🦴 Chapter 33: Arthritis

Nelson's Pediatric Symptom-Based Diagnosis | Juvenile Idiopathic Arthritis · Septic Arthritis · Lyme Arthritis · Systemic Lupus Erythematosus · Uveitis · Rheumatic Fever

🔍 Arthritis in Children: Key Concepts

📊 Definition of Arthritis
Joint swelling OR limitation of motion with pain/tenderness/warmth. Distinguish from arthralgia (pain without inflammation). Morning stiffness improves with activity.
🔄 Juvenile Idiopathic Arthritis (JIA)
Arthritis ≥6 weeks, onset <16 years. Subtypes: oligoarticular (most common, ANA+, uveitis risk), polyarticular (RF+ poor prognosis), systemic (quotidian fever, salmon rash), enthesitis-related, psoriatic.
⚠️ Red Flags for Septic Arthritis
Acute monoarthritis, fever, refusal to bear weight, high ESR/CRP. Most common pathogens: S. aureus, Kingella kingae (young children), N. gonorrhoeae (adolescents). Requires emergent drainage.
💧 Lyme Arthritis
Episodic monoarthritis (knee), history of tick bite/erythema migrans. Endemic NE/Midwest US. ELISA + Western blot. Treat with doxycycline (≥8y) or amoxicillin.
🧬 Systemic Lupus Erythematosus (SLE)
Malar rash, arthritis, nephritis, ANA+, anti-dsDNA, low C3/C4. Non-erosive arthritis. More severe in children than adults.
🚩 Rheumatic Fever Arthritis
Migratory polyarthritis of large joints, extremely painful, dramatic response to NSAIDs. Jones criteria: 2 major or 1 major + 2 minor + evidence of GAS infection.

💡 Key Takeaway: Differentiate inflammatory arthritis (swelling, morning stiffness) from mechanical pain. JIA is a diagnosis of exclusion. Septic arthritis and malignancy (leukemia) are emergencies that must be ruled out. All children with JIA need regular ophthalmology exams for uveitis.

🩺 Clinical Approach to Arthritis: Step-by-Step Algorithm

🔹 Step 1: Is it truly arthritis?
• Swelling OR limited ROM with pain/tenderness/warmth.
• Morning stiffness >30 min improves with activity.
• Arthralgia = pain without inflammation (normal exam, no swelling).
🔹 Step 2: Acute vs chronic (>6 weeks)
• Acute (<6 weeks): infection (septic, Lyme, viral), trauma, rheumatic fever, reactive arthritis.
• Chronic (≥6 weeks): JIA, SLE, vasculitis, inflammatory bowel disease arthritis.
🔹 Step 3: Monoarthritis vs polyarthritis
• Monoarticular: septic arthritis, Lyme, transient synovitis, osteomyelitis, trauma, JIA oligoarticular.
• Polyarticular: JIA, SLE, reactive arthritis, post-streptococcal, Henoch-Schönlein purpura.
🔹 Step 4: Red flags requiring urgent action
• Septic arthritis: fever, refusal to bear weight, toxic appearance → joint aspiration + IV antibiotics.
• Malignancy (leukemia): night pain, pallor, bruising, cytopenias → CBC, bone marrow.
• Spinal cord compression: weakness, bowel/bladder dysfunction → emergent MRI.
🔹 Step 5: Diagnostic workup
• CBC, ESR, CRP (inflammatory markers).
• ANA (JIA risk for uveitis; SLE screen), RF (only in polyarticular RF+).
• Lyme ELISA/Western blot if endemic area.
• Joint aspiration: cell count, Gram stain, culture, PCR for Kingella.
• X-ray (rule out fracture, tumor), ultrasound for hip effusion.