Paroxysmal nocturnal hemoglobinuria is a rare acquired hematologic disorder, the most serious complication of which is thrombosis. The increased incidence of thrombosis in paroxysmal nocturnal hemoglobinuria is still poorly understood, but unlike many other thrombotic disorders, predominantly involves complement-mediated mechanisms. This review article discusses the different factors that contribute to the increased risk of thrombosis in paroxysmal nocturnal hemoglobinuria. Paroxysmal nocturnal hemoglobinuria leads to a complex and multifaceted prothrombotic state due to the pathological effects of platelet activation, intravascular hemolysis and neutrophil/monocyte activation. Platelet and endothelial microparticles as well as oxidative stress may play a role. Impaired fibrinolysis has also been observed and may be caused by several mechanisms involving interactions between complement activation, coagulation and fibrinolysis. While many factors may affect thrombosis in paroxysmal nocturnal hemoglobinuria, the relative contribution of each mechanism that has been implicated is difficult to quantify. Further studies, including novel in vivo and in vitro thrombosis models, are required in order to define the role of the individual mechanisms contributing to thrombosis, impaired fibrinolysis and clarify other complement-driven prothrombotic mechanisms in paroxysmal nocturnal hemoglobinuria.
