What causes small round purplish spots known as petechiae?

Petechiae are small, round, purplish spots that appear on the skin or mucous membranes as a result of bleeding from capillaries. This type of bleeding occurs when capillaries, which are the smallest blood vessels in the body, become damaged and leak blood into the surrounding tissues. The pressure changes associated with certain medical conditions, such as thrombocytopenia (low platelet count) or certain infections, can contribute to this bleeding.

The distinguishing characteristic of petechiae is their small size and the fact that they do not blanch (turn white) when pressure is applied, which differentiates them from other types of bruising. Unlike arterial or venous bleeding, which typically results in larger areas of discoloration or pooling of blood, capillary bleeding accounts for the pinpoint nature of petechiae. Trauma-associated bleeding can lead to larger bruises and other types of bleeding, but petechiae specifically indicate damage to capillaries rather than larger vessels.