What are the differential diagnoses and initial tests for a 7-year-old presenting with hip pain and limping?

Differential Diagnosis and Initial Tests for a 7-Year-Old with Hip Pain and Limping

Initial Diagnostic Approach

Radiographs of the hip and lower extremities should be the first imaging test for a 7-year-old presenting with hip pain and limping to rule out fractures, osteonecrosis, or other bony pathologies. 1

Key Differential Diagnoses

• Transient synovitis - Most common cause of acute limp in children without trauma (80-85% of cases) 2, 3
• Septic arthritis - Medical emergency requiring urgent diagnosis and treatment to prevent permanent joint damage 1
• Osteomyelitis - May present with hip pain and limp, often associated with septic arthritis 1
• Legg-Calvé-Perthes disease - Idiopathic avascular necrosis of the femoral head 4, 3
• Slipped capital femoral epiphysis - More common in overweight adolescents but can occur in younger children 4, 3
• Fracture - Particularly spiral tibial fractures ("toddler's fracture") 1
• Developmental dysplasia of the hip - May present with limp in walking children 4
• Juvenile idiopathic arthritis - Inflammatory condition affecting joints 1, 2
• Tumor - Bone or soft tissue neoplasms can present with pain and limp 1, 3
• Pelvic abscess - Can present with symptoms similar to transient synovitis 5

Initial Diagnostic Tests

Clinical Evaluation

• Assess for fever, which raises concern for infection (septic arthritis, osteomyelitis) 1, 2
• Evaluate for history of trauma 1
• Assess for systemic symptoms suggesting inflammatory conditions 2
• Check for limitation of hip movement, especially internal rotation and abduction 4

Laboratory Tests

• Complete blood count (CBC) - Elevated WBC (>12,000/mm³) suggests infection 2, 3
• Erythrocyte sedimentation rate (ESR) - Values >40 mm/hr raise concern for septic arthritis 2
• C-reactive protein (CRP) - Values >20 mg/L suggest infection 2, 3
• Blood cultures - If infection is suspected 1

Imaging Studies

First-Line Imaging

• Plain radiographs of the hip and pelvis - To evaluate for fractures, Legg-Calvé-Perthes disease, slipped capital femoral epiphysis, and other bony abnormalities 1
• Ultrasound of the hip - Particularly useful for detecting joint effusions and guiding aspiration if septic arthritis is suspected 1, 6

Second-Line Imaging (if initial tests are inconclusive)

• MRI of the hip/pelvis - High sensitivity for detecting soft tissue pathology, bone marrow changes, early stress reactions, and infection 1
• Bone scan - May be useful when symptoms cannot be well localized or for detecting multifocal disease 1

Diagnostic Algorithm

1. If fever present (>101.3°F/38.5°C) or elevated inflammatory markers:

   • Urgent ultrasound of the hip to assess for effusion 1
   • If effusion present, ultrasound-guided aspiration for cell count, Gram stain, and culture 1, 3
   • Consider MRI if osteomyelitis is suspected 1

2. If no fever but focal hip pain:

   • Plain radiographs of the hip and pelvis 1, 6
   • Ultrasound if radiographs are normal but symptoms persist 1, 6

3. If non-localized symptoms:

   • Radiographs of the lower extremities 1
   • If negative but symptoms persist, consider MRI of the affected area 1, 4

Common Pitfalls and Caveats

• Pain due to hip pathology can be referred to the thigh, knee, or buttock, leading to misdiagnosis 1, 4
• Negative radiographs do not completely exclude fractures; approximately 10% of tibial fractures are only visible on follow-up radiographs 1
• Septic arthritis requires rapid diagnosis and intervention to prevent permanent joint damage 1
• If initial evaluation is negative but symptoms persist or worsen, follow-up clinical reassessment and further imaging are necessary 1
• Ultrasound may be falsely negative if performed within 24 hours of symptom onset 1