The fundamental lesion in idiopathic thrombotic thrombocytopenic purpura (TTP), sporadic hemolytic-uremic syndrome (HUS), and some forms of familial HUS/TTP is a thrombotic microangiopathy (1–3).Hyaline thrombi, composed predominantly of platelets and accompanied by localized endothelial cell (EC) activation, proliferation, and detachment, are found exclusively in the microvasculature of involved tissues (1). These thrombi subserve the clinical manifestations of both disorders. Initial treatment consists of plasmapheresis and plasma infusions, often requiring over 20 liters of plasma. Failure to institute prompt plasma exchange invariably results in death from renal failure, stroke, myocardial infarction, or other thrombotic events.

Although TTP is recognized classically by the pentad of thrombocytopenia, microangiopathic hemolytic anemia, fluctuating neurological signs, renal abnormalities, and fever, and HUS by a triad of signs – the hemolytic anemia and fever seen in TTP plus prominent renal disease (4) – pathological changes overlap significantly. Sporadic HUS patients often have multiple organ involvement, including dermal, ocular, cardiac, gastrointestinal, and neurological changes, similar to that seen in TTP (1–3). In one study, cerebral microthrombi were found in 13 of 26 HUS patients at autopsy; reciprocally, renal dysfunction occurs in 40% to 80% of TTP cases (3). Only two organs are absolutely or relatively spared in classic TTP/sporadic HUS: The lung is virtually never involved (5), and the liver is an infrequent site of pathology (2).

It is critical – in terms of pathophysiology and selection of appropriate therapeutic interventions – to distinguish between two clinical syndromes commonly labeled HUS. The epidemic type, occurring primarily in children younger than 5 years of age and typically preceded by bloody diarrhea, is linked to the shiga-like verocytotoxins (4).