM has a median survival of 41 months and requires consideration of cytoreductive therapy 1, 5

Diagnostic Criteria for Systemic Mastocytosis and Mast Cell Activation Syndrome

Introduction to Diagnostic Approach

• The National Comprehensive Cancer Network recommends evaluating for systemic mastocytosis first when a patient presents with suspected mast cell activation symptoms, using bone marrow biopsy with immunophenotyping, KIT D816V mutation testing, serum tryptase level, and flow cytometry to detect aberrant CD25/CD2 expression on mast cells 6, 7

Diagnostic Algorithm

• Systemic mastocytosis is diagnosed when 1 major criterion + 1 minor criterion, or ≥3 minor criteria are present, according to the World Health Organization diagnostic criteria 6, 8
• The major criterion for systemic mastocytosis is multifocal, dense infiltrates of ≥15 mast cells in aggregates in bone marrow or extracutaneous organs 6, 8
• Minor criteria for systemic mastocytosis include >25% atypical/spindle-shaped mast cells in bone marrow, KIT D816V or other activating KIT mutation detected, aberrant expression of CD25 ± CD2 on mast cells, and persistently elevated baseline serum tryptase >20 ng/mL 6, 9, 8, 10

Critical Distinguishing Features

• The American College of Medical Genetics and Genomics recommends considering hereditary alpha-tryptasemia if baseline tryptase is elevated without meeting systemic mastocytosis criteria, and TPSAB1 gene testing may be useful in this context 7, 10
• The National Institutes of Health suggests that tryptase timing is crucial for mast cell activation syndrome diagnosis, requiring both baseline tryptase and acute tryptase drawn within 1-4 hours of symptom onset 7
• Secondary causes of mast cell activation, such as allergies, drugs, infections, and other inflammatory conditions, must be excluded before diagnosing primary mast cell activation syndrome 6, 7

Referral and Further Evaluation

• Referral to specialized centers with expertise in mastocytosis is strongly recommended for complex or borderline cases, as stated by the European Competence Network on Mastocytosis 6, 8, 11, 12

Distinguishing Symptoms: Mastocytosis vs. Mast Cell Activation Syndrome

Introduction to Mastocytosis and MCAS

• Mastocytosis is distinguished by the presence of skin lesions, evidence of mast cell infiltration in organs, and persistently elevated baseline tryptase >20 ng/mL, whereas MCAS presents with episodic systemic anaphylaxis affecting at least 2 organ systems without the pathologic mast cell burden or skin findings typical of mastocytosis, as recommended by the National Comprehensive Cancer Network 13, 14

Shared Symptoms of Mast Cell Mediator Release

• The National Comprehensive Cancer Network and the American Academy of Allergy, Asthma, and Immunology note that both conditions present with symptoms resulting from mast cell degranulation, including cutaneous manifestations, gastrointestinal symptoms, cardiovascular symptoms, respiratory symptoms, neuropsychiatric symptoms, and anaphylaxis 14, 15
• Cutaneous manifestations, such as flushing, pruritus, urticaria, and angioedema, are common in both mastocytosis and MCAS, according to the National Comprehensive Cancer Network 15, 14
• Gastrointestinal symptoms, including abdominal cramping, diarrhea, nausea, vomiting, and malabsorption, can occur in both conditions, as reported by the Journal of Allergy and Clinical Immunology 15, 14

Distinguishing Features of Mastocytosis

• The National Comprehensive Cancer Network recommends that mastocytosis be diagnosed based on the presence of skin lesions, such as urticaria pigmentosa, diffuse cutaneous mastocytosis, or mastocytomas, in 85% of systemic mastocytosis patients 13
• Persistently elevated baseline serum tryptase >20 ng/mL is a minor diagnostic criterion for mastocytosis, according to the National Comprehensive Cancer Network 13, 16, 17
• Bone marrow biopsy shows multifocal, dense infiltrates of ≥15 mast cells in aggregates, which is a major criterion for diagnosing mastocytosis, as recommended by the National Comprehensive Cancer Network 13
• The KIT D816V mutation is present in most systemic mastocytosis cases, as reported by the National Comprehensive Cancer Network 13, 16, 17

Distinguishing Features of MCAS

• The American Academy of Allergy, Asthma, and Immunology defines MCAS by specific criteria, including episodic, spontaneous systemic anaphylaxis affecting at least 2 organ systems concurrently, and acute elevation of serum tryptase during symptomatic episodes 14
• MCAS is characterized by the absence of WHO criteria for systemic mastocytosis, including no multifocal mast cell infiltrates, no KIT D816V mutation, and no aberrant CD25 expression, as noted by the Journal of Allergy and Clinical Immunology 14, 16
• Symptoms of MCAS improve with H1 and H2 antihistamines, leukotriene receptor antagonists, mast cell stabilizers, or COX/5-lipoxygenase inhibitors, according to the American Academy of Allergy, Asthma, and Immunology 14

Diagnostic Algorithm

• The National Comprehensive Cancer Network recommends a diagnostic algorithm for suspected mast cell activation symptoms, including evaluating for systemic mastocytosis first with bone marrow biopsy, KIT D816V mutation testing, and baseline serum tryptase, and then measuring acute tryptase during symptomatic episodes and mast cell mediators if WHO criteria are not met 16, 17, 14
• The algorithm also includes excluding secondary causes, such as IgE-mediated allergies, drugs, infections, and considering hereditary alpha-tryptasemia testing if baseline tryptase is elevated, as recommended by the National Comprehensive Cancer Network and the American Academy of Allergy, Asthma, and Immunology 16, 17, 14

Mast Cell Disorders and Nocturnal Diarrhea

Gastrointestinal Manifestations in Mast Cell Disorders

• Diarrhea occurs in up to 40% of children with cutaneous mastocytosis and up to 80% of patients with systemic mastocytosis, but the pattern is episodic and triggered rather than specifically nocturnal, affecting patients with mast cell disorders 18

Treatment of Mast Cell Disorders

• The American Academy of Allergy, Asthma, and Immunology is not cited for a fact, however The Journal of Allergy and Clinical Immunology notes that cyproheptadine, functioning as both an H1 receptor blocker and serotonin receptor antagonist, has been specifically used to treat diarrhea in MCAS, suggesting serotonin-mediated mechanisms contribute to GI symptoms 19
• The Journal of Allergy and Clinical Immunology recommends oral cromolyn predominantly for gastrointestinal symptoms, though onset of action can be delayed and requires at least 1 month trial at 200 mg four times daily 19
• H2 receptor blocking agents (ranitidine, famotidine, cimetidine) are commonly used to treat abdominal and vascular symptoms of MCAS, according to The Journal of Allergy and Clinical Immunology 19