What are the differential diagnoses for a previously healthy 5‑year‑old who presents with an acute limp, upper‑thigh/hip pain, and inability to bear weight?

Differential Diagnoses for a 5-Year-Old with Acute Limp, Hip/Thigh Pain, and Inability to Bear Weight

In a previously healthy 5-year-old presenting with acute hip/thigh pain and inability to bear weight, the primary differential diagnoses are septic arthritis (most urgent), transient synovitis (most common), occult fracture (especially toddler's fracture), osteomyelitis, and less commonly Legg-Calvé-Perthes disease or malignancy.

Critical Life-Threatening Diagnoses to Rule Out First

Septic Arthritis

• Most urgent diagnosis requiring immediate intervention to prevent permanent joint damage 1
• Typically caused by hematogenous spread of Staphylococcus aureus to the hip joint 2
• Look for: fever >101.3°F, refusal to bear weight, ill appearance, elevated inflammatory markers (ESR ≥40 mm/hr, CRP >2.0 mg/dL, WBC ≥12,000) 2, 3
• If suspected, ultrasound of the hip is the immediate next step to identify joint effusion and guide urgent arthrocentesis 1
• Requires emergent surgical drainage and IV antibiotics 2

Osteomyelitis

• Consider especially if no hip effusion is present on ultrasound 1
• Can involve the femoral metaphysis or pelvis (including sacrum, which can mimic hip pathology) 4
• May present with fever, localized bone tenderness, elevated inflammatory markers 2
• Can lead to subperiosteal abscess and bone necrosis if untreated 2

Most Common Benign Diagnosis

Transient Synovitis

• Accounts for 80-85% of atraumatic limping in children 3, 5
• Self-limiting, viral-associated condition 5
• Distinguished by: absence of fever or ill appearance, normal or only mildly elevated inflammatory markers 3, 5
• Symptoms resolve within 1 week with NSAIDs and observation 5
• Critical pitfall: Must definitively exclude septic arthritis first, as presentations overlap 6, 5

Traumatic Causes

Occult Fractures

• Toddler's fracture (nondisplaced spiral tibial fracture) is the most common fracture in children <5 years presenting with nonlocalized limp 1
• 41% of toddler's fractures are not visible on initial radiographs and only appear on follow-up imaging 1
• Other locations: ankle, foot, femur 1
• Even with localized hip/thigh pain, consider referred pain from tibial injury 1

Age-Specific Considerations at Age 5

Legg-Calvé-Perthes Disease

• Avascular necrosis of the femoral head, typically ages 4-8 years 3, 7
• Insidious onset of limp with groin or thigh pain
• Radiographs show degenerative femoral head changes 3

Developmental Dysplasia of the Hip (DDH)

• Less likely at age 5 unless previously undiagnosed
• Look for history of breech presentation, leg-length discrepancy 3

Rare but Important Diagnoses

Malignancy

• Consider if pain is primarily nocturnal 3
• Leukemia, bone tumors (osteosarcoma, Ewing sarcoma)
• Requires CBC with differential, ESR, CRP if suspected 3

Pelvic/Spinal Pathology

• Discitis, epidural abscess (can present as hip pain with inability to bear weight) 1
• Sacral osteomyelitis can mimic hip pathology 4
• Psoas abscess or pyomyositis 1

Algorithmic Approach to Differentiation

Step 1: Assess for infection urgency

• Fever >101.3°F + ill appearance + inability to bear weight = septic arthritis until proven otherwise 1
• Obtain: CBC, ESR, CRP, blood cultures
• Immediate hip ultrasound → if effusion present → urgent arthrocentesis 1

Step 2: If no fever/systemic signs

• Likely transient synovitis or occult fracture 3, 5
• Check inflammatory markers: normal/mildly elevated = transient synovitis 3
• Obtain radiographs of hip/pelvis AND tibia/fibula (even if pain seems localized to hip) 1

Step 3: If initial workup negative but symptoms persist

• Follow-up radiographs in 7-10 days (to detect initially occult toddler's fracture) 1
• Consider MRI if symptoms worsen or don't improve, to evaluate for osteomyelitis, stress fracture, or other pathology 1

Step 4: Red flags requiring expanded workup

• Sacral or spinal tenderness → MRI pelvis/spine for discitis, epidural abscess, sacral osteomyelitis 1, 4
• Night pain → consider malignancy, obtain CBC with differential 3
• No improvement after 1 week → reassess clinically and consider MRI 1

Critical Pitfalls to Avoid

• Do not assume hip pain originates from the hip: Pain can be referred from tibia, spine, pelvis, or abdomen 1, 7, 4
• Do not rely solely on initial radiographs: Up to 41% of toddler's fractures are radiographically occult initially 1
• Do not discharge without clear follow-up plan if initial evaluation is negative but symptoms persist 1
• Do not miss septic arthritis: Even one missed case can result in permanent joint destruction 1, 2
• Obtain inflammatory markers before diagnosing transient synovitis: Cannot be a clinical diagnosis alone 3, 5