What is the differential diagnosis, appropriate workup, and management for a petechial rash limited to the ankles and shins with burning pain?

Petechial Rash Limited to Lower Extremities with Burning Pain

This presentation most likely represents cutaneous small-vessel vasculitis (leukocytoclastic vasculitis), though Rocky Mountain Spotted Fever must be urgently excluded if there is any fever, systemic symptoms, or tick exposure history.

Differential Diagnosis

The key differentiating features are the localized distribution to lower extremities and burning pain, which narrow the differential significantly:

Primary Considerations:

• Cutaneous Small-Vessel Vasculitis (Leukocytoclastic Vasculitis): Most likely given the classic distribution to dependent areas (ankles/shins), palpable purpura, and burning sensation 1, 2. This typically presents with symmetrical, non-blanching purpuric lesions concentrated on lower extremities, often with a visible "sock line" where pressure was applied 3.

• Rocky Mountain Spotted Fever (RMSF): Critical to exclude. The rash classically begins as small blanching macules on ankles, wrists, or forearms that become petechial 4. However, RMSF typically presents with fever, headache, and systemic symptoms 3-12 days after tick exposure, and the rash appears 2-4 days after fever onset 4. The absence of fever and systemic symptoms makes this less likely, but do not wait for the classic triad (fever, rash, tick bite) as it's present in only a minority of initial presentations 4.

• Disseminated Gonococcal Infection: Can present with petechial/purpuric rash on lower extremities, but typically includes tenosynovitis, migratory polyarthralgia, and fever 5. The burning pain alone without joint involvement makes this less likely.

• Thrombotic Thrombocytopenic Purpura (TTP): Rare but life-threatening. Petechial rash can be the presenting sign, though typically accompanied by thrombocytopenia and hemolytic anemia 6.

Immediate Workup Algorithm

Step 1: Assess for Life-Threatening Causes

• Temperature: Fever suggests RMSF, disseminated gonococcal infection, or meningococcemia
• Tick exposure history: Recent outdoor activities, travel to endemic areas (even without recalled tick bite) 4
• Neurological symptoms: Headache, altered mental status (RMSF, TTP)
• Joint symptoms: Tenosynovitis, arthralgia (gonococcal infection)

Step 2: Initial Laboratory Tests

• Complete blood count with differential: Look for thrombocytopenia (TTP, RMSF), leukopenia 4
• Comprehensive metabolic panel: Hyponatremia (RMSF), renal dysfunction 4
• Urinalysis with microscopy: Hematuria without proteinuria suggests vasculitis 3; dysmorphic RBCs >12% suggest glomerular involvement 2
• Inflammatory markers: ESR, CRP (elevated in vasculitis) 2
• Coagulation studies: If concerned for TTP or meningococcemia

Step 3: Additional Testing Based on Clinical Suspicion

• Skin biopsy (punch biopsy): Gold standard for vasculitis diagnosis, showing leukocytoclastic vasculitis with neutrophilic infiltrate and fibrinoid necrosis 1, 2. Note: May be negative if performed after acute inflammation resolves 2.
• Autoimmune serologies: ANA, ANCA, complement levels, cryoglobulins (if systemic vasculitis suspected)
• Infectious workup: Blood cultures, NAAT for gonorrhea/chlamydia (if sexually active), viral serologies if recent pharyngitis or URI 3

Management Approach

If Fever or Systemic Symptoms Present:

Empiric doxycycline 100 mg twice daily immediately while awaiting confirmatory testing for RMSF 4. Do not delay treatment waiting for rash to become generalized or for laboratory confirmation—RMSF has 5-10% case-fatality rate and early treatment is critical 4.

If Afebrile with Isolated Lower Extremity Petechiae:

1. Discontinue potential triggers: Review medications started within past 2-3 weeks (antibiotics like clindamycin are common culprits) 3

2. Supportive care initially:

   • Leg elevation to reduce dependent edema
   • Avoid prolonged standing
   • NSAIDs for pain (if no contraindications)

3. Steroid therapy if biopsy-confirmed vasculitis:

   • Prednisone taper (typically starting 40-60 mg daily, tapered over 2-4 weeks) 1
   • Reserve for symptomatic relief or extensive disease

4. Avoid unnecessary antibiotics: Misdiagnosis as cellulitis is common with lower extremity vasculitis 1, 2. Cellulitis is typically unilateral, warm, and has indistinct borders—not bilateral petechiae with sharp demarcation.

Critical Pitfalls to Avoid

• Do not dismiss based on absence of fever or tick bite history: <50% of RMSF patients have rash in first 3 days, and many don't recall tick exposure 4
• Do not wait for classic RMSF triad: Only present in minority of initial presentations 4
• Do not assume cellulitis: Bilateral, symmetrical petechiae with burning pain is not cellulitis 1, 2
• Do not delay skin biopsy: Perform within 24-48 hours of rash appearance for highest diagnostic yield 2
• Do not overlook medication history: Recent antibiotics, NSAIDs, or other drugs are common triggers for hypersensitivity vasculitis 3

In the absence of fever and systemic symptoms, with bilateral lower extremity distribution and burning pain, proceed with vasculitis workup including skin biopsy, but maintain high vigilance for evolving RMSF if any systemic symptoms develop.