Thrombotic thrombocytopenic purpura (TTP)

Widespread adhesion and aggregation of platelets lead to microvascular thrombosis and profound thrombocytopenia. This occurs due to deficiency of ADAMTS 13, a protease which is normally responsible for the degradation of vWF. ADAMTS 13 deficiency is congenital, sporadic, or autoantibody mediated (pregnancy, systemic lupus erythematosus, infection, drug treatment e.g. clopidogrel). There is florid purpura, fever, fluctuating cerebral dysfunction and haemolytic anaemia with red cell fragmentation, often accompanied by renal failure. The coagulation screen is usually normal but lactate dehydrogenase levels are markedly raised as a result of haemolysis. Treatment is with plasma exchange (to remove the antibody to ADAMTS13), methylprednisolone and rituximab. Platelet concentrates are contraindicated.