Pathophysiology

Endothelial cells normally synthesize vWF in the form of a high-molecular weight multimer composed of smaller identical monomers. Each monomer is able to bind platelets, and the number of monomers on the vWF multimer is directly proportional to its platelet-binding capacity. Consequently, particularly adherent ultralarge molecules of the vWF (ULvWF) are broken down to smaller size by vWF-cleaving proteases such as ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 repeats) to avoid undesired clot formation. TTP results from genetic or acquired deficiency in the vWF-cleaving protease ADAMTS13 activity. This, in turn, elevates circulating levels of ultralarge molecules of the factor (ULvWF) leading to inappropriate platelet agglutination. Thrombocytopenia develops because the rate of aggregated platelet consumption is faster than megakaryocyte bone marrow production. Microangiopathic hemolytic anemia generally follows as a consequence of red blood cell damage by platelet clumps occluding the microcirculation. Occlusive ischemia of the brain or GI tract is common, and renal dysfunction may occur.

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