Osteomyelitis and Septic Arthritis in Children
Osteomyelitis (infection of the bone) and septic arthritis (infection of a joint) are serious bacterial infections in children that require prompt diagnosis and management to prevent long-term complications, such as chronic osteomyelitis or joint destruction. A high index of suspicion is required, especially in a child with fever and a painful, non-weight-bearing limb.
Aetiology and Pathophysiology
The most common route of infection is haematogenous spread from a distant focus, such as a skin infection or a tonsillitis. The metaphyseal region of long bones is particularly susceptible due to its rich blood supply and slow-moving blood flow, which allows bacteria to settle and proliferate.
Common Organisms:
Staphylococcus aureus: The most frequent cause across all age groups. Methicillin-resistant S. aureus (MRSA) should be considered in high-risk groups.
Kingella kingae: An increasingly recognised cause in children under 4 years of age, often presenting with subtle symptoms.
Group A and B Streptococcus: Significant causes, particularly in neonates and young infants.
Pseudomonas aeruginosa: Should be considered in children with a history of penetrating foot injury.
Clinical Presentation
The classic presentation is a child who is unwell, febrile, and refuses to use a limb or bear weight.
Osteomyelitis: The child may present with localised pain, swelling, and tenderness over the affected bone. The pain is often severe, constant, and not relieved by rest.
Septic Arthritis: This is a surgical emergency. The child presents with a hot, red, and swollen joint that is exquisitely painful on passive movement. The most commonly affected joints are the hip and knee.
Investigations
A combination of blood tests and imaging is required for diagnosis.
Bloods:
Full Blood Count (FBC): A raised white cell count (WCC) may be present, but can be normal.
Inflammatory Markers: Both C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) are typically elevated. CRP is a more sensitive and dynamic marker of acute infection.
Blood Cultures: It is essential to obtain blood cultures before starting antibiotics. These will be positive in approximately 50% of cases.
Imaging:
X-ray: May be normal in the early stages of the disease (first 7-10 days). It’s useful for ruling out other causes like a fracture or tumour.
Ultrasound: The preferred imaging modality for diagnosing septic arthritis. It can show a joint effusion, which may be aspirated for culture.
MRI: The most sensitive and specific imaging modality for both osteomyelitis and septic arthritis. It can show early bone and soft tissue changes and is useful for detecting an abscess.
Management
Management is a medical and surgical emergency that requires a collaborative approach between paediatricians, orthopaedic surgeons, and microbiologists.
Start Empirical Antibiotics Immediately: Do not delay the administration of intravenous antibiotics while awaiting culture results.
First-line: Flucloxacillin is the antibiotic of choice for suspected S. aureus infection.
Consider a broader-spectrum agent: Ceftriaxone or clindamycin should be added if MRSA or other organisms are a concern, particularly in very unwell children.
Surgical Intervention:
Septic Arthritis: This is a surgical emergency requiring urgent joint washout and drainage to prevent cartilage destruction.
Osteomyelitis: Surgical drainage is indicated if there is a subperiosteal or intraosseous abscess.
Monitoring: Monitor clinical improvement with serial CRP measurements, which should start to fall within 24-48 hours of effective treatment.
Duration of Treatment:
The total duration of antibiotic therapy is typically 3-6 weeks and is guided by clinical response and CRP levels.
Children usually start with intravenous antibiotics and are switched to oral antibiotics once they are clinically well and their inflammatory markers are trending down.