What is the most common cause of bilateral lower extremity pain with fever and difficulty walking in a 1-year-old child?

Bilateral Lower Extremity Pain with Fever and Difficulty Walking in a 1-Year-Old

The most common cause is osteomyelitis with concurrent septic arthritis, which occurs in over 50% of pediatric bone and joint infection cases, particularly in children under 18 months of age where transphyseal vessels allow direct spread between bone and joint. 1, 2

Primary Diagnostic Considerations

Osteomyelitis with Septic Arthritis (Most Likely)

• In children ≤18 months, transphyseal vessels connect the metaphysis directly to the epiphysis and adjacent joint space, explaining why concurrent osteomyelitis and septic arthritis occur in >50% of cases in this age group 1, 2
• Staphylococcus aureus is the predominant pathogen across all pediatric age groups, responsible for the majority of acute hematogenous osteomyelitis cases 1
• At 1 year of age, Kingella kingae is also a common pathogen and should be considered 1, 2
• Half of all pediatric osteomyelitis cases occur in children <5 years of age, making this the highest-risk population 3, 1

Why Bilateral Involvement Suggests Infection

• Bilateral lower extremity involvement with fever and inability to walk strongly suggests a systemic infectious process rather than trauma or isolated mechanical causes 3
• In septicemic neonates and young infants, 32% have multiple joint involvement, and 76% of joint infections occur in association with generalized septicemic illness 4

Critical Clinical Features to Assess

Red Flags for Infection

• Inability to bear weight is significantly associated with bacterial infection (osteomyelitis, septic arthritis, or intramuscular abscess) 5
• Fever with elevated inflammatory markers (ESR >36 mm/hour or CRP >60 mg/L) strongly suggests osteomyelitis or septic arthritis 5
• Limitation of movement (64%) and local swelling (60%) are the most common presentations in neonates and young infants 4

Age-Specific Pathogen Considerations

• Group B streptococcus should be considered in neonates 1, 2
• Kingella kingae is common in children <4 years of age 1, 2
• Staphylococcus aureus remains the most common pathogen at all ages, averaging 53% of all cases 6

Diagnostic Approach

Initial Imaging Strategy

Radiographs of both lower extremities should be obtained first, though they are often normal in acute osteomyelitis as bone changes require 10-14 days to become visible 3, 1, 7

• Radiographs have low sensitivity for early osteomyelitis, with bone destruction not visible until >30% osseous matrix destruction has occurred 3
• However, radiographs are essential to exclude fractures or other bony pathologies 3, 7

Advanced Imaging When Infection Suspected

MRI is the diagnostic modality of choice with 82-100% sensitivity for osteomyelitis and septic arthritis 3, 1

• MRI can detect both osteomyelitis and septic arthritis simultaneously, which is critical given the >50% concurrent rate in this age group 3
• In children <18 months with community-acquired S. aureus infection, 7 of 9 cases demonstrated normal cartilage signal on noncontrast sequences, making contrast administration important for detecting epiphyseal involvement 3
• Contrast-enhanced MRI improves detection of infection in unossified growth cartilage (chondritis) in children <6 years, where cartilage commonly appears normal on unenhanced sequences 3
• Large field-of-view MRI covering both lower extremities can identify contralateral involvement in 11-20% of cases 3

Ultrasound Considerations

• Ultrasound is sensitive for detecting joint effusions and can guide aspiration if septic arthritis is suspected 7
• However, ultrasound cannot evaluate bone marrow and is not sensitive for osteomyelitis 3
• Ultrasound may detect pyomyositis and subperiosteal collections associated with osteomyelitis 3

Critical Pitfalls to Avoid

Delayed Diagnosis Consequences

• Delayed diagnosis results in premature physeal arrest or permanent joint damage, making prompt recognition essential despite the fact that the classic triad of fever, pain, and diminished mobility occurs in only ~50% of cases 1
• The poorest long-term prognosis is in neonates and infants, especially when the hip joint is involved either alone or with concomitant osteomyelitis 6

Diagnostic Challenges

• Pain from hip pathology can be referred to the thigh, knee, or buttock, potentially leading to misdiagnosis 7
• In young children <4 years, inability to verbalize and localize symptoms is common, requiring broader imaging coverage 3
• Initial radiographs may be negative; approximately 10% of fractures are only visible on follow-up radiographs 3, 7

Management Urgency

• Septic arthritis is an orthopedic emergency requiring urgent diagnosis and treatment to prevent permanent joint damage 2, 7
• When infection is clinically suspected, empiric antibiotic therapy should be started as soon as possible after obtaining cultures 8
• Empiric treatment for a 1-year-old should cover Staphylococcus aureus and Kingella kingae, typically with anti-staphylococcal penicillin or cephalosporin 8