Although relatively rare (prevalence unknown), thromboembolic complications do occur in paediatric patients and cause significant morbidity with occasional mortality. The literature regarding thrombosis in childhood is limited to a few case reports, therefore most of the information derives from studies in adults population. The main inherited abnormalities that predispose to thrombosis are deficiencies of natural inhibitors of haemostasis(antithrombin III-ATIII -, protein C-PC -, protein S-SP), and dysfibrinogenaemia. Other inherited abnormalities for which an association has not been proved are: plasminogen deficiency (plg), heparin cofactor II (HC-II) deficiency, histidine-rich acid glycoprotein, increased levels of plasminogen activator inhibitor (PAI) or reduced levels of tissue plasminogen activator(t-PA). Recently, inherited resistance to activated protein C (APCr) was described as a pathogenic risk factor of thrombosis. APCr is caused by a single point mutation in the gene for factor V in more than 90% of the cases.

There is strong evidence that the frequency of thrombotic events increases with age. The risk of thrombosis for heterozygous ATIII, PC or PS deficiency is extremely low in the first decade of life, begins to increase in the second decade and by the fifth decade 70-80% of the patients with any type of defiency will develop thrombosis with or without provocation. Children with inherited deficiencies of inhibitors usually develop thrombosis in the presence of a second thrombotic stimulus such as trauma, prolonged immobilization, local infection, cancer or its treatment. Some young children with PS deficiency present with strokes for reasons that remain unclear. The diagnosis of the inherited abnormality which predisposes to thrombosis is established by specific assays. However the heterozygocity for ATIII, PC or PS is difficult to be diagnosed in the sick neonate, since physiological levels for these components can be extremely low (as low as 12%). In contrast neonates with the recessive homozygous or double heterozygous state of PC or PS is expressed clinically in the newborn as purpura fulminans. These neonates have very low or undetectable levels of PC or PS. Besides, the disorder is often associated with severe DIC.