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Indolent systemic mastocytosis
Indolent systemic mastocytosis (ISM) is a benign form of systemic mastocytosis (SM; see this term) characterized by an abnormal proliferation of mast cells either only in bone marrow or in numerous tissues.
- Synonym(s): -
- Prevalence: 1-9 / 100 000
- Inheritance: Not applicable
- Age of onset: All ages
- ICD-10: D47.0
- OMIM: -
- UMLS: C0272203
- MeSH: -
- GARD: -
- MedDRA: 10056452
It represents 90% of cases of SM and the global prevalence is estimated at between 1/40,000 and 1/20,000. ISM mainly affects adults.
Unlike in purely cutaneous forms in infants, patients rarely present with skin lesions, usually in the form of urticaria pigmentosa. Symptoms related to the release of mast cell mediators are minor but may be multiple, including pruritus, flushing, syncope, headache and gastric events (vomiting and diarrhea). Patients do not show clinical or biological signs of aggressive systemic mastocytosis, mast cell leukemia (see these terms) or other non mast cell hematological diseases.
The etiology is not understood but there is evidence of an activating mutation of KIT, usually D816V, in the mast cells of nearly 80% of patients with ISM.
Diagnosis is based on histological and cytological analysis of bone marrow (a major criterion for diagnosis). Cytological analysis reveals a proportion of bone marrow mast cells less than 5% and, sometimes, bone marrow mast cells with an elongated oval nucleus and hypogranulous cytoplasm. Minor diagnostic criteria include the level of serum tryptase (above 20ng/mL), the identification of activating mutations of KIT, and the abnormal expression of CD2 and/or CD25 by bone marrow mast cells.
In forms without skin involvement, the differential diagnosis with myelofibrosis may be difficult. It should be possible to distinguish Waldenström disease (see this term), where excess mast cells are found in association with lymphocytic infiltration and the D816V mutation of KIT is absent.
Management and treatment
Treatment of ISM is usually symptomatic and includes antihistamines, corticosteriods or disodium cromoglycate. The resulting osteoporosis may benefit from treatment with bisphosphonates.
The evolution of ISM is slow and benign. The prognosis is generally good and life expectancy is similar to that of the general population.
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