Palpable Purpura is Most Consistent with Small-Vessel Vasculitis
A palpable purpural rash is the hallmark clinical finding of small-vessel vasculitis, specifically indicating leukocytoclastic vasculitis affecting postcapillary venules in the upper dermis. 1, 2, 3
Why Small-Vessel Vasculitis
The key distinguishing feature is palpability—this indicates inflammation and hemorrhage within dermal vessels that creates raised, non-blanching lesions. 2, 4
- Palpable purpura specifically indicates involvement of small vessels (postcapillary venules) in the upper dermis, distinguishing it from other vascular pathology. 5, 4
- The purpuric component reflects extravasation of red blood cells due to vessel wall damage and increased vascular permeability. 2
- Small-vessel vasculitis manifests most frequently as palpable purpura, typically on the lower extremities where hydrostatic pressure is greatest. 2, 4
Distinguishing from Other Vessel Sizes
Large-vessel vasculitis does NOT typically present with palpable purpura. Large vessels (aorta and its major branches) when inflamed produce different manifestations:
- Constitutional symptoms, claudication, absent pulses, and bruits—not purpuric rashes. 1
Medium-vessel vasculitis (affecting muscular arteries in the deep dermis and subcutis) presents differently:
- Livedo racemosa, deep nodular lesions, and skin ulceration rather than palpable purpura. 2, 5
- These lesions are deeper and more indurated than the superficial palpable purpura of small-vessel disease. 5
Clinical Context
The differential diagnosis for palpable purpura includes multiple etiologies of small-vessel vasculitis:
- Cutaneous leukocytoclastic angiitis (idiopathic small-vessel vasculitis) 2, 3
- Henoch-Schönlein purpura (IgA-mediated vasculitis) 5
- ANCA-associated vasculitides (granulomatosis with polyangiitis, microscopic polyangiitis) when they involve the skin 1, 5
- Drug-induced hypersensitivity vasculitis 2, 3
- Infection-associated vasculitis 6
- Connective tissue disease-associated vasculitis 2
Important Caveats
- While palpable purpura indicates small-vessel vasculitis, not all purpura is vasculitic—thrombocytopenia, coagulopathy, and embolic phenomena can cause non-palpable purpura. 7
- The presence of palpable purpura warrants skin biopsy extending to the subcutis to confirm leukocytoclastic vasculitis histologically. 2, 5
- Systemic evaluation is essential as cutaneous small-vessel vasculitis can be the presenting sign of serious systemic disease requiring urgent intervention. 7, 2