What are the differential diagnoses for a 10-year-old boy presenting with mild fever and hip pain?

Differential Diagnoses for a 10-Year-Old Boy with Mild Fever and Hip Pain

The most critical diagnosis to exclude immediately is septic arthritis, as it requires urgent intervention to prevent permanent joint damage, followed by consideration of transient synovitis (the most common cause in this age group), osteomyelitis, pyomyositis, and less commonly slipped capital femoral epiphysis or Legg-Calvé-Perthes disease. 1, 2

Immediate Exclusion: Life-Threatening and Limb-Threatening Conditions

Septic Arthritis (Medical Emergency)

• Must be ruled out first as it can cause permanent joint destruction if not treated urgently 1, 2, 3
• Suspect when fever is present with hip pain, particularly if temperature >37.5°C (99.5°F) 4
• Key clinical features suggesting septic arthritis over benign causes:
  • Severe pain or spasm with hip movement or palpation 4
  • Fever >38.5°C (101.3°F) 3
  • Inability to bear weight 3
  • ESR ≥40 mm/hr 3
  • WBC ≥12,000 cells/mm³ 3
  • C-reactive protein >2.0 mg/dL 3

Osteomyelitis

• Often associated with or mimics septic arthritis 1, 2
• May present with fever and localized bone tenderness 2
• Can occur in the femur or pelvis, causing hip pain 1

Pyomyositis

• Increasingly important differential in areas with high community-acquired MRSA prevalence 5, 6
• Infection of muscles around the hip (particularly obturator internus, iliopsoas) can mimic septic arthritis 6
• Presents with fever and hip pain but involves periarticular muscles rather than the joint itself 1, 6

Most Common Benign Diagnosis in This Age Group

Transient Synovitis (Coxitis Fugax)

• Most common cause of acute hip pain in children aged 3-10 years 4, 7
• Typically presents with hip pain for 1-3 days, limping, or refusal to bear weight 4
• Usually has milder fever (<38.5°C) or no fever 4, 7
• Diagnosis of exclusion after ruling out septic arthritis 4

Other Important Differentials

Legg-Calvé-Perthes Disease

• Idiopathic avascular necrosis of the femoral head 2, 7
• Common in children aged 4-10 years 7
• Usually presents with gradual onset of limp and hip pain, though can be acute 7
• Fever is not typical—its presence should raise concern for other diagnoses 7

Slipped Capital Femoral Epiphysis (SCFE)

• More common in children ≥10 years and adolescents 8, 7
• At the younger end of typical age range for a 10-year-old 7
• Classic presentation includes external rotation deformity of the affected leg 8
• Fever is not typical of SCFE 8

Referred Pain from Other Sources

• Hip pain can be referred from lumbar spine, sacroiliac joints, or knee pathology 1
• Appendicitis can present as hip pain in children, creating diagnostic confusion 9
• Pain from hip pathology can be referred to thigh, knee, or buttock 1, 2

Tumor (Rare but Must Consider)

• Bone tumors must always be considered in the differential 7
• Usually presents with night pain, progressive symptoms, and constitutional symptoms 7

Juvenile Idiopathic Arthritis

• Inflammatory condition that can present with hip pain and limp 2
• May have associated fever, though typically with more chronic presentation 2

Diagnostic Algorithm

Step 1: Initial Assessment

• Assess fever severity and pattern 2, 4
• Evaluate ability to bear weight 3, 4
• Examine for severe pain with hip movement (suggests septic arthritis) 4
• Check for external rotation deformity (suggests SCFE) 8

Step 2: Laboratory Tests (if infection suspected)

• CBC with differential, ESR, CRP 3, 4
• Blood cultures if septic arthritis suspected 2

Step 3: Initial Imaging

• Plain radiographs of hip and pelvis should be first imaging test 2, 3
  • Rules out fractures, SCFE, Perthes disease, tumors 2
• Ultrasound of the hip if infection suspected 2, 3
  • Detects joint effusion rapidly 1, 2
  • Can guide aspiration if needed 1, 2

Step 4: If Effusion Present on Ultrasound

• Ultrasound-guided aspiration for cell count, Gram stain, and culture 2, 3
• Synovial fluid WBC >50,000 cells/mm³ strongly suggests septic arthritis 3

Step 5: Advanced Imaging (if diagnosis unclear)

• MRI if osteomyelitis or pyomyositis suspected 1, 2, 6
• MRI shows decreased femoral head enhancement in septic arthritis (not seen in transient synovitis) 3

Critical Pitfalls to Avoid

• False-negative ultrasound can occur if performed within 24 hours of symptom onset 1
• Hip effusion is not specific for septic arthritis—can occur with fractures, transient synovitis, Perthes disease, or juvenile idiopathic arthritis 1
• Pain localization in children is unreliable—hip pathology commonly presents as knee or thigh pain 1, 2
• Mild fever does not exclude septic arthritis, though higher fever is more concerning 3, 4
• Normal radiographs do not exclude serious pathology—MRI may be needed for osteomyelitis, early Perthes disease, or stress fractures 1, 2