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The hemolytic uremic syndrome (HUS) is a clinical entity characterized by thrombocytopenia, non-immune hemolytic anemia and renal impairment. Kidney pathology shows thrombotic microangiopathy (TMA) with endothelial cell injury leading to thrombotic occlusion of arterioles and capillaries. Traditionally, HUS was classified in 2 forms: Typical HUS, most frequently occurring in children and caused by Shiga-toxin-producing bacteria, and atypical HUS (aHUS). aHUS is associated with mutations in complement genes in 50-60% of patients and has worse prognosis, with the majority of patients developing end stage renal disease. After kidney transplantation HUS may develop as a recurrence of aHUS or as de novo disease. Over the last years, many studies have demonstrated that complement dysregulation underlies the endothelial damage that triggers the development of TMA in most of these patients. Advances in our understanding of the pathogenic mechanisms of aHUS, together with the availability of novel therapeutic options, will enable better strategies for the early diagnosis and etiological treatment, which are changing the natural history of aHUS. This review summarizes the aHUS clinical entity and describes the role of complement dysregulation in the pathogenesis of aHUS. Finally, we review the differential diagnosis and the therapeutic options available to patients with aHUS., (Copyright © 2014 Elsevier España, S.L.U. All rights reserved.)

Haemolytic uraemic syndrome (HUS) is a clinical entity defined as the triad of nonimmune haemolytic anaemia, thrombocytopenia, and acute renal failure, in which the underlying lesions are mediated by systemic thrombotic microangiopathy (TMA). Different causes can induce the TMA process that characterizes HUS. In this document we consider atypical HUS (aHUS) a sub-type of HUS in which the TMA phenomena are the consequence of the endotelial damage in the microvasculature of the kidneys and other organs due to a disregulation of the activity of the complement system. In recent years, a variety of aHUs-related mutations have been identified in genes of the the complement system, which can explain approximately 60% of the aHUS cases, and a number of mutations and polymorphisms have been functionally characterized. These findings have stablished that aHUS is a consequence of the insufficient regulation of the activiation of the complement on cell surfaces, leading to endotelial damage mediated by C5 and the complement terminal pathway. Eculizumab is a monoclonal antibody that inhibits the activation of C5 and blocks the generation of the pro-inflammatory molecule C5a and the formation of the cell membrane attack complex. In prospective studies in patients with aHUS, the use of Eculizumab has shown a fast and sustained interruption of the TMA process and it has been associated with significative long-term improvements in renal function, the interruption of plasma therapy and important reductions in the need of dialysis. According to the existing literature and the accumulated clinical experience, the Spanish aHUS Group published a consensus document with recommendations for the treatment of aHUs (Nefrologia 2013;33[1]:27-45). In the current online version of this document, we update the aetiological classification of TMAs, the pathophysiology of aHUS, its differential diagnosis and its therapeutic management., (Copyright © 2015 Sociedad Española de Nefrología. Published by Elsevier España, S.L.U. All rights reserved.)

Haemolytic uraemic syndrome (HUS) is a clinical entity defined as the triad of nonimmune haemolytic anaemia, thrombocytopenia, and acute renal failure, in which the underlying lesions are mediated by systemic thrombotic microangiopathy (TMA). Atypical HUS (aHUS) is a sub-type of HUS in which the TMA phenomena are the consequence of decreased regulation of the alternative complement pathway on cell surfaces due to a genetic cause. aHUS is an extremely rare disease that, despite the administration of standard treatment with plasma therapy, often progresses to terminal chronic renal failure with a high associated rate of mortality. In recent years, research has established the key role that the complement system plays in the induction of endothelial damage in patients with aHUS, through the characterisation of multiple mutations and polymorphisms in the genes that code for certain complement factors. Eculizumab is a monoclonal antibody that inhibits the terminal fraction of the complement protein, blocking the formation of a cell membrane attack complex. In prospective studies in patients with aHUS, administering eculizumab produces a rapid and sustained interruption in the TMA process, with significant improvements in long-term renal function and an important decrease in the need for dialysis or plasma therapy. In this document, we review and bring up to date the important aspects of this disease, with special emphasis on how recent advancements in diagnostic and therapeutic processes can modify the treatment of patients with aHUS.