Atypical hemolytic uremic syndrome (aHUS) is a disease that causes abnormal blood clots to form in small blood vessels in the kidneys. These clots can cause serious medical problems if they restrict or block blood flow, including hemolytic anemia, thrombocytopenia, and kidney failure. It can occur at any age and is often caused by a combination of environmental and genetic factors. Genetic factors involve genes that code for proteins that help control the complement system (part of your body’s immune system). Environmental factors include certain medications (such as anticancer drugs), chronic diseases (e.g., systemic sclerosis and malignant hypertension), viral or bacterial infections, cancers, organ transplantation, and pregnancy. In about 60% of aHUS, a gene mutation may be identified. The genes associated with genetic aHUS include C3, CD46 (MCP), CFB, CFH, CFHR1, CFHR3, CFHR4, CFI, DGKE, and THBD. Mutations in these genes increase the likelihood (predisposition) to developing aHUS, rather than directly causing the disease. Most cases are sporadic. In familiar cases, predisposition to aHUS is inherited in an autosomal dominant or an autosomal recessive pattern of inheritance.
Atypical hemolytic uremic syndrome differs from a more common condition called typical hemolytic uremic syndrome. The two disorders have different causes and different signs and symptoms. TMA consists of reduced platelet count (thrombocytopenia), hemolytic anemia (as a result of hemolysis [destruction of red blood cells]) and acute kidney injury (AKI). If left untreated, significant proportions of adults (46%) and children (16%) can progress to end-stage renal disease (ESRD) or die during first clinical manifestations of aHUS despite supportive care, including plasma exchange or plasma infusion (PE/PI). One year following clinical manifestations, 56% of adults and 29% of children can progress to ESRD or die, if left untreated. Early and careful diagnosis of aHUS is critical as many coexisting diseases and events are known or suspected to activate the complement cascade, and as patients may not necessarily present with the classic TMA triad of thrombocytopenia, hemolytic anemia and renal impairment or may have less severe renal involvement. Available tests can help distinguish aHUS from other hemolytic diseases with similar symptoms such as HUS caused by Shiga toxin-producing Escherichia coli (STEC-HUS) and thrombotic thrombocytopenic purpura (TTP).