Wheal

A wheal is a firm, elevated, rounded or flat topped, generally pale red papule or plaque , that is evanescent, typically disappearing within 24-48 hours; a lesion that is well demarcated , and may be round, oval, gyrate, annular or irregular/serpiginous with pseudopods. This dermatologic finding is due to the infiltration of edema into the papillary body of the dermis. Therefore, while the borders are sharp, they are not stable, and as this short-lived papillary edema shifts, each wheal transiently moves from involved to adjacent uninvolved areas over a period of hours. Also note, while this finding is usually pale red, if the amount of edema is adequate to compresses superficial vessels, the wheal may have a white center. The epidermis is not affected and there is no scaling. A rash consisting of wheals may be called a uticarial exanthem, or urticaria, and may be an allergic response to innumerable initiating agents such as drugs or insect bites. Wheals may also be produced with Darier's sign, and by stroking the skin of certain normal persons who demonstrate the phenomenon called dermatographism.