What is the primary cause of osteomyelitis in pediatric patients?

Primary Causes of Osteomyelitis in Pediatric Patients

The most common cause of osteomyelitis in pediatric patients is hematogenous spread of bacteria, with Staphylococcus aureus being the predominant pathogen. 1

Pathophysiology and Microbiology

Predominant Pathogens

• Staphylococcus aureus is the most common causative organism across all pediatric age groups 1, 2, 3, 4
• Age-specific pathogens include:
  • Group B streptococcus in neonates 1
  • Kingella kingae in children under 4 years of age 1, 3
  • Salmonella species in patients with sickle cell disease 1
• Methicillin-resistant S. aureus (MRSA) has emerged as a significant pathogen, accounting for up to 31% of cases in more recent studies 2, 4

Mechanism of Infection

• Hematogenous seeding is the primary mechanism in children, particularly affecting the metaphyses of long bones 1
• The metaphysis is the most frequent site of involvement due to its unique vascular anatomy with looping nutrient vessels that do not traverse the physis 1
• In children under 18 months, infection can spread to the epiphysis due to the presence of transphyseal vessels 1
• Although hematogenous spread is most common, a history of minor trauma is frequently elicited 1, 4

Anatomical Distribution

• Most cases occur in the long bones 2
• Common sites include:
  • Femur (20-24%)
  • Tibia (16%)
  • Foot bones (23%)
  • Pelvis (7%) 4
• Over 50% of cases occur in children 5 years and younger 1

Clinical Presentation and Diagnosis

• The classic triad of fever, pain, and diminished mobility is present in just over 50% of cases of acute hematogenous osteomyelitis 1
• Localized pain (84%), fever (67%), and swelling (62%) are the most common presenting symptoms 4
• Diagnosis can be challenging, particularly in young children who may present with nonspecific symptoms like limp 1
• MRI is the imaging modality of choice with high sensitivity (82-100%) and specificity (75-96%) 1, 3
• Plain radiographs should be the initial imaging test, though they are often not diagnostic in acute cases 1

Complications and Prognosis

• Local complications are common, particularly with MRSA infections (49% of complicated cases) 2
• Hematogenous complications may include deep venous thrombosis, septic pulmonary emboli, and endophthalmitis 2
• Delayed diagnosis may result in premature physeal arrest or joint damage 1
• MRSA osteomyelitis is associated with worse clinical outcomes, including longer hospital stays, extended antibiotic therapy, and higher complication rates 4

Special Considerations

• Concurrent infections: Concomitant joint and bone infections are common in children and may occur in >50% of cases 1
• Rare causes: While bacterial causes predominate, fungal osteomyelitis should be considered in cases that don't respond to conventional treatment, even in immunocompetent children 5
• Age distribution: The distribution of osteomyelitis varies by age:
  • Children under 2 years are more likely to have septic arthritis than osteomyelitis
  • Children 2-10 years old have slightly more osteomyelitis than septic arthritis
  • Children 10-18 years old have slightly more septic arthritis than osteomyelitis 1

Management Approach

• Most cases of early osteomyelitis without drainable abscess can be treated with a short course of intravenous antibiotics followed by at least 3 weeks of oral antibiotics 3
• Surgical management is indicated when there is an abscess or failure of antibiotic therapy 3
• Empiric antibiotic coverage should include activity against MRSA given its increasing prevalence 2
• Operative cultures have high yield (84%) even after antibiotics have been started, but treatment should not be delayed until cultures are obtained 2

Understanding the primary bacterial cause and age-specific pathogens is crucial for prompt diagnosis and appropriate treatment of pediatric osteomyelitis, which can significantly reduce morbidity and improve outcomes.