Acute Knee Swelling and Limp in a 10-year-old Child

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A 10-year-old Caucasian male, with no significant past medical history, presented with 6 days of limp and right knee swelling in August 2016. The patient lived in a suburb of Buffalo and also spent regular time at his father’s residence in rural central New York. The limp had initially been mild, but while walking around a county fair, his right knee suddenly became more swollen and painful. There was no improvement after 2 days of compression, ice and elevation. He had no history of trauma, fever or rash. He also had no nausea, vomiting, chills or other constitutional or focal symptoms.
In the emergency department, the patient was well-appearing, with a temperature of 37.1°C, heart rate of 125 beats/minute, respiratory rate of 20 breaths/minute and blood pressure of 115/84 mm Hg. His right knee was 2–3 times larger than his left knee, with no pain at baseline. He had tenderness with palpation above and throughout the joint, decreased flexion to 40 degrees and near-normal extension. All other joints were normal. He had no palpable lymphadenopathy, and the remainder of his physical exam was unremarkable.
Notable laboratory results included a white blood cell count of 8.4 × 109/L with 62.3% neutrophils, 25.5% lymphocytes, 10.4% monocytes, 1.4% eosinophils and 0.2% basophils; erythrocyte sedimentation rate of 88 mm/h; and C-reactive protein of 11.8 mg/dL. A radiograph of the right knee showed a joint effusion without bony abnormalities. Joint aspiration, which required multiple attempts, ultimately yielded 5 mL of serosanguinous drainage, containing 134,900/mm3 erythrocytes and 13,033/mm3 leukocytes, with 84% neutrophils, 10% bands, 4% lymphocytes and 2% monocytes.
The patient then underwent magnetic resonance imaging (MRI) with and without intravenous gadolinium, which demonstrated a large knee joint effusion with possible septations within its superior aspects, diffuse synovial enhancement, edema in the adjacent tissues, including the anterior thigh, and mildly enlarged lymph nodes within the superior popliteal fossae (Figures 1 and 2). These findings were interpreted by the radiologists as consistent with septic arthritis without osteomyelitis.
An arthroscopic incision and drainage was performed the next morning, and the joint fluid now contained 160,000/mm3 erythrocytes and 79,000/mm3 leukocytes, with 73% neutrophils, 18% bands, 3% lymphocytes and 6% monocytes. The patient reported that he felt much better immediately after this procedure. He was empirically treated with intravenous oxacillin. Admission blood culture and joint fluid Gram stain and culture were negative at 2 days. An additional test confirmed the diagnosis.
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