Adult hemolytic uremic syndrome with renal arteriolar deposition of IgM andC3.

Two cases of idiopathic adult hemolytic uremic syndrome in which deposits of IgM and C3 were identified in renal arterioles showing fibrinoid necrosis are reported. Fibrin was also identified in the lumina of the involved vessels, but there was no laboratory evidence of disseminated intravascular coagulation. In both cases, serum C3 was decreased and C4 was normal, suggesting involvement of the alternate pathway of complement activation. These two cases suggest that in some instances the adult hemolytic uremic syndrome may be immunologically mediated, and that renal vascular thrombosis is a secondary phenomenon.