A toddler with a limp and fever
A 2-year-old girl is brought in with refusal to weight-bear on the right leg for two days, low-grade fever of 37.8°C and irritability. She has no history of trauma and no recent illness. On examination she is reluctant to move the right hip, with pain on passive internal rotation and limited abduction. There is no swelling or erythema of the knee, and the overlying skin is intact. Temperature is 37.8°C, pulse 120/min, and she is otherwise well appearing. White cell count is 13.5 × 10⁹/L, C-reactive protein is 40 mg/L, and erythrocyte sedimentation rate is 35 mm/h. Hip ultrasound shows a small joint effusion.
1. How do you distinguish transient synovitis from septic arthritis in this child, and what is the next step?
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The distinction matters because septic arthritis requires urgent drainage and intravenous antibiotics, whereas transient synovitis is self-limiting. Validated prediction tools such as the Kocher criteria use four features: inability to bear weight, fever above 38.5°C, white cell count above 12 × 10⁹/L and erythrocyte sedimentation rate above 40 mm/h. This child has two criteria: refusal to weight-bear and a white cell count of 13.5 × 10⁹/L, with a temperature below 38.5°C and an ESR below 40 mm/h. Two criteria make septic arthritis less likely but do not exclude it, and C-reactive protein is a useful additional marker. The next step is to perform a diagnostic hip aspiration under ultrasound guidance, sending fluid for cell count, Gram stain and culture, because this is the definitive test and also therapeutic if pus is obtained. Do not rely on ultrasound alone, since an effusion occurs in both conditions; ultrasound cannot reliably distinguish them. If aspiration is not immediately available, start empirical intravenous antibiotics after blood cultures and involve orthopaedics urgently, because delay in septic arthritis causes joint destruction. Admission for observation and serial examination is reasonable if aspiration is deferred, but a low threshold for intervention is essential in a child this age.
2. If the aspirate is purulent, outline the ongoing management and the factors that determine duration of antibiotics.
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Purulent aspirate confirms septic arthritis. Urgent orthopaedic drainage and washout of the hip is required, followed by intravenous antibiotics guided by Gram stain and culture, initially broad-spectrum and narrowed on sensitivities. Monitor clinical response with temperature, pain, range of movement and serial inflammatory markers. The duration of intravenous therapy depends on the organism, the adequacy of drainage, clinical improvement and inflammatory marker trends; typical regimens continue intravenous treatment for one to several weeks with a total course of several weeks, often transitioning to oral therapy once the child is afebrile, improving and markers are falling. Physiotherapy and follow-up are needed to detect sequelae such as avascular necrosis or growth disturbance.