Petechiae is the term given to the individual small red or red-blue spots about 1 to 5mm in diameter which make up the rash. They are caused by a small local amount of bleeding within the skin. Deeper bleeding beneath the skin may be seen as larger spots (ecchymoses).
The difference between petechiae and abnormally prominent blood vessels can be shown by applying pressure to a red spot. If it is caused by an abnormal blood vessel the redness disappears temporarily. By contrast when pressure is applied to purpura the spots do not pale.
Petechia
Ecchymosis
www.netdoctor.co.uk/diseases/facts/purpura.htm
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