INTRODUCTION AND DISEASE DEFINITIONS
Thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS) are both acute syndromes with abnormalities in multiple organ systems and evidencing microangiopathic hemolytic anemia and thrombocytopenia. Although some studies appear to distinguish between TTP and HUS, the presenting features are essentially the same in most adult patients:
●In a few patients, neurologic abnormalities are dominant and acute renal failure is minimal or not present; these patients are considered by some to represent idiopathic or "classical" TTP.
●When acute renal failure is dominant, the disorder is considered by some to represent HUS.
●Among patients with the diagnosis of TTP supported by the presence of severe ADAMTS13 deficiency (activity <10 percent), severe neurologic abnormalities and renal failure are both uncommon (table 1). These data document that many patients with classical TTP have no or minimal neurologic abnormalities and renal function abnormalities, and that the presence of the “classic pentad” of clinical features is now rare, and may only occur in patients with additional or alternative diagnoses.
However, some patients present with severe neurologic abnormalities, such as seizures and coma, together with acute renal failure; these patients can best be described by the comprehensive term TTP-HUS. Common clinical usage has been to describe adults with these syndromes as having TTP, and children as having "typical" HUS if preceded by diarrhea caused by E. coli O157:H7 . (See "Causes of thrombotic thrombocytopenic purpura-hemolytic uremic syndrome in adults", section on 'Evolution of disease definitions' and "Causes of thrombotic thrombocytopenic purpura-hemolytic uremic syndrome in adults", section on 'Pathogenesis'.)