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Summary Sporadic essential thrombocythaemia (ET) is rare in paediatrics, and the diagnostic and clinical approach to paediatric cases cannot be simply copied from experience with adults. Here, we assessed 89 children with a clinical diagnosis of ET and found that 23 patients (25·8%) had a clonal disease. The JAK2 V617F mutation was identified in 14 children, 1 child had the MPL W515L mutation, and 6 had CALR mutations. The monoclonal X-chromosome inactivation pattern was seen in six patients (two with JAK2 V617F and two with CALR mutations). The other 66 patients (74·2%) had persistent thrombocytosis with no clonality. There were no clinical or haematological differences between the clonal and non-clonal patients. The relative proportion of ET-specific mutations in the clonal children was much the same as in adults. The higher prevalence of non-clonal cases suggests that some patients may not have myeloproliferative neoplasms, with significant implications for their treatment.

Essential thrombocythaemia (ET) is usually considered a disease of the middle-aged but, with the advent of automated platelet counting, ET is diagnosed with increasing frequency in young adults and, even more rarely, in children. We report five paediatric patients (four girls and one boy, mean age 89 months) diagnosed with ET in agreement with Polycythaemia Vera Study Group criteria. The patients had a persistent thrombocytosis over 900 x 10(9)/l and, at the time of diagnosis, their platelet count ranged between 1,112 and 3,178 x 10(9)/l. A 9-month-old girl had thrombosis of the inferior cava vein, two children had headaches and two others remained asymptomatic throughout the follow-up period. Megakaryocytes in the bone marrow were increased in number. The chromosomal analysis was normal, and bcr rearrangement was always negative. None of the patients had spontaneous BFU-E or altered levels of serum erythropoietin and thrombopoietin. Two patients showed alteration of platelet aggregation, and all had decreased levels of intraplatelet serotonin. In spite of the diagnosis of ET in our patients, we are not sure that the cases reported here are really myeloproliferative disorders. The features could suggest that the cases observed may be affected by an 'idiopathic thrombocytosis' without myeloproliferation. Possible variants of ET are described in young adults, and the heterogeneous nature of ET is also suggested by our paediatric patients. Only careful long-term follow-up of patients such as these will clarify the natural history of these disorders and suggest therapeutic management.

We studied factor VIII related properties in 24 patients with increased platelet number. Twenty-one were affected by myeloproliferative disorders (eight had polycythaemia vera, 13 had essential thrombocythaemia) and three had secondary thrombocytosis. Normal levels of VIII:C and VIIIR:Ag were found while a significant (P less than 0.05) decrease of VIIIR:RCOF (43 +/- 13%) related to a lack of larger multimers of VWF (39 +/- 12%) was observed in 57% of patients with myeloproliferative disorders. A normal VWF pattern was found in the three patients with secondary thrombocytosis. The highest incidence of VWF abnormalities occurred in patients with essential thrombocythaemia (70%) in comparison with polycythaemic patients (38%). A significant (P less than 0.03) correlation between platelet count and the values of both VIIIR:RCOF and VWF multimeric pattern was observed only in patients with polycythaemia vera. The lowest levels of VIIIR:RCOF and the greatest loss of larger VWF multimers (less than 30%) were observed in two patients who presented bleeding symptoms at the time of study and a prolonged bleeding time. In addition, the relationship between VWF pattern and bleeding diathesis was supported by the fact that 75% of the patients with VWF abnormalities had bleeding history.