MudPhud20XX

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So based on Pathoma and others, palpable purpura seems to be associated with vasculitis such as HSP, whereas "purpura" not palpable (I am assuming that's the case, pls correct me if I am wrong.) is associated with disorders of primary hemostasis such as ITP, TTP, and HUS. Dr. Sattar did point out "palpable purpura" is specifically associated with small vessel vsculitis, but I don't recall him explaining it in detail. So what is the deal with purpura being "palpable?" Many thanks in advance.
 
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Seph

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From what I understood, purpura is a manifestation of disorders of primary hemostasis. Basically, small vessels get damage and blood leaks into cutaneous tissue, and since proper thrombus formation is lacking, that manifests as major bleeding (echymosis and purpura). With vasculitis, blood vessels become inflamed by an autoimmune process which, besides damaging vessels and leading to bleeding (purpura), there is also massive inflammation which leads to cutaneous edema from vessels that are not necessarily ruptured, and that manifests as raised purple plaques (hence, palpable purpura). I might be wrong, so wait for someone with more knowledge on the subject than me.
 
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SBR249

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I think it's to do with the underlying pathological process. Purpura caused by leakage of blood due to insufficient clotting factors or platelets (ITP, TTP, and HUS) would be macular and non-palpable. However, leakage of blood caused by damage to the blood vessel due to inflammatory processes would be palpable. The inflammation associated with the purpura is what causes the area of skin to be raised and thus palpable.
 
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Transposony

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Purpura is caused by bleeding into the skin.

Palpable purpura results from blood vessel inflammation (vasculitis) due to immune-mediated vessel injury (thrombosis) and immune complex deposition in vessel walls leading to fibrinoid necrosis. Thrombosis of affected small vessels is what makes it palpable.
Usually associated with systemic symptoms.
 
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Transposony

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Here is a good explanation of all kinds of purpura.