What kind of rash blanches with pressure?

[Saman49]

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I know scarlet fever has a rash that blanches with pressure but how is this any different from Nikolsky sign?

Thanks

 

[Myxedema]

Nikolsky's sign is not "blanching", it is denuding of the skin with minimal pressure. If the integrity of the epidermis is destroyed (pemphigus vulgaris, burns, Stevens-Johnson syndrome, TEN, staphylococcal scalded skin syndrome, etc.), Nikolsky's sign becomes positive.
If a rash blanches with pressure, it means there is extravasation of RBCs. If it does not blanch with pressure (i.e. purpura), that means there's something wrong with the blood vessel (such as vasculitis, meningococcal septicemia, etc.).

 

[MerickManual]

Blanching rash means the rash is due to hypervascularization or vasodilation. There is no extravasation of RBCs with blanching rashes, the blood is still in the vessels.

Extravasation of RBCs cause non-blanching rashes (ie: something wrong with blood vessels)