Mastocytosis¶
Chapter 366 | Part 11: Immune-Mediated, Inflammatory, and Rheumatologic Disorders
KEY CLINICAL POINTS¶
- Mastocytosis is defined by clonally expanded mast cells in tissues, with KIT D816V mutation in >90% of cases
- Classification includes cutaneous mastocytosis (most common in children), systemic mastocytosis (ISM/ASM), and mast cell sarcoma
- Clinical manifestations include urticaria, flushing, gastrointestinal symptoms, and anaphylaxis
- Diagnosis requires tryptase elevation (>200 ng/mL) and tissue infiltration with exclusion of other disorders
- Management includes antihistamines, avoidance of triggers, and targeted therapies for systemic disease
1. DEFINITION & OVERVIEW¶
Mastocytosis is characterized by clonally expanded mast cells in tissues such as skin, bone marrow, liver, spleen, and gut. The disease spectrum includes cutaneous, systemic, and malignant forms. KIT gene mutations (D816V most common) drive mast cell proliferation. Diagnosis requires histopathology, tryptase levels, and exclusion of other disorders.
Table 366-1: Classification of Mastocytosis¶
| Category | Subtypes | Key Features |
|---|---|---|
| Cutaneous Mastocytosis (CM) | Maculopapular (MPCM), Solitary Mastocytoma, Diffuse Cutaneous | Skin lesions with Darier’s sign; common in children |
| Systemic Mastocytosis (SM) | Indolent (ISM), Smoldering (SSM), Aggressive (ASM), SM-AHNMD | Extracutaneous infiltration; organ dysfunction; KIT mutations |
| Malignant Forms | Mast Cell Leukemia (MCL), Mast Cell Sarcoma (MCS) | Lethal; circulating atypical mast cells; solid tumors |
1.1 Pathophysiology¶
KIT mutations (D816V in >90% cases) activate mast cells, leading to degranulation and release of mediators. Somatic mutations in KIT, TET2, SRSF2, and other genes are associated with disease progression. Mast cell infiltration causes tissue damage and systemic symptoms.
1.2 Classification¶
Classification includes cutaneous mastocytosis (CM), systemic mastocytosis (SM), and mast cell sarcoma. SM is further divided into indolent (ISM), smoldering (SSM), and aggressive (ASM) subtypes. Mast cell leukemia (MCL) is a rare malignant form.
2. EPIDEMIOLOGY¶
Prevalence estimated at ~1 in 10,000. Most common in infancy and young adulthood. Familial cases are rare. Atopy is not more common than general population. SM-AHNMD and ASM have worse prognosis due to associated hematologic disorders.
2.1 Demographics¶
Peak incidence in infancy and young adults. Cutaneous forms dominate in children; systemic disease more common in adults. MCL and MCS are rare but associated with poor prognosis.
2.2 Risk Factors¶
KIT mutations (D816V most common), prior mast cell activation, and genetic predisposition. Environmental triggers (e.g., NSAIDs, Hymenoptera stings) exacerbate symptoms.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Somatic KIT mutations (D816V in >90%) drive mast cell proliferation. Mutations in TET2, SRSF2, and other genes contribute to disease progression. Mast cell degranulation releases mediators causing tissue damage and systemic symptoms.
3.1 Molecular Mechanisms¶
KIT mutations activate mast cells, leading to uncontrolled proliferation and degranulation. Mutations in TET2, SRSF2, and RUNX1 are associated with advanced disease and hematologic malignancies.
3.2 Tissue Involvement¶
Mast cells infiltrate skin (CM), bone marrow, liver, spleen, and gastrointestinal tract. Fibrotic changes occur in liver and spleen, leading to portal hypertension and organ dysfunction.
4. CLINICAL FEATURES¶
Symptoms include urticaria, flushing, gastrointestinal issues, and anaphylaxis. Cutaneous lesions (maculopapular, plaques) respond to trauma with urticaria (Darier’s sign). Systemic disease presents with organ dysfunction, cytopenias, and fibrosis.
4.1 Cutaneous Manifestations¶
Maculopapular lesions (MPCM), solitary mastocytomas, or diffuse cutaneous mastocytosis. Lesions may resolve spontaneously in children. Darier’s sign (urticaria after trauma) is diagnostic.
4.2 Systemic Symptoms¶
Gastrointestinal: diarrhea, abdominal pain, gastric hypersecretion. Hematologic: cytopenias, splenomegaly. Neurological: memory impairment, migrainous headaches. Anaphylaxis from triggers (NSAIDs, insect stings).
5. DIFFERENTIAL DIAGNOSIS¶
Distinguish from urticaria, allergic reactions, and other mast cell disorders. Exclude other causes of tryptase elevation (e.g., mast cell tumors, infections). Differentiate ISM from SM-AHNMD based on hematologic findings.
5.1 Non-Mast Cell Disorders¶
Urticaria, allergic reactions, and other mast cell activation syndromes. Exclude infections, neoplasms, and other hematologic disorders.
5.2 Diagnostic Pitfalls¶
Avoid misdiagnosis of SM-AHNMD as chronic myeloproliferative neoplasms. Confirm KIT mutations and tissue infiltration patterns.
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnosis requires tryptase elevation (>200 ng/mL), tissue infiltration, and exclusion of other disorders. Bone marrow biopsy and genetic testing for KIT mutations are critical.
Table 366-2: B and C Findings for Diagnosis of SSM and ASM¶
| Category | Findings |
|---|---|
| B Findings (SSM) | High mast cell burden (>30% bone marrow), basal tryptase >200 ng/mL, KIT D816V >10% VAF |
| B Findings (SSM) | Hypercellular bone marrow with dysmyelopoiesis, palpable hepatosplenomegaly |
| C Findings (ASM) | Cytopenias (ANC <1000/mL, Hb <10 g/dL, PLT <100,000/mL), ascites, portal hypertension |
| C Findings (ASM) | Malabsorption, skeletal lesions (osteolysis, pathologic fractures), hypersplenism |
6.1 Laboratory Tests¶
Elevated serum tryptase (>200 ng/mL), baseline tryptase >200 ng/mL, and KIT D816V mutation analysis. Cytopenias, hypoalbuminemia, and liver dysfunction indicate systemic involvement.
6.2 Imaging and Biopsy¶
Bone marrow biopsy for mast cell infiltration. Imaging for splenomegaly, hepatomegaly, and skeletal lesions. Histopathology confirms mast cell proliferation.
7. MANAGEMENT & TREATMENT¶
First-line: antihistamines (H1/H2 blockers), avoidance of triggers. Systemic disease requires targeted therapies (e.g., imatinib, midostaurin). Supportive care for complications (e.g., corticosteroids for anaphylaxis).
7.1 Pharmacologic Therapy¶
H1/H2 antihistamines for symptoms. Interferon-alpha for cutaneous disease. Targeted therapies (imatinib, midostaurin) for systemic disease. Corticosteroids for acute anaphylaxis.
7.2 Non-Pharmacologic¶
Avoidance of triggers (NSAIDs, Hymenoptera, alcohol). Skin care for lesions. Regular monitoring for hematologic complications.
8. PROGNOSIS & COMPLICATIONS¶
Cutaneous mastocytosis has normal life expectancy. SM-AHNMD and ASM have poor prognosis due to associated hematologic disorders. Complications include anaphylaxis, fibrosis, and organ failure.
8.1 Disease Progression¶
Progression from ISM to ASM occurs in ~5% of patients. Spontaneous resolution in children with cutaneous forms. MCL and MCS are invariably fatal.
8.2 Long-Term Risks¶
Risk of developing myeloid neoplasms, leukemias, or other hematologic malignancies. Fibrotic complications in liver and spleen.
9. SPECIAL CONSIDERATIONS¶
Pregnancy: Safe with careful monitoring. Pediatrics: Spontaneous resolution in many children. Elderly: Higher risk of systemic complications. Avoid NSAIDs and anaphylactic triggers.
9.1 Pregnancy¶
Desensitization protocols for drug allergies may allow safe continuation of therapies. Monitor for anaphylaxis and maternal complications.
9.2 Pediatric Management¶
Cutaneous forms often resolve spontaneously. Monitor for progression to systemic disease. Avoid triggers and use antihistamines for symptoms.
10. KEY POINTS & CLINICAL PEARLS¶
- KIT D816V mutation is diagnostic in >90% of cases
- Tryptase elevation (>200 ng/mL) is critical for diagnosis
- Cutaneous forms are common in children, systemic in adults
- Avoid NSAIDs and Hymenoptera stings to prevent anaphylaxis
- Targeted therapies (imatinib, midostaurin) improve outcomes in systemic disease