What could cause painful purple bumps on the thigh and glutes?

Painful Purple Bumps on Thigh and Glutes: Diagnostic Approach

Primary Diagnosis: Cutaneous Vasculitis

The most likely diagnosis is cutaneous small-vessel vasculitis (palpable purpura), which requires immediate evaluation to exclude systemic disease and identify the underlying trigger. 1, 2

Key Diagnostic Features

Clinical Presentation

• Palpable purpura is the hallmark finding—raised purple lesions that do not blanch with pressure, distinguishing them from simple bruising 1, 2
• The thigh and gluteal distribution is consistent with dependent areas where hydrostatic pressure increases vascular inflammation 3
• Pain accompanying the lesions suggests active vasculitic inflammation rather than benign vascular malformations 1

Critical Historical Elements

• Recent medication exposure (antibiotics, NSAIDs, diuretics) is the most common identifiable trigger for cutaneous vasculitis 1
• Recent infection (particularly streptococcal pharyngitis or upper respiratory infection) can precipitate IgA vasculitis (Henoch-Schönlein purpura) 3
• Systemic symptoms including fever, joint pain, abdominal pain, or hematuria indicate potential systemic vasculitis requiring urgent evaluation 3
• Drug use history is essential, as levamisole-contaminated cocaine causes distinctive purpuric vasculopathy 2

Differential Diagnosis to Consider

Infectious Etiologies

• Disseminated gonococcal infection presents with painful pustules and hemorrhagic lesions on extremities, though typically involves distal limbs more than proximal thighs 4
• Necrotizing fasciitis manifests with purple bullae and skin sloughing, but requires systemic toxicity, marked edema, and rapid progression over hours 5
• Strongyloidiasis causes pruritic dermatitis over legs and buttocks during larval migration, presenting as linear urticarial rash rather than purpura 6

Non-Infectious Causes

• IgA vasculitis (Henoch-Schönlein purpura) classically presents with palpable purpura on lower extremities and buttocks, often with arthritis and abdominal pain 3
• Polyarteritis nodosa causes painful subcutaneous nodules along medium-sized arteries, which can appear purple 2
• Warfarin-induced skin necrosis or heparin-induced thrombocytopenia should be considered if the patient is anticoagulated 2
• Calciphylaxis presents with painful purple-black lesions but occurs almost exclusively in patients with end-stage renal disease 2

Essential Diagnostic Workup

Laboratory Evaluation

• Complete blood count with differential to assess for thrombocytopenia, eosinophilia, or leukocytosis 6
• Comprehensive metabolic panel to evaluate renal function (uremic vasculopathy) and liver function (hepatobiliary disease) 6
• Urinalysis to detect hematuria or proteinuria suggesting renal involvement in systemic vasculitis 3
• Antistreptolysin O titer if recent pharyngitis or upper respiratory infection 7
• Coagulation studies (PT/INR, PTT) if anticoagulation-related necrosis is suspected 2
• Cryoglobulins and hepatitis C serology if cryoglobulinemic vasculitis is considered 2

Skin Biopsy

• 4mm punch biopsy from an early lesion (less than 24-48 hours old) provides definitive diagnosis 1
• Request direct immunofluorescence on a separate specimen to identify IgA deposition (IgA vasculitis) or other immune complex patterns 1, 3
• Histopathology showing leukocytoclastic vasculitis with fibrinoid necrosis of vessel walls confirms cutaneous vasculitis 1

Imaging Considerations

• Not routinely indicated for isolated cutaneous findings 1
• Consider CT or MRI only if necrotizing fasciitis is suspected based on systemic toxicity, rapid progression, or gas in tissues 5

Management Algorithm

Immediate Actions

1. Discontinue any potentially offending medications immediately, as drug-induced vasculitis typically resolves within weeks of cessation 1
2. Assess for systemic involvement by checking vital signs, examining for joint swelling, palpating abdomen for tenderness, and reviewing urinalysis 3
3. Obtain skin biopsy before initiating treatment to confirm diagnosis 1

Treatment Based on Severity

Mild, Localized Disease (No Systemic Involvement)

• Supportive care with leg elevation and compression stockings to reduce hydrostatic pressure 1
• NSAIDs for pain control if not contraindicated 1
• Observation as many cases are self-limited and resolve within 2-4 weeks 1

Moderate Disease (Extensive Cutaneous Involvement)

• Colchicine 0.6 mg twice daily as first-line agent for chronic or recurrent vasculitis 1
• Dapsone 50-100 mg daily (after checking G6PD level) for refractory cases 1

Severe Disease (Systemic Involvement or Rapidly Progressive)

• Systemic corticosteroids (prednisone 0.5-1 mg/kg/day) for IgA vasculitis with renal involvement or other systemic manifestations 1, 2
• Urgent rheumatology or nephrology consultation if systemic vasculitis is confirmed 2

Critical Pitfalls to Avoid

• Do not assume trauma or simple bruising without examining for palpability—palpable purpura always warrants investigation 1
• Do not delay biopsy in atypical presentations, as histopathology is essential for distinguishing vasculitis from vasculopathy and identifying specific subtypes 1
• Do not miss necrotizing fasciitis—if the patient appears systemically ill with fever, tachycardia, or severe pain out of proportion to examination findings, obtain immediate surgical consultation 5
• Do not overlook medication history—carefully review all prescription medications, over-the-counter drugs, and supplements from the past 4-6 weeks 1
• Do not ignore systemic symptoms—joint pain, abdominal pain, or hematuria mandate evaluation for systemic vasculitis with potential organ-threatening complications 3