Measles

Septic arthritis is bacterial infection of a synovial joint and is an orthopaedic emergency. Rapid bacterial proliferation and intense inflammation can destroy articular cartilage within days, particularly in the hip.

Staphylococcus aureus is the major pathogen. Kingella kingae is important in toddlers, while pneumococcus, group A Streptococcus and Gram negative organisms occur less commonly. Adjacent osteomyelitis may coexist.

The classic presentation is acute painful restriction of a single joint with fever.

• Severe joint pain
• Refusal to move the limb
• Limp or complete refusal to weight bear
• Joint swelling, warmth and effusion
• Marked pain on passive movement
• Hip held flexed, abducted and externally rotated when the hip is involved

Infants may present only with irritability, pseudoparalysis or poor feeding.

Septic arthritis should be considered urgently in any febrile child who refuses to move a joint.

• Obtain blood cultures, FBC, CRP and ESR.

• Ultrasound is useful for detecting joint effusion, particularly at the hip, but cannot distinguish sterile from infected fluid.

• Joint aspiration is central. Send synovial fluid for Gram stain, cell count, differential, culture and susceptibility; molecular testing can increase yield, particularly for K. kingae.

• MRI is valuable when adjacent osteomyelitis, deep infection or pelvic disease is suspected.

• A high synovial white cell count with neutrophil predominance strongly supports bacterial arthritis, but no single numerical threshold completely confirms or excludes infection.

• Obtain cultures and begin IV antibiotics promptly. Empirical treatment must cover S. aureus.

• Where MRSA prevalence is low, cefazolin 50 mg/kg IV every 8 hours, maximum 2 g per dose, is appropriate.

• Where MRSA is significant, clindamycin 10 to 13 mg/kg IV every 8 hours, maximum 900 mg per dose, can be used if susceptibility is reliable; otherwise use vancomycin with therapeutic drug monitoring.

• Drainage is required for most substantial purulent joint infections, by repeated aspiration, arthroscopic washout or open drainage depending on joint, disease severity and local expertise.

• Switch to appropriate oral therapy when fever resolves, joint function clearly improves and CRP is falling.

• Primary bacterial arthritis without adjacent osteomyelitis may require only 10 to 14 days total therapy when source control is adequate and clinical and CRP response is rapid. Slower response or inadequate drainage generally requires 21 to 28 days.