What is the appropriate evaluation and management for a patient presenting with ankle pain and purpuric lesions over the lower extremities?

Evaluation and Management of Ankle Pain with Purpuric Lesions on Lower Extremities

Immediate Clinical Assessment

This presentation strongly suggests Henoch-Schönlein purpura (HSP) or another form of cutaneous vasculitis, and requires urgent evaluation to exclude systemic involvement and rule out critical limb ischemia or infection. 1, 2, 3

Key History Elements to Obtain

• Recent infections: Group A streptococcal pharyngitis, Mycoplasma, or other upper respiratory infections within the past 1-3 weeks 3
• New medications: Particularly antibiotics (clindamycin, penicillins), NSAIDs, or other drugs started within days to weeks of symptom onset 1, 3, 4
• Associated symptoms:
  • Abdominal pain or cramping (suggests gastrointestinal vasculitis) 2, 3
  • Gross hematuria or tea-colored urine (suggests renal involvement) 1, 3
  • Joint pain or swelling beyond the ankle (polyarticular involvement) 2, 3
  • Fever or systemic symptoms (suggests infection or severe vasculitis) 4
• Diabetes, peripheral neuropathy, or vascular disease history: Critical to exclude limb-threatening ischemia 5

Critical Physical Examination Findings

• Rash characteristics: Palpable purpura (raised, non-blanching lesions) concentrated on lower extremities, particularly below the knees, with possible "sock line" distribution 1, 2, 3
• Vascular assessment: Palpate bilateral pedal pulses (dorsalis pedis and posterior tibial) to exclude peripheral arterial disease 5, 6
• Skin integrity: Examine for ulcerations, necrosis, bullae, or crepitus that would suggest necrotizing infection or critical ischemia 6, 7
• Joint examination: Assess for effusion, warmth, or limited range of motion in ankles and other joints 2, 3
• Abdominal examination: Palpate for tenderness, guarding, or organomegaly 3

Diagnostic Algorithm

Step 1: Rule Out Life-Threatening Conditions

If pedal pulses are absent or diminished, immediately measure ankle-brachial index (ABI) and consider urgent vascular imaging to exclude critical limb ischemia (CLI), as patients with purpura and vascular compromise are at high risk for limb loss. 5, 6

If fever, systemic toxicity, or signs of necrotizing infection are present (crepitus, bullae, rapid progression), obtain blood cultures and start empiric IV antibiotics immediately while arranging urgent surgical consultation. 7

Step 2: Initial Laboratory Workup

• Urinalysis with microscopy: Essential to detect hematuria or proteinuria indicating renal involvement 1, 3
• Complete blood count: Assess for thrombocytopenia (which would suggest alternative diagnoses like thrombocytopenic purpura) 4
• Comprehensive metabolic panel: Evaluate renal function (BUN, creatinine) 3
• Inflammatory markers: ESR and CRP to assess disease activity 4
• Coagulation studies: PT/INR and PTT if bleeding disorder is suspected 4

Step 3: Imaging

Plain radiographs of the ankle (AP, lateral, and mortise views) are the initial imaging study to exclude fracture, particularly if trauma history is present or if joint effusion is suspected. 5, 8

If vascular disease is suspected (absent pulses, diabetes, age >50), proceed with:

• ABI measurement (normal 0.91-1.30; <0.9 indicates PAD; <0.5 suggests CLI) 5, 6
• Toe pressures if ABI is falsely elevated (>1.30) due to arterial calcification 5, 6
• Duplex ultrasound of lower extremities if revascularization may be needed 5

Step 4: Skin Biopsy (If Diagnosis Unclear)

Punch biopsy of a fresh purpuric lesion should be performed if the diagnosis remains uncertain after initial evaluation, particularly to distinguish between HSP and other forms of vasculitis or to exclude malignancy. 4 Direct immunofluorescence showing IgA deposition confirms HSP. 3

Management Strategy

For Henoch-Schönlein Purpura (Most Likely Diagnosis)

Treatment is primarily supportive, as HSP is typically self-limited in adults, but close monitoring for systemic complications is essential. 3

• Supportive care:

  • NSAIDs for joint pain (if renal function is normal) 3
  • Elevation of lower extremities to reduce edema 1
  • Adequate hydration 3

• Corticosteroids: Consider prednisone 1 mg/kg/day (max 60-80 mg) if severe abdominal pain, significant joint involvement, or progressive renal disease is present 3, 4

• Close renal monitoring: Weekly urinalysis for the first month, then monthly for 6 months, as renal involvement can develop or progress after initial presentation 3

For Drug-Induced Vasculitis

Immediately discontinue the suspected offending medication (e.g., clindamycin if recently started). 1, 3, 4 Most drug-induced cases resolve within days to weeks after drug cessation. 4

For Peripheral Arterial Disease with Purpura

If ABI <0.9 or absent pulses are confirmed, urgent vascular surgery consultation is required for consideration of revascularization, as tissue loss is imminent with CLI (ABI <0.5, ankle pressure <50 mmHg, or toe pressure <30 mmHg). 5, 6

• Initiate antiplatelet therapy (aspirin 75-325 mg daily or clopidogrel 75 mg daily) 5
• Aggressive cardiovascular risk factor modification 5, 6
• Wound care with appropriate offloading if ulceration is present 5, 6

Common Pitfalls to Avoid

• Assuming purpura is always benign: Failure to assess vascular status in patients with diabetes or age >50 can miss critical limb ischemia requiring urgent revascularization 5, 6
• Relying solely on ABI in diabetic patients: Arterial calcification can falsely elevate ABI; always measure toe pressures or TcPO2 if ABI >1.30 5, 6
• Missing renal involvement in HSP: Up to 50% of adults with HSP develop renal disease, which may not be apparent at initial presentation; serial urinalysis is mandatory 3
• Delaying skin biopsy when diagnosis is uncertain: Early biopsy prevents diagnostic delay and guides appropriate treatment, particularly when systemic vasculitis or malignancy is possible 4
• Treating with corticosteroids before excluding infection: Always rule out septic arthritis or necrotizing soft tissue infection before initiating immunosuppression 7, 4

Follow-Up Protocol

• Weekly visits for the first month to monitor for progression of purpura, development of new systemic symptoms, or worsening renal function 3
• Serial urinalysis: Weekly for 1 month, then monthly for 6 months to detect delayed renal involvement 3
• If vascular disease is present: Twice-yearly evaluation by vascular specialist due to high recurrence risk 5, 6