Add POCUS to Pediatric Septic Hip Arthritis Assessments

A 2-year-old boy presents with a limp. His mom reports that he has seemed reluctant to put weight on his left leg since the morning. No trauma is reported. His vital signs are HR 135, RR 25, T 100.6. Exam of the affected extremity reveals no obvious deformity, but range of motion is limited. Plain films of the hip and femur are negative.Figure 1: Ultrasound of a normal pediatric hip (A) and hip with effusion (B, arrow).From Fleisher & Ludwig's Textbook of Pediatric Emergency Medicine (Chapter 142) by Bachur, R; Shaw K, 2015, Wolters Kluwer Health and Pharma.In this patient, the diagnosis of septic arthritis of the hip must be considered. This condition often presents with atraumatic hip pain, reduced range of motion, inability or refusal to bear weight and systemic symptoms such as fever and malaise. However, multiple other conditions can present similarly, making these patients a diagnostic challenge for emergency medicine physicians. Getting “routine labs” or advanced imaging on a 2-year-old is much more of an undertaking than in an adult. Additionally, physicians in the community may need to transfer patients with a high degree of suspicion to a tertiary center for testing or orthopedic evaluation. How can we add some weight to our suspicions? Certainly, obtaining blood work and plain radiographs may be helpful. Patients with septic arthritis often have elevated white blood cell counts (WBC) and inflammatory markers such as erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP). In one study, the findings of fever, inability to bear weight, ESR more than 40 mm/h, serum WBC count greater than 12,000 cells/l, and CRP greater than 1 mg/dL had predicted probabilities with 3, 4, or 5 of these factors present to be 83%, 93%, and 98%, respectively for septic arthritis.1 Although plain films can at times indicate an increased synovial space to suggest septic arthritis, they are often normal.2 Ultrasound can be very helpful in identifying a hip effusion, which in conjunction with clinical suspicion, can be useful in planning a hip aspiration for a definitive diagnosis. Although traditionally, these ultrasounds were performed by radiologists, this is often not practical as many departments do not have 24-hour coverage, and many sonographers do not perform musculoskeletal exams emergently. A recent systematic review and meta-analysis evaluated ultrasound performed by emergency medicine to identify pediatric hip effusions. This review included more than 500 pediatric patients and found that the specificity of emergency medicine-performed ultrasound was quite high, at 97%, with a sensitivity of 88.3 Identifying an effusion resulted in a positive likelihood ratio of 35. Ultrasound of the hip is relatively easy to perform. Next month, we will walk through how to do it.