Presentation: Purpuric Rash

See also Presentation: Bruising and Bleeding for the fuller systemic bleeding-disorder work-up.

Purpura is discolouration of the skin/mucous membranes from haemorrhage of small blood vessels. Petechiae are small purpuric lesions up to 2mm across. Ecchymoses (bruises) are larger extravasations. Palpable purpura can be felt, reflecting vasculitis. Pigmented purpura reflects petechial haemorrhages associated with capillaritis.

Mechanisms

• Thrombocytopenic purpura (platelet destruction) — primary (idiopathic) immune thrombocytopenic purpura (ITP); secondary thrombocytopenic purpura from drugs, infections, or systemic disease; DIC (ranging from a severe, rapidly fatal purpura fulminans to a relatively minor disorder); heparin-induced thrombocytopenia (anti-platelet antibodies inducing platelet plugs); warfarin-induced necrosis (from relative protein C deficiency early in treatment).
• Non-thrombocytopenic purpura (leakage through the vessel wall) — damage to small vessels, increased intraluminal pressure, or deficient vascular support (e.g. senile purpura in aged/sun-damaged skin).

The platelet count itself is a useful guide to how urgently a purpuric patient needs work-up, independent of the underlying cause: patients with platelet counts greater than 50,000/µL usually remain asymptomatic, whereas those with platelet counts below 20,000/µL often present with spontaneous bleeding. Where a prior platelet count is available for comparison, a recent decline raises concern even if the current count is not yet critically low.

Secondary ITP is common enough to actively look for a cause rather than assume primary/idiopathic disease: management of secondary ITP focuses on treating the underlying condition — systemic lupus erythematosus and other autoimmune disease, lymphoproliferative disorders, and Helicobacter pylori infection (eradication therapy can itself improve the platelet count) are the drug-history-negative causes worth screening for.

Investigations

FBC and platelet count/morphology are first-line for any purpuric rash. Beyond that, clinical assessment has high sensitivity, although low specificity, for the presence of a bleeding disorder. By contrast, both the sensitivity and specificity of routine laboratory screening are low — so a well-directed bleeding history (see Presentation: Bruising and Bleeding) still does more work than reflexively ordering a coagulation panel on every purpuric patient with no suggestive history.