How should a child presenting with fever and knee pain be evaluated and managed?

Child with Fever and Knee Pain: Evaluation and Management

A child presenting with fever and knee pain must be presumed to have septic arthritis until proven otherwise, requiring immediate joint aspiration, blood cultures, and empiric IV antibiotics after cultures are obtained, because bacterial proliferation can cause irreversible cartilage damage within hours to days. 1

Initial Clinical Assessment

Critical Red Flags Requiring Immediate Action

• Fever >101.3°F (38.5°C), inability to bear weight, and acute monoarticular joint pain with swelling constitute the classic triad, though this occurs in only 50% of cases 1
• Assess for signs of sepsis including altered consciousness, severe lethargy, petechial/purpuric rash, or hemodynamic instability 2
• Document accurate rectal temperature (≥38.0°C/100.4°F defines fever in children) 2, 3
• Evaluate for respiratory distress, oxygen saturation ≤92%, or signs of meningism 2

Diagnostic Criteria for Septic Arthritis

Meeting all of the following Kocher criteria approaches 100% likelihood of septic arthritis: 1

• Fever >101.3°F (38.5°C)
• Erythrocyte sedimentation rate (ESR) ≥40 mm/hour
• White blood cell count ≥12,000 cells/mm³
• Inability to bear weight
• C-reactive protein (CRP) >2.0 mg/dL

In a Lyme-endemic area, fever at presentation, negative anti-streptolysin-O (ASO), ESR >40 mm/hr, and CRP >20 mg/L are most predictive of septic arthritis versus transient synovitis or Lyme arthritis 4

Diagnostic Algorithm

Laboratory Evaluation

• Obtain blood cultures before antibiotics if serious bacterial infection is suspected 2
• Send complete blood count, ESR, and CRP 1, 4
• Joint aspiration is the definitive diagnostic procedure and must not be delayed 1
• Synovial fluid white blood cell count ≥50,000 cells/mm³ is suggestive of septic arthritis 1
• Synovial fluid culture is positive in approximately 80% of non-gonococcal cases 1
• Gram stain and culture have sensitivity 0.76 and specificity 0.96 for distinguishing septic from crystal arthropathy 1

Critical pitfall: Negative synovial fluid culture does not exclude infection—if clinical suspicion remains high, consider percutaneous bone biopsy to evaluate for concurrent osteomyelitis 1

Imaging Protocol

Step 1: Plain Radiographs 1

• Always obtain knee radiographs first to exclude fractures, tumors, and other bony pathology 1
• Radiographs have low sensitivity for septic arthritis and are often normal in early infection (<14 days) 1
• Normal radiographs should not delay further evaluation when clinical suspicion remains high 1

Step 2: Ultrasound 1

• Musculoskeletal ultrasound can identify joint effusions and guide aspiration 1
• Bedside aspiration can be performed for the knee joint, unlike the hip which requires ultrasound guidance 1
• Ultrasound may yield false-negative results if performed within 24 hours of symptom onset 1

Step 3: MRI with Contrast 1

• Order MRI when clinical suspicion remains high despite negative aspiration, concern for concurrent osteomyelitis exists, or need to assess soft tissue abscess 1
• MRI has 82-100% sensitivity and 75-96% specificity for diagnosing septic arthritis and osteomyelitis 1
• Over 50% of pediatric patients with septic arthritis have infection beyond the joint space on MRI 1
• Early post-contrast MRI showing decreased femoral head enhancement is specific for septic arthritis and distinguishes it from transient synovitis 1

Critical pitfall: MRI should complement, not replace, joint aspiration 1

Immediate Management

Surgical Intervention

Immediate surgical drainage combined with appropriate antibiotic therapy is mandatory 1

• Surgical drainage is required in all cases of septic arthritis 1
• Repeated needle aspiration alone fails in 46% of cases 1
• Surgical drainage is mandatory when symptoms persist >7 days, patient has severe sepsis, or hip/shoulder involvement 1

Empiric Antibiotic Therapy

Start empiric antibiotics immediately after cultures are obtained: 1

Pediatric dosing:

• IV vancomycin 15 mg/kg/dose every 6 hours for MRSA coverage 1
• Alternative: Clindamycin 10-13 mg/kg/dose IV every 6-8 hours if local clindamycin resistance is low 1
• Consider adding coverage for Kingella kingae in children <4 years old 1
• Staphylococcus aureus is the most common pathogen overall 1, 5

Transition to Oral Therapy

Oral antibiotics are not inferior to IV therapy and can be initiated after 2-4 days if: 1

• Patient is clinically improving
• Afebrile
• Tolerating oral intake

Treatment Duration

• 3-4 weeks total for uncomplicated bacterial arthritis 1
• Recent evidence suggests 2 weeks may be adequate after surgical drainage in select cases 1
• Longer treatment required for concomitant osteomyelitis 1

Monitoring and Follow-up

Response Assessment

• Monitor CRP and ESR to assess treatment response 1
• Monitor vancomycin trough levels to adjust for toxicity 1
• In children whose symptoms worsen or persist despite initial treatment, obtain repeat MRI, as this leads to management changes in approximately 21% of cases 1

Concurrent Osteomyelitis

Critical consideration: Concurrent osteomyelitis occurs in approximately 58% of pediatric elbow septic arthritis cases and 40% of septic elbow arthritis patients who don't respond to antibiotics have concurrent osteomyelitis 1

MRI should be obtained when clinical response to antibiotics is suboptimal because concurrent osteomyelitis is present in >50% of pediatric patients with septic arthritis 1

Differential Diagnosis to Consider

While septic arthritis must be presumed, consider these mimics: 1, 6

• Transient synovitis (most common mimic in children, but cannot be assumed without excluding septic arthritis first) 1
• Lyme arthritis (in endemic areas—check Lyme serology) 4
• Crystal arthropathies (perform crystal analysis even if infection suspected) 1
• Reactive arthritis (typically preceded by GI/GU infection) 1
• Osteomyelitis (may present with similar findings) 1, 5

Disposition

Hospitalization is indicated for: 2

• Any red flag signs
• Suspected septic arthritis requiring parenteral antibiotics and surgical drainage
• Inability of caregivers to monitor or return for follow-up

Parent education for outpatient follow-up (if discharged after initial treatment): 2

• Return immediately for worsening symptoms, development of respiratory distress, altered consciousness, signs of dehydration, fever persisting ≥5 days, or any rash development