Introduction and Clinical Indications

Acute onset of limp or refusal to bear weight is a common presenting complaint in the pediatric emergency department (PED).¹ History and physical examination may be limited by the child’s age and ability to cooperate. With a broad differential, including infectious, traumatic, inflammatory, intra-abdominal, hematologic, and other musculoskeletal disorders as etiologies, it is imperative that the emergency department workup be thorough.

Even when the pain can be localized to the hip, the differential diagnosis remains broad (Table 1). The history and physical examination can help guide the differential diagnosis. When there is a high clinical suspicion for infectious or inflammatory pathology (fevers, painful range of motion, overlying erythema), laboratory studies, including a blood culture, complete blood cell count, C-reactive protein, and an erythrocyte sedimentation rate, are indicated.

Plain radiographs can screen for fractures, avascular necrosis, and destructive lesions but have limited utility for detecting joint effusions. Ultrasound is an excellent modality for identifying joint effusions, and effusion detection focuses the differential diagnosis toward osteomyelitis, transient synovitis, or septic arthritis and away from neoplasms, avascular necrosis, slipped capital femoral epiphysis (SCFE), or Legg-Calve-Perthes disease.

Point-of-care ultrasound to detect hip effusion can serve as an adjunct to the history and physical examination in the evaluation of hip pain in the pediatric population. It is an ideal imaging modality in pediatric patients due to its ease of use, portability, reproducibility, low cost, and, perhaps most important, lack of radiation exposure.

Radiologists conducted one of the first studies evaluating hip ultrasound in 1989, where they analyzed 500 consecutive cases. They found ultrasound to have a higher sensitivity and specificity in detecting hip effusions, compared with radiographs, when final results of arthrocentesis or follow-up sonography were compared at 2 weeks.² Since that initial publication, there have been a number of case

reports demonstrating the ability of Pediatric Emergency Medicine (PEM) physicians to use point-of-care ultrasound to detect hip effusions and diagnose transient synovitis and septic arthritis,^3,4,5 as well as to guide arthrocentesis to obtain synovial fluid for analysis.^4,5

Most recently, a prospective study from Children’s Hospital Boston evaluated 28 children who required hip ultrasound as part of their PED management.⁶ PEM physicians with focused training in point-of-care hip ultrasound evaluated patients’ symptomatic hips and were able to detect hip effusions with a sensitivity of 85%, specificity of 93%, positive predictive value of 92%, and negative predictive value of 88%, compared with radiology department ultrasound. This study was the first of its kind to demonstrate that PEM physicians with focused training could use point-of-care ultrasound to identify hip effusions in PED patients.

Performing the Ultrasound

Patients should be placed supine with legs extended in the neutral position. The hip to be scanned should be exposed, with drapes placed to preserve patient comfort. A high-frequency