🦠 Septic Arthritis (SURGICAL EMERGENCY)
• Acute monoarthritis, fever, refusal to bear weight
• Kocher criteria: fever >38.5, non-weight-bearing, ESR >40, WBC >12,000
• Pathogens: S. aureus, K. kingae (<5y), Group A Strep, N. gonorrhoeae (adolescents)
• Synovial fluid WBC >50,000-100,000/mm³
🩺 Juvenile Idiopathic Arthritis (JIA)
• Arthritis ≥6 weeks, onset <16 years, exclude other causes
• Oligoarthritis (≤4 joints): most common subtype, ANA+, risk of uveitis
• Polyarthritis (≥5 joints): RF+ or RF-
• Enthesitis-related arthritis: HLA-B27+, sacroiliitis, enthesitis
⚠️ Other Inflammatory Arthritides
• Transient synovitis: post-viral, self-limited, normal CRP/ESR
• Lyme arthritis: late manifestation (weeks-months after tick bite), ELISA + Western blot
• Reactive arthritis: post-GI/GU infection, urethritis, conjunctivitis
• HSP arthritis: large joints, self-limited, resolves in weeks
📋 Diagnostic Keys
• Synovial fluid Gram stain and culture (critical for septic arthritis)
• JIA: diagnosis of exclusion; ANA, RF, anti-CCP, HLA-B27
• Lyme serology in endemic areas
• Imaging: ultrasound for effusion, x-ray for chronic changes, MRI for osteomyelitis
📌 Decision strategy: Acute monoarthritis + fever + non-weight-bearing → aspirate for septic arthritis. Chronic arthritis (>6 weeks) → JIA. Lyme arthritis in endemic areas; treat with doxycycline or amoxicillin.