What is the difference between palpable and non-palpable purpura, including definitions, causes, and treatment options?

Differentiating Palpable vs Non-Palpable Purpura

Palpable purpura indicates inflammation in vessel walls (leukocytoclastic vasculitis in most cases), while non-palpable purpura reflects simple extravasation of red blood cells without vessel wall inflammation, typically from thrombocytopenia or platelet dysfunction.

Definitions

Palpable Purpura

• Raised, firm lesions that can be felt on palpation, representing antigen-antibody deposition along small cutaneous vessels with inflammatory infiltrate in the vessel wall 1
• Results from leukocytoclastic vasculitis affecting cutaneous postcapillary venules in the majority of cases 2, 1
• The palpability reflects edema and inflammatory cell infiltration around damaged vessels 1

Non-Palpable Purpura

• Flat, macular lesions that cannot be felt, representing simple hemorrhage into skin without vessel wall inflammation 3
• Results from extravasation of red blood cells due to low platelet count, platelet dysfunction, or vascular fragility without inflammatory component 3
• Commonly seen in thrombocytopenic conditions like ITP, where platelet counts are reduced but vessels remain intact 3

Clinical Differentiation

Physical Examination Findings

Palpable Purpura:

• Raised, firm papules or plaques that you can feel with your fingertips 2, 1
• Often distributed in dependent areas (lower extremities) due to hydrostatic pressure 2
• May have a retiform (net-like) pattern when presenting as plaques, suggesting more severe vascular involvement 4
• Splenomegaly is typically absent in primary cutaneous vasculitis 3, 5

Non-Palpable Purpura:

• Flat, macular lesions that do not elevate above skin surface 3
• Distributed in areas of trauma or pressure 3
• Splenomegaly should be absent in ITP (present in <3% of cases, similar to healthy population) 3, 5
• If spleen is palpable, consider alternative diagnoses like thalassemia, Gaucher's disease, or myeloproliferative disorders 5

Causes

Palpable Purpura Causes

Primary Cutaneous Leukocytoclastic Vasculitis:

• Drug-induced vasculitis (most common identifiable cause): sulfonamides, penicillins, NSAIDs 2, 6
• Idiopathic (many cases are self-limited) 2

Henoch-Schönlein Purpura:

• Systemic involvement with gastrointestinal, renal, and/or joint manifestations in addition to skin 6
• More likely to have multi-organ involvement compared to drug-induced cases 6

Other Causes:

• Infections (bacterial, viral) 2, 1
• Collagen vascular diseases (SLE, rheumatoid arthritis) 2, 1
• Malignancies (hematologic or solid tumors) 1
• Cryoglobulinemia 2

Retiform Palpable Purpura (Aggressive Variants):

• Calciphylaxis 4
• Warfarin-induced skin necrosis 4
• Antiphospholipid antibody syndrome 4
• Heparin-induced skin necrosis 4

Non-Palpable Purpura Causes

Thrombocytopenic Disorders:

• Idiopathic thrombocytopenic purpura (ITP) - most common 3, 7
• Drug-induced thrombocytopenia 3
• Bone marrow failure syndromes 3
• HIV-associated thrombocytopenia 7

Platelet Dysfunction:

• Medication effects (aspirin, NSAIDs) 3
• Uremia 3

Vascular Fragility:

• Senile purpura 3
• Corticosteroid-induced purpura 3
• Scurvy 3

Diagnostic Approach

For Palpable Purpura:

• Skin biopsy is highly recommended to confirm leukocytoclastic vasculitis, though not absolutely essential if clinical presentation is typical 8
• The positive predictive value of palpable purpura for vasculitis is 86%, with sensitivity of 82% 8
• Medication history is critical - review all drugs, especially sulfonamides and penicillins 6
• Assess for systemic involvement: check urinalysis, renal function, stool guaiac, and joint examination 6

For Non-Palpable Purpura:

• Complete blood count with peripheral smear is essential first step 7
• Peripheral smear should show thrombocytopenia with normal-sized or slightly larger platelets, normal RBC morphology, and normal WBC morphology 3, 7
• HIV testing if risk factors present 7
• Imaging for splenomegaly only if spleen is palpable on examination 7
• Bone marrow aspiration generally not needed unless atypical features present (only 4% of suspected ITP cases had alternative diagnoses) 3

Common Pitfalls

• Do not assume all palpable purpura is vasculitis: 14% of palpable purpuras in one study were not vasculitis 8
• Do not miss retiform patterns: these indicate more aggressive disease requiring urgent intervention 4
• Do not overlook drug causes: carefully review all medications, including over-the-counter drugs 6
• Do not forget systemic evaluation: palpable purpura limited to skin has different prognosis than multi-organ involvement 6
• Splenomegaly argues strongly against ITP: if present, pursue alternative diagnoses like lymphoproliferative disorders, storage diseases, or hemolytic conditions 3, 5