✓ Purpura is caused by extravasation of red blood cells into the skin or mucous membranes due to disorders of blood vessels or some component or disorder of the hemopoietic system.
✓ Purpura can present as petechiae, ecchymosis, palpable lesions or retiform lesions.
✓ Purpura may be associated with serious diseases of several organ systems.
✓ A skin biopsy of a new lesion less than 2 days old and laboratory studies are usually needed to confirm the diagnosis of purpura and its underlying cause.
Purpura is extravasation of red blood cells into the skin or mucous membranes. For this reason, purpuric lesions do not blanch on diascopy (pressing on the lesion with a glass slide or finger). The differential diagnosis for purpura is broad, but it can be quickly narrowed by classifying the lesions based on their morphology, as well as other clinical and laboratory findings. A flowchart for the differential diagnosis of purpura is in Figure 24-1.
Flowchart for the differential diagnosis of purpura. BM, bone marrow; DIC disseminated intravascular coagulation; EDS, Ehlers-Danlos syndrome; EGPA, eosinophilic granulomatosis with polyangiitis; GPA, granulomaatosis with polyangiitis; ITP, immune thrombocytopenic purpura; PPD, pigmented purpuric dermatitis; TTP, Thrombotic thrombocytopenic purpura.
The clinical descriptive terms for purpura are listed below, and their respective tables describing the differential diagnosis are referenced.
Petechiae: Flat lesions, macules ≤ 4 mm (Figure 24-2). Typically, initially bright red and then fade to a rust color (Tables 24-1 and 24-2).
Ecchymosis: Flat lesions, macules and/or patches, >5 mm (Figure 24-3). Typically, initially red or purple, but may fade to yellow, brown, or green (Table 24-3).
Palpable purpura: Elevated, round or oval, red or purple papules and/or plaques (Figure 24-4), sometimes barely palpable (Table 24-4).
Retiform purpura: Stellate or branching lesions, with angular or geometric borders (Figure 24-5). These are often palpable plaques but can present as non-palpable patches as well (Tables 24-5 and Table 24-6).
Petechiae. Bright red macules < 4 mm on abdomen.
Ecchymosis. Resolving purpura leaving red, purple, blue, and yellow patch.
Palpable purpura. Multiple red papules on leg of patient with Henoch-Schönlein Purpura.
Retiform purpura. Purpura on toes with retiform pattern on foot in patient with cholesterol emboli.
Table 24-1.Causes of petechiae (primary lesion is a macule ≤ 4mm) with platelets <150,000 μL...