Thrombotic thrombocytopenic purpura and other thrombotic microangiopathic hemolytic anemias: diagnosis and classification.

Thrombotic microangiopathies (TMAs) include several diseases, most prominently are thrombotic thrombocytopenic purpura (TTP) and hemolytic-uremic syndrome (HUS). TMAs are characterized by profound thrombocytopenia, microangiopathic hemolytic anemia and organ ischemia. In most cases TTP results from deficiency of ADAMTS13, the von Willebrand factor-cleaving protease leading to increase of ultra-large von Willebrand factor (ULVWF) multimers. Congenital TTP is due to mutations in the gene of ADAMTS13 whereas acquired TTP is due to production of autoantibodies against ADAMTS13. In both cases severe deficiency of ADAMTS13 exists. However, the presence of ADAMTS13 activity does not rule out TTP. Diagnostic criteria of TTP are based on clinical features of neurologic and renal disfunction along with anemia and thrombocytopenia, low ADAMTS13 activity, and the presence of ULVWF. The standard treatment of TTP includes plasma exchange, protein A immunoabsobtion, immunosuppressive drugs, CD20 antibodies against B cells, and splenectomy. HUS is commonly caused by infection with Shiga-toxin produced by Escherichia coli. HUS is characterized by thrombocytopenia, anemia, renal impairment and diarrhea. Rarely, atypical HUS appears as a consequence of mutations related to the alternative pathway for the compliment system. Plasmapheresis in HUS is not efficient. Alternatively, plasma therapy and in some cases dialysis are used. TMA diseases may be associated with other infections, bone marrow transplantation, pregnancy, systemic vasculitis, and certain drugs.