purpura is visible bleeding into the skin or mucous membranes which is non-blanching when pressure is applied
the patient with an acute petechial or purpuric rash should be immediately considered for the life threatening emergency of meningococcal septicaemia, particularly if they are unwell or febrile - see Fever with petechial or purpuric rash
types of purpuric lesions
petechiae - < 4mm spots
macular purpura - non-palpable spots 4-10mm
many of the conditions which cause petechiae can progress to macular purpura
macular ecchymoses - non-palpable spots larger than macular purpura
livedo reticularis which is usually a functional response to cold as skin has zones of arterial predominance and zones of venous predominance between them
livido racemos
petechiae
DDx
increased venous pressure:
head - asphyxia / strangulation
upper trunk/face (superior vena cava distribution above the nipple line) - vomiting / severe coughing / etc
arms distal to BP cuff / tourniquet application
neonates due to delivery (NB. they can also have petechiae from acquired infection or maternal antiplatelet antibodies)