Investigation and diagnosis

• Diagnosis of cutaneous mastocytosis (usually in children) based on typical clinical and histological skin lesions and absence of definitive signs of systemic involvement.

• Diagnosis of systemic mastocytosis (SM):

- Bone marrow trephine biopsy: essential for diagnosis; multifocal lesions consisting of foci of spindle shaped mast cells with eosinophils and lymphocytes in a fibrotic stroma (90%); in advanced SM diffuse mast cell infiltration may occur.

- Bone marrow aspirate: increased numbers of mast cells; clusters of confluent mast cells are a more specific finding (<30%); features of accompanying haematological disorder may be present, usually 261 myelodysplastic or myeloproliferative disorder rarely lymphoprolif-erative.

- Biopsy of other extracutaneous tissue: notably liver or lymph node; rarely necessary.

- Serum mast cell tryptase and/or histamine:elevated in SM but not in isolated urticaria pigmentosa.

- 24 hour urine for mediators (histamine metabolites, tryptase, PGD2 metabolites).

- Bone scan/skeletal survey: bone lesions in 60%; generalised osteosclerosis, focal sclerosis or generalised osteopenia.

- FBC and film: no characteristic features; circulating mast cells rare (2%) unless very advanced disease or mast cell leukaemia; mild to moderate anaemia in about 45%; eosinophilia up to 25%; thrombocytopenia about 20%; monocytosis about 15%; pancytopenia may develop due to BM infiltration or hypersplenism.

- GI studies: as necessary.

- EEG: if necessary.

Variants of SM

• Indolent SM: commonest form of SM; associated with maculopapular skin lesions (90%), slow involvement of target organs and good prognosis.

• Aggressive SM: characterised by impaired organ function due to infiltration of BM, liver, spleen, GI tract or skeletal system and predisposition to severe mediator release attacks with haemorrhagic complications.

• SM with associated haematological non-mast cell disease: <50% have urticaria pigmentosa; generally CML or CMML; classified according to FAB/WHO criteria; poorer survival.

• Mast cell leukaemia: defined by >20% MC in BM aspirate and >10% in PB; diffuse infiltration on trephine biopsy; no skin lesions, severe peptic ulcer disease, hepatosplenomegaly, anaemia, multiorgan failure and short survival.

• Mast cell sarcoma: a tumour consisting of atypical MC; locally destructive growth; no systemic involvement.

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