Thrombotic microangiopathy in renal transplantation.

The term thrombotic microangiopathy (TMA) encompasses syndromes of thrombocytopenia, microangiopathic haemolytic anaemia, neurologic deficits, renal dysfunction and variable signs of organ impairment. Childhood cases of TMA with predominant renal failure are usually referred as Haemolytic Uremic Syndrome (HUS), and adult cases with major neurological involvement as Thrombotic Thrombocytopenic Purpura (TTP). Exotoxins, produced in most cases by E. Coli O 157:H7, have been related to diarrhea associated HUS(D + HUS). Anticancer (mitomycin), immunosuppressive drugs (cyclosporin, tacrolimus and OKT3) and as well as some antiplatelet agents (ticlopidine, clopidrogel) have been associated with both HUS and TTP. Defective factor H or vWF protease activity have been found with familiar and recurrent forms. Endothelial damage and dysfunction is most likely the initial event of the pathogenic process that eventually leads to platelet aggregation, microvascular thrombosis and tissue ischemia. TMA may occur de novo in the native kidneys of patients who received a non-kidney transplant or in the transplanted kidney of patients who progressed to ESRD because of a disease other than HUS. Calcineurin inhibitors and vascular rejection are most often involved in these cases. The disease may also recur on the transplanted kidney in patients who progressed to ESRD because of HUS/TTP. The risk of postransplant recurrence is negligible for D + HUS but is close to 100% in familial/recurrent forms associated with low C3 and decreased factor H bioavailability or activity. Withdrawal or treatment of precipitating factors are the most effective approach. Plasma therapy is usually attempted with the rationale to limit the microangiopathic process, but its efficacy for improving graft survival is unproven. The outcome of recurrent forms is almost invariably poor.