What is the most appropriate next step in managing a febrile (feverish) child with a swollen, erythematous (red), and painful joint?

Management of a Child with Suspected Septic Arthritis

Arthrocentesis is the most appropriate next step in management for this 4-year-old girl with fever, erythematous and swollen knee, and refusal to walk. 1

Clinical Presentation Analysis

The child presents with classic signs of septic arthritis:

• Fever (38.6°C/101.5°F)
• Joint pain (crying when the knee is moved)
• Erythema and swelling of the knee
• Refusal to walk/bear weight
• Holding the knee in flexion
• Resistance to movement

These findings strongly suggest an infectious process in the joint that requires immediate diagnosis and treatment to prevent serious complications.

Diagnostic Approach

Why Arthrocentesis is Necessary

Arthrocentesis (joint aspiration) is essential because:

• It provides definitive diagnosis of septic arthritis versus other conditions 1
• It allows for identification of causative organisms through synovial fluid culture 1
• It helps distinguish between septic arthritis and other conditions like transient synovitis 2
• Delay in diagnosis and treatment of septic arthritis can lead to permanent joint damage, growth abnormalities, and significant morbidity 1, 3

Synovial Fluid Analysis

The aspirated fluid should be analyzed for:

• Cell count and differential (elevated WBC with neutrophil predominance suggests infection)
• Gram stain and culture
• Crystal analysis (to rule out crystal-induced arthritis)
• Glucose and protein levels 1, 2

Why Other Options Are Less Appropriate

• Acetaminophen with codeine (A): Only provides symptomatic relief without addressing the underlying infection
• Bone marrow aspiration (C): Not indicated for suspected septic arthritis
• Bone scan (D): Less sensitive than MRI and delays diagnosis of a potential emergency 1
• Immobilization and traction (E): Supportive measure only, doesn't provide diagnosis
• Lyme titer (F): While Lyme disease can cause arthritis, the acute presentation with fever and significant pain/swelling is more consistent with septic arthritis requiring immediate diagnosis 1
• MRI of spine (G): Not indicated for knee pathology
• Physical therapy (H): Contraindicated in acute infectious arthritis
• Reassurance (I): Inappropriate given the concerning clinical presentation
• Serum rheumatoid factor (J): Not helpful in acute presentation of suspected septic arthritis
• Systemic antibiotic therapy (K): Should be initiated after arthrocentesis to ensure proper organism identification and targeted therapy 1, 4

Management Algorithm

1. Perform arthrocentesis immediately:

   • Ultrasound guidance may be helpful, especially for difficult joints 2, 5
   • Send fluid for cell count, Gram stain, culture, crystal analysis 1

2. After arthrocentesis:

   • Start empiric antibiotics based on most likely pathogens (typically Staphylococcus aureus, Streptococcus species, or Kingella kingae in this age group) 1
   • Adjust antibiotics based on culture results and sensitivities

3. Additional management:

   • Joint rest and immobilization
   • Pain control
   • Monitor inflammatory markers (CRP, ESR)
   • Consider surgical drainage if poor response to antibiotics or if hip joint is involved 4

Common Pitfalls to Avoid

• Delaying arthrocentesis in favor of imaging studies
• Starting antibiotics before obtaining synovial fluid for culture
• Misdiagnosing as transient synovitis without proper fluid analysis
• Inadequate follow-up after initial treatment

Arthrocentesis remains the gold standard for diagnosis of joint infections and should be performed promptly in this clinical scenario to prevent serious complications and ensure optimal outcomes for the child.