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. Most cases are sporadic. Less than 20 percent of all cases have been reported to run in families. When the disorder is familial, it can have an autosomal dominant or an autosomal recessive pattern of inheritance.
Atypical hemolytic-uremic syndrome should be distinguished from a more common condition called typical hemolytic-uremic syndrome. The two disorders have different causes and different signs and symptoms. Unlike the atypical form, the typical form is caused by infection with certain strains of Escherichia coli bacteria that produce toxic substances called Shiga-like toxins. The typical form is characterized by severe diarrhea and most often affects children younger than 10. The typical form is less likely than the atypical form to involve recurrent attacks of kidney damage that lead to ESRD.
Some of the signs and symptoms of aHUS include:
- Nausea and vomiting
- Shortness of breath
- Heart symptoms
- Acute kidney injury
Atypical hemolytic-uremic syndrome often results from a combination of environmental and genetic factors. Mutations in at least seven genes appear to increase the risk of developing the disorder. Mutations in a gene CFH are most common; they have been found in about 30 percent of all cases of atypical hemolytic-uremic syndrome. Mutations in the other genes have each been identified in a smaller percentage of cases.
Mutations in the genes associated with atypical hemolytic-uremic syndrome lead to uncontrolled activation of the complement system. The overactive system attacks cells that line blood vessels in the kidneys, causing inflammation and the formation of abnormal clots. These abnormalities lead to kidney damage and, in many cases, kidney failure and ESRD.
Although gene mutations increase the risk of atypical hemolytic-uremic syndrome, studies suggest that they are often not sufficient to cause the disease. In people with certain genetic changes, the signs and symptoms of the disorder may be triggered by factors including certain medications (such as anticancer drugs), chronic diseases, viral or bacterial infections, cancers, organ transplantation, or pregnancy.
Some people with atypical hemolytic-uremic syndrome do not have any known genetic changes or environmental triggers for the disease. In these cases, the disorder is described as idiopathic. Most cases of atypical hemolytic-uremic syndrome are sporadic, which means that they occur in people with no apparent history of the disorder in their family. Less than 20 percent of all cases have been reported to run in families.
Combined kidney-liver transplantation and prophylactic plasma exchange have been used to prevent posttransplant recurrences. More recent data have provided evidence about the efficacy of the anti-C5 monoclonal antibody eculizumab in the prevention and treatment of posttransplant aHUS recurrences.
- Red blood cell and platelet counts
- Creatinine levels
- The ADAMTS13 laboratory test can help differentiate aHUS from other diseases
Patients with aHUS have an extremely poor prognosis. Among those with the most commonly identified aHUS genetic mutation, the proportion of patients experiencing negative outcomes (e.g., need for dialysis, permanent kidney damage, death) within the first year rises to 70%. However, sudden morbidity and mortality can occur regardless of mutational status. aHUS can arise at any age, with more than 40% of cases first reported after 18 years of age. The oldest presentation in one study was at age 83. As noted above, kidney transplantation for aHUS patients with ESRD was rarely considered because of a high incidence of graft loss due to TMA recurrence in the transplanted organ in up to 90% of patients. Consequently, most aHUS patients with ESRD undergo chronic dialysis, which is associated with significant morbidities and worsened prognosis. Combined liver-kidney transplantation has been attempted in patients with aHUS, although this high-risk procedure has a mortality rate approaching 50%.
Quality of life is very poor for patients with aHUS, who are burdened with fatigue, renal complications, hypertension, neurological impairment, gastrointestinal distress, clotting at the site of venous access, and ultimately, death. PE/PI is also reported to be associated with significant safety risks and is highly disruptive to patients’ lives due to the requirements for extensive vascular access and frequent administration.
aHUS is generally treated with plasma exchange. In addition a new therapy has been approved and is available for patients with aHUS.
Eculizumab (Soliris) - FDA-approved indication: For the treatment of atypical Hemolytic Uremic Syndrome (aHUS)