Histologic distribution of lesions may have some prognostic significance (see below). Age has a major impact on prognosis. Mortality of TTP in adults was nearly 100% before advent of plasma therapy. Children have a much more benign course, with less than 10% mortality even when only symptomatic treatment was given. Improved survival in the last 10 years is associated with use of a combination of antiplatelet agents and plasmapheresis (21). In some series, plasma exchange has resulted in better prognosis than plasma infusion, but the results are not clear-cut. New molecular insights (see above) suggest that plasmapheresis could be useful when acquired inhibitors of ADAMTS13 are present, whereas plasma replacement theoretically could be indicated in patients with deficiency of this protease or factor H mutation, with normal plasma presumably correcting the deficiency (19). ADAMTS13 testing has been advocated as a means to distinguish between HUS and TTP, with TTP proposed to result from ADAMTS13 mutation and resulting deficiency (7). However, there may be overlap both clinically and at a molecular level. Hemolytic uremic syndrome accounts for about half of cases of acute renal failure in HIV patients, and has a poor outcome (15). The pathogenesis of this association is not known, but animal studies do not support direct HIV infection of intrinsic renal cells as a cause of this lesion.
Long-term follow-up 10 years after HUS has shown a decrease in the glomerular filtration rate (GFR) in half of patients (22). Histologic distribution of lesions may have some prognostic significance. Degree of histo-logic damage, rather than initial clinical severity, was the best predictor of long-term prognosis in HUS (23). Predominantly glomerular involvement has a better outcome than larger vessel involvement. Glomerular predominant injury is the most frequent pattern of injury in children. Hypertension is more frequent with larger vessel, rather than glomerular, injury. Poor prognosis was predicted by cortical necrosis or thrombotic microangio-pathy involving >50% of glomeruli at time of presentation. Segmental sclerosis was associated with decreased GFR long term. Recurrence in the transplant is very common in familial forms of HUS, and is most often associated with graft loss. Initial levels of serum plasminogen activator inhibitor-1 (PAI-1) in patients with HUS also correlated with worse long-term outcome, perhaps because high PAI-1 promotes thrombosis and also inhibits matrix breakdown (24). Recently, serum laminin P levels have been proposed to be an index of injury (25).
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