Purpura is visible hemorrhage in the skin or mucous membranes. Purpura can be either macular (flat) or palpable (raised or indurated). There can be varying degrees of inflammation associated with purpura, and the ratio of erythema (inflammation) to purpura (hemorrhage) can help identify the most likely cause of purpura. Noninflammatory macular purpura ≤4 mm in diameter (petechiae) is often due to thrombocytopenia or abnormal platelet function, while noninflammatory macular purpura ≥1 cm in diameter (ecchymosis) often occurs due to coagulation defects or decreased support of dermal blood vessels. Palpable purpura with associated inflammation (erythema) typically occurs in the setting of cutaneous small-vessel vasculitis. In contrast, noninflammatory retiform purpura (branched or reticulated shape) usually indicates microvascular occlusion. Less commonly, retiform purpura with associated inflammation may occur, suggesting IgA cutaneous small-vessel vasculitis or vasculitis affecting both small- and medium-sized vessels. This chapter describes the definition of purpura and its key associated features (e.g., the presence or absence of inflammation and palpability); the differential diagnosis of purpura based upon the morphology of purpura observed by the clinician at the bedside; how the anatomic location of the purpura can be used to refine the differential diagnosis of purpura; and an approach to correctly diagnosing the underlying cause of purpura guided by clinical history and physical examination findings while utilizing skin biopsy and targeted laboratory studies when the cause of purpura is uncertain.
KeywordsLeukocytoclastic Vasculitis Palpable Purpura Ecthyma Gangrenosum Microvascular Occlusion Stasis Dermatitis
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