Petechiae are nonblanching, usually red, macules <1 mm in diameter. They are a form of purpura, which as a general term refers to rashes caused by the extravasation of red blood cells into the skin. Other purpuric lesions are ecchymoses (nonblanching macules >1 cm), and purpura (macules from 1 mm to 1 cm in diameter). Thus, the term purpura (somewhat confusingly) can be used either to refer to all of these purpuric rashes as a group, or specifically to mean purpura per se. Purpura fulminans refers to large, confluent “lakes” of ecchymoses, which become necrotic. Broadly, purpuric rashes are caused by disorders of coagulation, platelet disorders, or disorders affecting the walls of blood vessels. Thus, the process leading to purpura may be thrombocytopenia (immune-mediated, infectious, malignant, Kasabach-Merritt syndrome) or platelet dysfunction, coagulopathy (inherited [e.g., protein C or S deficiency or other factor deficiency] or acquired [e.g., hemorrhagic disease of the newborn, disseminated intravascular coagulation]), vasculitis, or connective tissue disease (scurvy, Ehlers-Danlos). Petechiae are most often the result of platelet dysfunction or thrombocytopenia. The differential diagnosis of purpura is enormous and includes everyone’s favorite suspect, meningococcemia, as well as many other infections, noninfectious acquired diseases, congenital disorders, and trauma.

In neonates, a common cause of petechia is immune-mediated thrombocytopenia, due either to maternal alloimmunization against fetal platelets or to transplacental passage of maternal autoantibodies (e.g., idiopathic thrombocytopenic purpura, lupus, or drug reactions). Kasabach-Merritt syndrome refers to thrombocytopenia from the sequestration or coagulative consumption of platelets within a hemangioma or similar vascular anomaly. Other causes of thrombocytopenia in infants are congenital disorders of platelet number (Wiskott-Aldrich, Fanconi, thrombocytopenia-absent radii syndromes) or function (Bernard-Soulier, Glanzmann thrombasthenia), and heparin-induced thrombocytopenia. Infectious possibilities
include TORCH (toxoplasmosis, other infections, rubella, cytomegalovirus, herpes simplex virus) infections, human immunodeficiency virus, parvovirus B19, and bacterial sepsis.