Anything that can cause a hemolytic anemia, thrombocytopenia, ecchymosis, purpura, acute renal failure, etc. (either occurring in concert or as isolated symptoms) should certainly be included in your differential diagnosis. You can come up with a whole host of events that could easily lead to one or more of the signs/symptoms described above.
As far as TTP and HUS are concerned, the etiologies are distinctly different, although the symptoms sometimes blur the lines between which syndrome the patient actually has (i.e. the presence of neurological symptoms in a child with ARF does not automatically rule out HUS). The rather distinct etiologies are what I personally consider as the main difference between the two. TTP is due to a deficiency in ADAMTS13, a metalloprotease that breaks vWF multimers into smaller fragments. The deficiency in ADAMTS13 (either because of an inherited deficiency or antibodies that neutralize the protein) leads to larger vWF multimers which activate the clotting cascade. It is also associated with a classic pentad of symptoms: fever, CNS deficiencies, acute renal failure, microangiopathic hemolytic anemia and thrombocytopenia. If you see any combination of these five symptoms, your radar should sound for TTP.
HUS is classically seem in children who have previously eaten undercooked meat (this may also be called Typical HUS). It also generally has marked renal failure and with the absence of neurological/CNS symptoms. The rest is pretty similar to TTP. The E. coli in the meat produces Shiga-like toxin, which injures endothelial cells and leads to activation of the clotting cascade, and so on and so forth.
You probably knew all of this, but just for completeness, this is what I consider as the main difference between the two.
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This probably doesn't answer your question, but I don't have any real good mnemonics; you've just got to go over and over it until you commit it to memory.
