Hemolytic uremic syndrome is a severe, life-threatening complication of an E. coli bacterial infection that was first described in 1955, and is now recognized as the most common cause of acute kidney failure in childhood. E. coli O157:H7 is responsible for over 90% of the cases of HUS that develop in North America. In fact, some researchers now believe that E. coli O157:H7 is the only cause of HUS in children. HUS develops when the toxin from E. coli bacteria, known as Shiga-like toxin (SLT) [1,2], enters cells lining the large intestine. The Shiga-toxin triggers a complex cascade of changes in the blood. Cellular debris accumulates within the body’s tiny blood vessels and there is a disruption of the inherent clot-breaking mechanisms. The formation of micro-clots in the blood vessel-rich kidneys leads to impaired kidney function and can cause damage to other major organs.
About ten percent of individuals with E. coli O157:H7 infections (mostly young children) goes on to develop Hemolytic Uremic Syndrome, a severe, potentially life-threatening complication. HUS is an extremely complex process that researchers are still trying to fully explain.
Its three central features describe the essence of Hemolytic Uremic Syndrome: destruction of red blood cells (hemolytic anemia), destruction of platelets (those blood cells responsible for clotting, resulting in low platelet counts, or thrombocytopenia), and acute renal failure. In HUS, renal failure is caused when the nephrons, or filtering units, become occluded (blocked) by micro-thrombi, which are tiny blood clots. In almost all cases, the filtering ability of the kidneys recovers as the body of the patient slowly dissolves the micro-thrombi within the microvessels.
A typical person is born with about one million filtering units, called nephrons, in each kidney. The core of the nephron is a bundle of tiny blood vessels, called a glomerulus, where osmotic exchange allows for the filtration of wastes that eventually collect in the urine and are excreted. During Hemolytic Uremic Syndrome, the lack of blood flow to the nephrons can cause them to die or be damaged, just as heart muscle can die as the result of coronary vessel occlusion during a heart attack. Dead nephrons do not regenerate.
In general, the longer a patient suffers kidney failure, the greater the loss of filtering units as a result. At some point, the damage to the kidneys’ filtering units can be so severe that the patient will, over a period of years, lose kidney function and suffer end-stage renal disease (ESRD), which requires chronic dialysis or transplantation.
HUS can also cause transient or permanent damage to other organs, which include the pancreas, liver, brain, and heart. The essential pathogenic process is the same regardless of the organ affected: microthrombi inhibit necessary blood flow and cause tissue death or damage. During the acute stage of Hemolytic Uremic Syndrome, patients must be carefully monitored for these extra-renal complications. It is very difficult to predict the severity and course of HUS once it initiates.