Hemolytic uremic syndrome (HUS) usually occurs in children following an episode of diarrheal infection caused by STEC strain O157:H7, and occasionally following infection with Shigella dysenterieae or neuraminidase-producing Streptococcus pneumoniae. This is also called typical HUS. Diagnosis of this condition depends upon the clinical features and laboratory evidence of the pathognomonic blood changes.
There is a typical prodromal diarrheal episode. There are no specific signs in the gastroenteritis phase to identify patients who may be at high risk for HUS, but the features to watch for may include:
The earliest signs of HUS include those of acute renal failure:
Associated complications may be manifested, including CNS complications such as jitteriness, seizures and disorientation.
Laboratory investigations are far more specific, and include:
In the remaining cases, there is no history of diarrhea, and stool cultures as well as Shiga toxin testing are negative. These are called atypical HUS.
Investigations of these patients are similar to the above. However there are certain additional risk factors whose presence needs to be looked for - these are mostly associated with alternative complement pathway dysregulation. One or more of these factors will be detected in approximately 60% of cases. These include:
In other forms of atypical HUS, the alternative complement pathway is not affected.
Laboratory tests to distinguish between these two categories include:
It is important to distinguish complement-mediated forms of atypical HUS from the second category because only the first type is likely to respond to the administration of the humanized monoclonal anti-complement antibody, eculizumab. The second type should be treated with plasma exchange transfusions first, unless the patient is a child below the age of 2 years. In this subgroup, eculizumab may be the first option due to the general non-feasibility of plasma exchange in such small children.