Introduction

Children often present to the emergency department with a limp, a painful joint, or refusal to weight bear. The differential is broad, including transient synovitis, septic arthritis and trauma. Previously, a formal ultrasound done by the radiology department has been used to determine the presence or absence of an effusion in order to help narrow the differential. PoCUS has been increasingly used by pediatric emergency physicians to quickly assess for the presence of a joint effusion at the bedside. The rapid identification of an effusion can expedite necessary interventions, including consultation of subspecialists or arranging for arthrocentesis.

 

Why Ultrasound?

Currently, most children presenting with a limp, refusal to weight bear or joint pain and swelling receive some combination of x-ray, blood work and formal ultrasound in the radiology department. Ultrasound as an imaging modality has many advantages, including low cost and no radiation [1,2]. The use of ultrasound at the bedside provides many advantages in its ease of access, as well as speed and portability [1-2]. This is especially useful when children are presenting to the department outside the hours in which a formal ultrasound is readily available.

Physical examination for joint effusions can be limited by pain, patient cooperation or soft tissue changes. POCUS is superior to physical exam for the detection of effusions and can be used to differentiate swelling at a joint due to effusion and soft tissue change (e.g. cellulitis) [3]. From an imaging perspective, X-ray has been shown to have a sensitivity for identifying an effusion ranging between 20-28%, whereas the sensitivity of a formal ultrasound approaches 100% [4-5]. Many studies have shown that PoCUS can been used by emergency physicians to easily identify fluid or a joint effusion in the hip, knee and ankle [1,5,6,7]. For example, a study in 2010 comparing trained PEM physicians performing hip ultrasound to a formal ultrasound read by radiologists showed a sensitivity of 80% and specificity of 98%. Moreover, when the physician reported confidence in their exam, the specificity and positive predictive value improved to 100% [7]. In 2018 Cruz et al showed that PEM physicians were able to detect hip joint effusions with a sensitivity of 85% and specificity of 98% [2]. In 2010 Adhikari and Blavais examined the role of PoCUS in patients presenting with joint complaints. In patients with clinically suspected effusions in whom arthrocentesis was planned, management changed in 69.2% after PoCUS was performed. In those in whom arthrocentesis was not planned, management changed in 53.3% of cases after PoCUS was performed [8]. Clinically, the absence of a joint effusion can effectively rule out septic arthritis, which can prevent unnecessary interventions on patients.

Overall, the use of POCUS to identify joint effusions is safe, easy to perform, and a quick way to begin the work up for the child presenting to the emergency department with a limp.