Febrile Infant with a Limp
A 14-month-old boy is brought to the emergency department with 2 days of fever (38.9°C), irritability, and refusal to bear weight on the left leg. He has no rash, no respiratory symptoms, and no known sick contacts. He is up to date on immunizations. On examination he is fussy but consolable, holds the left leg flexed, and resists passive hip extension. There is no obvious swelling, erythema, or joint effusion. White blood cell count is 15,200/µL, CRP is 4.2 mg/dL, and ESR is 45 mm/h. Blood culture is pending. He has no prior imaging.
1. What is the differential diagnosis, and how would you use imaging to narrow it?
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The differential for a febrile, non-weight-bearing toddler includes septic arthritis, transient synovitis, osteomyelitis, and less commonly a fracture or malignancy. The Kocher criteria (fever >38.5°C, non-weight-bearing, ESR >40, WBC >12,000) are used to estimate the probability of septic arthritis; this child meets at least four, which is a high-probability score.
Imaging strategy:
- Plain radiographs of the hip and pelvis are a reasonable first step to exclude fracture or obvious osseous abnormality, but they are often normal early in osteomyelitis or septic arthritis.
- Ultrasound of the hip is the key next test: it can detect a joint effusion and guide aspiration. In septic arthritis, an effusion is expected, and joint aspiration is the definitive test. Ultrasound cannot distinguish septic from sterile effusion, so aspiration is required if an effusion is present.
- MRI with contrast is the modality of choice for osteomyelitis and for evaluating the extent of soft tissue involvement, but it should not delay aspiration if septic arthritis is suspected.
The judgment: in a child with a high Kocher score, do not let imaging delay joint aspiration. Ultrasound first, aspirate if effusion, and MRI if osteomyelitis is suspected after aspiration or if symptoms persist.
2. If ultrasound shows a small hip effusion and the joint aspirate has a white count of 60,000/µL with a negative Gram stain, how does that change management and imaging?
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A joint fluid WBC of 60,000/µL is in the septic arthritis range (typically >50,000/µL with neutrophil predominance), even with a negative Gram stain, which is negative in a substantial fraction of culture-proven cases. The child should be treated empirically for septic arthritis with IV antibiotics and orthopedic consultation for possible arthrotomy or serial aspiration, and blood and joint cultures should be followed.
Imaging after aspiration: MRI with contrast of the pelvis and left hip is indicated to assess for contiguous osteomyelitis (which can coexist with septic arthritis, especially in young children) and to evaluate for abscess or soft tissue extension. The presence of osteomyelitis changes the duration of antibiotic therapy (typically 3–4 weeks vs. 2–3 weeks for isolated arthritis) and may require surgical debridement.
If the aspirate had been sterile and the child improved, transient synovitis would be more likely, and MRI could be deferred with clinical follow-up. The judgment point is that the aspirate result, not the imaging, drives the decision to escalate.