Abstract • 126

The sudden occurrence of a thrombotic or hemorrhagic disorder in children, may be due to the development of auto-antibodies specifically directed against platelets or coagulation related proteins. Anti-protein S auto-antibodies have been described during the course of viral infections such as smallpox, and are associated with purpura fulminans or deep vein thrombosis and pulmonary embolism. Anti-FVIII or anti-prothrombin auto-antibodies most often occur during the course of systemic lupus erythematosus (SLE) and are associated with bleeding disorders. The mechanism of the coagulation factor deficiency is the neutralization of FVIII activity in case of FVIII inhibitor, whereas anti prothrombin auto-antibody is reported to induce a rapid turn-over of prothrombin-antiprothrombin complexes. Whatever the protein target of the antibody, it is often associated with lupus anticoagulant (LA), an antiphospholipid antibody which interferes with phospholipid dependent coagulation tests such as activated partial thromboplastin time (APTT). Although LA is very frequently in children, it seems to be associated with thrombotic events only in the cases of SLE. Our experience differs from these clasical presentations. Among the 6 cases of auto-immune induced coagulation abnormalities we observed in the last 2 years, only 3 were classical : anti protein S antibody occurring during the course of a malignant smallpox infection in a 18 month-year-old girl was associated with an arterial thrombosis ; in 2 teenager girls, anti FVIII or anti-prothrombin antibody, associated with hemorrhagic disorder, led to SLE diagnosis. In contrast, no pathology prone to induce immune disorder was evidenced in the last 3 cases, among which one case of anti protein S antibody, associated with LA and deep venous thrombosis in a 9 year-old boy. These results suggest that anti protein S auto-antibody and LA can be the only manifestations of an immune dysfunction leading to thrombosis. They address the question of the mechanism of immune disorder leading to the sudden occurence of symptomatic coagulation abnormalities in childhood.