What is the appropriate management and treatment for a 6-month-old infant diagnosed with pyogenic arthritis of the left hip?

Pyogenic Arthritis of the Left Hip in a 6-Month-Old: Infectious Disease Workup

For a 6-month-old with pyogenic hip arthritis, obtain blood cultures, joint fluid for Gram stain, culture, and PCR (especially for Kingella kingae), complete blood count, inflammatory markers (ESR/CRP), and consider whole-body MRI if clinical deterioration occurs or multifocal infection is suspected.

Immediate Diagnostic Workup

Microbiological Studies

• Blood cultures must be obtained before antibiotics to identify the causative organism, as only 64% of pediatric septic arthritis cases have an etiologic agent determined despite multiple culture sites 1
• Joint fluid aspiration is mandatory for Gram stain, aerobic and anaerobic cultures, and cell count with differential 2, 1
• PCR testing for Kingella kingae is critical in this age group (6 months to 4 years), as this organism is the most common pathogen in children 6 months to 4 years of age and has been greatly improved by targeted PCR techniques 2
• Staphylococcus aureus remains the most widespread pathogen at all ages and must be covered empirically 2, 1

Laboratory Studies

• Complete blood count with differential - though inflammatory markers are not always abnormal in septic arthritis 1
• C-reactive protein (CRP) - essential for monitoring treatment response and determining duration of therapy 3
• Erythrocyte sedimentation rate (ESR) - though platelet count and ESR frequently rise as clinical improvement occurs rather than at presentation 1
• Note that laboratory tests measuring inflammatory response are not always abnormal, so normal values do not exclude septic arthritis 1

Imaging Studies

Initial Imaging

• Ultrasound of the hip is the first-line imaging modality to confirm joint effusion and guide aspiration 4
• US allows quick and accurate diagnosis of joint effusion, though false-negatives can occur if performed within 24 hours of symptom onset 4
• Radiographs of the pelvis have low sensitivity and specificity for septic hip but should be obtained to exclude other pathology 4

Advanced Imaging for Complications

• MRI is especially useful for diagnosing complications when the child's condition worsens, including detection of concomitant osteomyelitis 2, 5
• MRI has high sensitivity for soft-tissue and bone marrow pathology, with high accuracy in diagnosing septic arthritis, osteomyelitis, and pyomyositis 4
• Whole-body MRI should be considered if multifocal osteomyelitis is suspected, as it provides total-body screening sensitive for detecting osseous abnormalities 4
• Tc-99m bone scan has high sensitivity for osteomyelitis diagnosis but lacks soft-tissue evaluation and anatomic detail 4

Critical Age-Specific Considerations

High-Risk Features in Infants <6 Months

• Development of sequelae is significantly associated with infection at age less than 6 months 1
• In neonates and infants, septic arthritis may occur secondary to spread of osteomyelitis into the adjacent joint 4
• Hip involvement in infants carries particularly high risk of permanent damage including growth disturbances, articular cartilage destruction, osteonecrosis, and lifelong deformities 2, 1
• Delay of 4 or more days in institution of treatment is significantly associated with sequelae 1

Expected Pathogens by Age

• At 6 months of age, expect Kingella kingae (most common 6 months to 4 years) or Staphylococcus aureus (all ages) 2, 1
• Haemophilus influenzae type b was historically common (82% of cases between 6-24 months), though less common now with vaccination 1

Common Pitfalls and Caveats

• Do not delay treatment waiting for culture results - empirical antibiotics must be started immediately after obtaining cultures 2
• Normal inflammatory markers do not exclude septic arthritis - clinical suspicion remains paramount 1
• Hip and shoulder joints require more aggressive surgical management due to high risk of sequelae 1, 3
• Concomitant osteomyelitis significantly worsens prognosis and should be actively sought with MRI if clinical response is poor 1, 5
• The functional prognosis depends highly on time elapsed before diagnosis and treatment initiation 2

Monitoring Treatment Response

• CRP levels should be monitored to guide duration of therapy, with treatment continuing until CRP decreases to ≤20 mg/L 3
• Clinical response should be assessed with preset criteria including resolution of fever, improved range of motion, and decreased pain 3
• Follow-up imaging may be needed if clinical deterioration occurs to assess for osteomyelitis development 5