Focus On: Pediatric Hip Ultrasound

Contributors
Dr. Jennifer Martin is Ultrasound Fellow in the Department of Emergency Medicine at St. Luke’s Roosevelt Hospital. Dr. Lorraine Ng is Pediatric Emergency Medicine Fellow at Children's Hospital at Montifiore Medical Center. Dr. Turandot Saul is Fellowship Director and Ultrasound Fellow in the Department of Emergency Medicine at St. Luke’s Roosevelt Hospital. Dr. Resa Lewiss is Division Director and Ultrasound Fellow in the Department of Emergency Medicine at St. Luke’s Roosevelt Hospital.

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Introduction and Clinical Indications

Acute onset of limp or refusal to bear weight is a commonpresenting complaint in the Pediatric Emergency Department(PED).¹ History and physical examination may be limitedby the child's age and ability to cooperate. With a broaddifferential, including infectious, traumatic, inflammatory,intra-abdominal, hematologic, and other musculoskeletal disordersas etiologies, it is imperative that the emergency department workup be thorough.

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

Plain radiographs can screen for fractures, avascular necrosisand destructive lesions, but have limited utility for detectingjoint effusions.  Ultrasound is an excellent modality foridentifying joint effusions, and detection of an effusion focusesthe differential diagnosis toward osteomyelitis, transientsynovitis, or septic arthritis, and away from neoplasms, avascularnecrosis, slipped capital femoral epiphysis (SCFE) orLegg-Calvé-Perthes disease.

Point of care ultrasound to detect hip effusion can serve as anadjunct to the history and physical examination in the evaluationof hip pain in the pediatric population.  It is an idealimaging 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 hipultrasound in 1989, where they analyzed 500 consecutivecases.  They found ultrasound to have a higher sensitivity andspecificity in detecting hip effusions than plain radiographs whencompared to final results of arthrocentesis or follow-up sonographyat 2 weeks.²   Since that initial publication,there have been a number of case reports demonstrating the abilityof pediatric emergency medicine (PEM) physicians to use point ofcare ultrasound to detect hip effusions; diagnose transientsynovitis and septic arthritis;^3,4,5 and guidearthrocentesis to obtain synovial fluid foranalysis.^4,5

Most recently, a prospective study from Children's HospitalBoston evaluated 28 children who required hip ultrasound as part oftheir PED management.⁶ PEM physicians with focusedtraining in point of care hip ultrasound evaluated patients'symptomatic hips and were able to detect hip effusions with asensitivity of 85%, specificity of 93%, positive predictive valueof 92% and negative predictive value of 88% when compared toradiology department ultrasound.  This study was the first ofits kind to demonstrate that PEM physicians with focused trainingcould use point of care ultrasound to identify hip effusions in PEDpatients.

Performing the Ultrasound

Patients should be placed supine with legs extended in theneutral position. The hip to be scanned should be exposed, withdrapes placed to preserve patient comfort. A high-frequency (5-10MHz) linear transducer is the preferred transducer to scan therelatively superficial pediatric hip. The transducer should beplaced in the sagittal oblique plane, parallel to the long axis ofthe femoral neck (Figure 1).

In this view, the femoral head, femoral neck, capsule, andiliopsoas muscle are visualized (Figure 2). The capsular-synovialthickness should be measured from the anterior concavity of thefemoral neck to the posterior surface of the iliopsoas muscle (Figure 3). When performing hip ultrasound, both hips should beevaluated to allow for comparison widths of the anterior synovialfluid space.  The split screen function is particularly usefulto compare the two hips for the presence of joint effusion. (Figure4)

US Findings 

With correct patient positioning and probe placement, thesonographer should readily identify the sonographic landmarks ofthe pediatric hip. These landmarks include the femoral head andneck, joint capsule, and iliospoas muscle (Figure 2). The jointcapsule extends anteriorly to the femoral neck, and the iliopsoasmuscle is anterior to the joint capsule. Normally, a small amountof physiologic fluid is present within the jointspace.⁸

The effusion may have variable echogenicity, appearing eitherhypoechoic, anechoic, or hyperechoic, and distends the jointcapsule.⁹  The presence of a hip effusion isdefined as an anterior synovial space thickness > 5 mm, measuredfrom the concavity of the femoral neck to the posterior surface ofthe iliopsoas muscle, or > 2mm difference when compared to theasymptomatic 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 inflammatoryprocess on the affected side, or the presence of an anechoiceffusion may be difficult to distinguish from the joint capsule.These reasons, again, highlight the importance of scanning thecontralateral side to determine the normal anatomy and tissuearchitecture.

The presence of an effusion is concerning for a pathologicalprocess.  However, the distinction between sterile andinflammatory effusions cannot be determined based solely on theultrasonographic appearance.⁵  The presence of aseptic hip can result in serious morbidity and should prompt eitheremergent orthopedic consultation, or depending on the practicesetting, immediate joint aspiration by the emergency physician,followed by microscopic evaluation of the joint fluid withmicroscopy and Gram staining.⁷  Aspiration ofsterile fluid should steer the diagnosis toward transient synovitisand away from a septic joint. 

Pearls and Pitfalls

When performing musculoskeletal ultrasound, comparison views areimportant to help differentiate between a potential pathologicprocess and an anatomical variant.    Bilateraleffusions are possible, especially in patients with transientsynovitis, where they occur in up to a fourth of patientsultimately diagnosed with this diseaseentity.⁸

Ensuring that the probe is positioned perpendicular to the skinwill help avoid anisotropy, which is an artifact that can falselysuggest fluid when none is present.  This is especiallyimportant in toddlers, who have a large hypoechoic cartilaginouszone between the ossified nucleus of the femoral head and the jointcapsule.⁷

Missing slipped capital femoral epiphysis is a theoreticalpitfall when performing hip ultrasound on the older child with apainful hip or limp.  The physician should additionallyconsider obtaining plain radiography in this clinicalsetting.⁸

References

1. Singer JL.  The cause of gait disturbance in 425 pediatricpatients.  Pediatric Emergency Care.  1985, 1:7.
2. Miralles M, Gonzalez G, Pulpeiro JR et al.  Sonography ofthe Painful hip in Children: 500 Consecutive Cases.  AmericanJournal of Radiology.  March 1989, 152:579-582.
3. Shavit I, Eidelman M, Galbraith R.  Sonography of the HipJoint by the Emergency Physician: Its Role in the Evaluation ofChildren Presenting with Acute Limp.  Pediatric EmergencyCare.  August 2006, 22(8):579-573.
4. Minardi JJ, Lander OM.  Septic Hip Arthritis: Diagnosisand Arthrocentesis Using Bedside Ultrasound.  Journal ofEmergency Medicine.  2012, Article in Press.
5. Tsung JW, Blaivas M.  Emergency Department Diagnosis ofPediatric hip Effusion and Guided Arthrocentesis UsingPoint-of-Care Ultrasound.  Journal of EmergencyMedicine.  2008, 35(4):393-399.
6. Vieira RL, Levy JA.  Bedside Ultrasonography to IdentifyHip Effusions in Pediatric Patients.  Annals of Emergency Medicine.  March 2010, 55(3):284-289.
7. De Bruyn R. Pediatric Ultrasound: How, Why and When. 2ndEdition. 2010, Edinburgh.  Churchill Livingstone, Elsevier. p.332 - 336.
8. Navaro OM, Parra DA.  Pediatric MusculoskeletalUltrasound.  Ultrasound Clinics October 2009, 4 (4): 457-470.
9.  Siegel MJ. Pediatric Sonography. 4th Ed. 2011,Philadelphia.  Lippincott Williams & Wilkins. p. 624 -626

Table 1: Differential Diagnosis of Acute Limp