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

(5-10 MHz) linear transducer is the preferred transducer to scan the relatively superficial pediatric hip. The transducer should be placed in the sagittal oblique plane, parallel to the long axis of the femoral neck (Fig. 1).

In this view, the femoral head, femoral neck, capsule, and iliopsoas muscle are visualized (Fig. 2). The capsular-synovial thickness should be measured from the anterior concavity of the femoral neck to the posterior surface of the iliopsoas muscle (Fig. 3). When performing hip ultrasound, both hips should be evaluated to allow for comparison widths of the anterior synovial fluid space. The split screen function is particularly useful to compare the two hips for the presence of joint effusion (Fig. 4).

Ultrasound Findings

With correct patient positioning and probe placement, the sonographer should readily identify the sonographic landmarks of the pediatric hip. These landmarks include the femoral head and neck, joint capsule, and iliopsoas muscle (Fig. 2). The joint capsule extends anteriorly to the femoral neck, and the iliopsoas muscle is anterior to the joint capsule. Normally, a small amount of physiologic fluid is present within the joint space.⁷

The effusion fluid may have variable echogenicity, appearing either hypoechoic, anechoic, or hyperechoic, which distends the joint capsule.⁸ The presence of a hip effusion is defined as an anterior synovial space thickness greater than 5 mm, measured from the concavity of the femoral neck to the posterior surface of the iliopsoas muscle, or greater than 2 mm difference when compared to the asymptomatic contralateral hip.⁵

These criteria stress the importance of comparison views, especially in the pediatric patient. Additionally, capsular thickening may be present secondary to an inflammatory process on the affected side, or the presence of an anechoic effusion may be difficult to distinguish from the joint capsule. These reasons, again, highlight the importance of scanning the contralateral side to determine the normal anatomy and tissue architecture.

The presence of an effusion is concerning for a pathological process. However, the distinction between sterile and inflammatory effusions cannot be determined based solely on the ultrasonographic appearance.⁵ The presence of a septic hip can result in profound morbidity and should prompt either emergent orthopedic consultation or, depending on the practice setting, immediate joint aspiration by the emergency physician, followed by microscopic evaluation of the joint fluid with microscopy and gram staining.⁹ Aspiration of sterile fluid should steer the diagnosis toward transient synovitis and away from a septic joint.

Pearls and Pitfalls

When performing musculoskeletal ultrasound, comparison views are important to help differentiate between a potential pathologic process and an anatomical variant. Bilateral effusions are possible, especially in patients with transient synovitis, where they occur in up to one-fourth of patients ultimately diagnosed with this disease entity.⁷

Ensuring that the probe is positioned perpendicular to the skin will help avoid anisotropy, which is an artifact that can falsely suggest fluid when none is present. This is especially important in toddlers, who have a large hypoechoic cartilaginous zone between the ossified nucleus of the femoral head and the joint capsule.⁹

Missing slipped capital femoral epiphysis is a theoretical pitfall when performing hip ultrasound on the older child with a painful hip or limp. The physician should additionally consider obtaining plain radiography in this clinical setting.⁷

References

1. Singer JL. The cause of gait disturbance in 425 pediatric patients. Pediatr. Emerg. Care 1985;1:7-10.
2. Miralles M, Gonzalez G, Pulpeiro JR, et al. Sonography of the painful hip in children: 500 consecutive cases. AJR 1989;152:579-82.
3. Shavit I, Eidelman M, Galbraith R. Sonography of the hip joint by the emergency physician: Its role in the evaluation of children presenting with acute limp. Pediatr. Emerg. Care 2006;22(8):570-3.
4. Minardi JJ, Lander OM. Septic hip arthritis: Diagnosis and arthrocentesis using bedside ultrasound. J. Emerg. Med. 2012 Jan. 26 [Epub ahead of print].
5. Tsung JW, Blaivas M. Emergency department diagnosis of pediatric hip effusion and guided arthrocentesis using point-of-care ultrasound. J. Emerg. Med. 2008;35(4):393-9.
6. Vieira RL, Levy JA. Bedside ultrasonography to identify hip effusions in pediatric patients. Ann. Emerg. Med. 2010;55(3):284-9.
7. Navarro OM, Parra DA. Pediatric musculoskeletal ultrasound. Ultrasound Clinics 2009;4(4):457-70.
8. Siegel MJ. Pediatric sonography. 4th ed. 2011. Philadelphia: Lippincott Williams & Wilkins. p. 624-6.
9. De Bruyn R. Pediatric ultrasound: How, why and when. 2nd ed. 2010. Edinburgh: Churchill Livingstone. p. 332-6.