What is the diagnosis and appropriate management for a patient presenting with a palpable rash on the legs and tender knees and ankles?

Palpable Rash on Legs with Tender Knees and Ankles

The most likely diagnosis is Henoch-Schönlein purpura (IgA vasculitis), which classically presents with palpable purpura on the lower extremities and arthritis/arthralgia of the knees and ankles, though life-threatening conditions including Rocky Mountain Spotted Fever and meningococcemia must be immediately excluded.

Immediate Life-Threatening Exclusions

Before considering benign causes, you must rule out conditions with high mortality:

Rocky Mountain Spotted Fever (RMSF)

• Start empiric doxycycline 100 mg twice daily immediately if RMSF cannot be excluded, as 50% of deaths occur within 9 days of illness onset 1, 2
• The rash typically begins as small blanching pink macules on ankles, wrists, or forearms evolving to maculopapular lesions with petechiae by day 5-6, though up to 20% lack rash entirely 3, 2
• Tick exposure history is present in only 60% of cases, so absence does not exclude diagnosis 2
• Associated symptoms include high fever, severe headache, myalgias, and the classic triad is present in only a minority at initial presentation 3, 2

Meningococcemia

• Presents with petechial or purpuric rash that can rapidly progress to purpura fulminans, typically with high fever, severe headache, and altered mental status 1, 2
• Add ceftriaxone empirically if meningococcemia cannot be excluded based on clinical presentation 1, 2
• Progresses more rapidly than RMSF 2

Critical Assessment Points

• Check vital signs immediately for fever, tachycardia, hypotension, or altered mental status indicating systemic toxicity 1
• Obtain detailed exposure history including tick exposure, recent trauma/surgery, IV drug use, travel to endemic areas, and new medications 1
• Examine for systemic toxicity signs: if present, hospitalize immediately and start empiric antibiotics without waiting for laboratory confirmation 2

Most Likely Diagnosis: Henoch-Schönlein Purpura (IgA Vasculitis)

Once life-threatening causes are excluded, the combination of palpable purpura on lower extremities with tender knees and ankles strongly suggests HSP:

Clinical Features

• Palpable purpura is the hallmark, representing small vessel vasculitis with leukocytoclastic changes 2
• Joint involvement occurs in 64-100% of cases, most frequently affecting knees, wrists, and ankles 3
• The arthritis is typically symmetric and associated with fever spikes 3

Distinguishing Features from Other Causes

• Distribution pattern: HSP characteristically affects lower extremities and dependent areas, while RMSF begins peripherally (ankles/wrists) and spreads centrally 3
• Palpability: True palpable purpura indicates vasculitis, while RMSF produces flat petechiae initially 3
• Systemic toxicity: HSP patients are generally less toxic-appearing than those with RMSF or meningococcemia 3, 1

Other Differential Diagnoses to Consider

Adult-Onset Still's Disease (AOSD)

• Can present with vasculitic purpuric rash and has been associated with mixed cryoglobulinemia 3, 4
• Arthritis affects knees, wrists, and ankles in symmetric polyarticular pattern 3
• Typically accompanied by high-spiking quotidian fevers (>39°C), salmon-pink evanescent rash on trunk/proximal limbs, and sore throat 3
• The typical AOSD rash is maculopapular and evanescent, not palpable purpura 3

Rheumatoid Arthritis with Vasculitis

• RA can present with vasculitic purpuric rash, though this is uncommon 4, 5
• Rheumatoid neutrophilic dermatitis presents as symmetric erythematous papules, plaques, or nodules over joints and extensor surfaces 6
• RA typically involves metacarpophalangeal, proximal interphalangeal joints, and wrists more than knees/ankles initially 7
• Morning stiffness lasting >1 hour is characteristic 7

Secondary Syphilis

• Can cause rash on palms and soles with systemic symptoms 3, 2
• Typically presents with maculopapular rash rather than palpable purpura 3

Drug Hypersensitivity Reactions

• Can cause rash on extremities including palms and soles 3
• Obtain medication history for new drugs 1

Diagnostic Workup

Essential Laboratory Studies

• Complete blood count with differential: assess for thrombocytopenia, leukopenia, or bandemia 1, 2
• Comprehensive metabolic panel: check for hyponatremia and elevated hepatic transaminases 1, 2
• Erythrocyte sedimentation rate and C-reactive protein: elevated in virtually all inflammatory conditions including AOSD and RA 3, 8
• Rheumatoid factor and anti-citrullinated protein antibody: if RA suspected 8
• Blood cultures: obtain before antibiotics if possible, but do not delay treatment 2

Skin Biopsy Considerations

• Biopsy the right lesion at the right time to obtain relevant histological information 5
• Palpable purpura shows leukocytoclastic vasculitis with perivascular inflammation 3, 5

Treatment Algorithm

If Systemic Toxicity Present

1. Start doxycycline 100 mg twice daily immediately (even in children <8 years old due to high RMSF mortality) 2
2. Add ceftriaxone if meningococcemia cannot be excluded 1, 2
3. Hospitalize for systemic toxicity, rapidly progressive rash, or diagnostic uncertainty 1, 2
4. Expect clinical improvement within 24-48 hours if rickettsial disease 1

If No Systemic Toxicity

• HSP is typically self-limited and requires supportive care
• Monitor for complications including renal involvement
• NSAIDs for joint pain if no contraindications
• Consider rheumatology referral if RA or AOSD suspected 8, 9

Critical Pitfalls to Avoid

• Do not wait for the classic triad of fever, rash, and tick bite in RMSF, as it is present in only a minority at initial presentation 2
• Do not exclude serious disease based on absence of rash, as up to 20% of RMSF cases and 50% of early meningococcal cases lack rash 2
• Do not delay doxycycline while awaiting laboratory confirmation if RMSF is in the differential 1, 2
• Rash on palms and soles is not pathognomonic for RMSF and occurs in multiple conditions 3
• The wooden-hard feel of subcutaneous tissues distinguishes necrotizing fasciitis from cellulitis, though this is unlikely with the described presentation 3