Tis the Season… For Hemolytic Uremic Syndrome
Hemolytic uremic syndrome (HUS) flourishes in the summer, for reasons we do not understand. This common cause of acute kidney injury in children also demonstrates geographic hot-spots, and the disorder can reach epidemic rates in July and August. HUS has two major components:
- Hemolytic: Little clots form in the smallest blood vessels of the body, which use up platelets and break up red blood cells, the oxygen carrying units of the blood. Doctors call this a microangiopathic process. Low platelet counts can cause bruising and bleeding. Low red blood cell counts can cause fatigue. When the blood breaks down, the skin and eyes can develop a yellow, jaundiced color.
- Uremic: One of the major places that clots form in HUS is the kidney. This process can cause mild problems with kidney function detectable only in a laboratory, full-blown kidney failure, or anything in between.
Clots can form in other organs and cause malfunction and damage. This process may affect all organs, but especially the pancreas, liver, lung, and brain. Clots in the brain may lead to fatal outcomes in a small number of affected children.
HUS in children occurs in two forms:
- Diarrhea + (typical): The most common form of HUS in children follows an illness with diarrhea, usually with blood in the stool. A number of germs can cause this, but they all produce a toxin that can enter the blood and attack the kidneys and other organs. The most common cause is a form of E. coli (O157:H7).
- Atypical: This type of HUS does not follow another illness. It may be associated with inherited problems of the immune system or the blood clotting system.
So how can bad diarrhea result in HUS? Bacteria such as E. coli and Shigella can produce a toxin called shiga toxin or vero toxin. This toxin can be absorbed into the bloodstream from the gut, where it damages the lining of small blood vessels called capillaries, as shown in the diagram. The damaged capillary lining (which we medical types call the endothelium) tells platelets of its damage through chemical signals. Platelets then clump there to prevent blood loss and promote repair of the damage. That is what platelets do.
In this case, the damage can be extensive. Platelets can be used up faster than the bone marrow can produce them. The platelets form a web across the small capillaries; as red blood cells try to squeeze through they get damaged. Damaged red blood cells often appear in children with HUS, helping us make the diagnosis. As these cells break down, the child may become anemic. As the body metabolizes these broken-down red blood cells, bilirubin results. This chemical can cause a yellow color to the skin and eyes called jaundice.
Red blood cells carry oxygen to tissues so they can function. As the platelet clumping in the capillaries becomes more extensive, oxygen delivery to kidneys and other organs can result in failure of these organs to function. Sometimes this damage reverses when the HUS resolves. Other times, the organ damage may be permanent.
No specific treatment makes HUS better. Doctors must support the child suffering from HUS until their own body heals itself. Careful attention must be directed to the child’s fluid balance and nutrition. Transfusions can be given to treat severe anemia and bleeding due to low platelet counts. If kidney failure occurs, dialysis may be necessary. Drugs may be needed to treat high blood pressure.
Typical HUS may follow a bacterial infection, but antibiotics play no role in its treatment. Children who receive antibiotics to treat the E. coli diarrhea have a higher risk of developing HUS. Atypical HUS may sometimes be treated with plasmapheresis, a special type of blood therapy.
In typical HUS, 95% of children survive. Of these children, 95% recover sufficient renal function that they do not need dialysis. The other 5% go on to long-term dialysis and kidney transplantation. Typical HUS almost never occurs again in the same child. Long-term follow-up is essential, since some children develop kidney problems 5 or 10 years later. These include protein in the urine, high blood pressure, or loss of renal function. At this time, we cannot tell who will develop later problems, so we watch everybody.
Atypical HUS has a worse prognosis. This type tends to occur over and over again in the same patient, and it often leads to permanent kidney failure. Even with kidney transplantation, this type of HUS may happen again in the new kidney. As we identify the genes and proteins that contribute to this form, we are developing new therapies that help us predict and prevent recurrent forms of this disorder.
For a more technical review of typical HUS, see Pediatric Nephrology 23:1749-60, 2008. PMID: 18704506