by Kathryn H Pade, MD, Viveta Lobo MD and Laleh Gharahbaghian, MD, FACEP
Pediatric limp, refusal to bear weight or leg pain, is a common chief complaint presenting to pediatric emergency departments and accounts for approximately 4 in 1000 visits.1 The differential diagnosis includes trauma, infection, inflammatory conditions, bony deformity or malignancy. In particular, the diagnosis is often difficult in patients with no history of trauma, and it is important to distinguish between benign, severe and life-threatening conditions. When a child has an acutely painful hip, the most common diagnoses are transient synovitis and septic arthritis. In patients less than 14 years of age, transient synovitis accounts for a significant portion of patients presenting with hip pain and limp (up to 40%), however, it can be a difficult diagnosis as it is typically made clinically.2,3
Although plain radiographs may show widening of the joint space, ultrasound is considered the gold standard for the diagnosis of hip effusions. It is noninvasive and has been shown to be more sensitive than plain radiographs in the diagnosis of a joint effusion (seen in both transient synovitis and septic arthritis.4,5 Traditionally it is performed by ultrasound technicians in the radiology department. However, a study by Viera et al showed that with limited focused training, emergency physicians can use point-of-care ultrasound to accurately identify hip effusions in pediatric patients (sensitivity of 85% and specificity of 100%).6
Although ultrasound cannot definitively distinguish between septic arthritis and transient synovitis, it can exclude the diagnosis if no effusion is detected. Thus, the clinician’s concern for septic arthritis should be based on history, clinical suspicion and available laboratory results.
Here we describe the technique and some tips and tricks to evaluate for a pediatric hip effusion.
1.Patient Position: Supine.
a. Tip: Expose the hip with drapes for patient comfort.
b. Tip: If the patient will tolerate it, position the leg in slight abduction and external rotation.
2. Probe: High frequency linear probe (or the curvilinear probe if increased depth is required)
a. Tip: For larger or older patients, use the linear probe with the larger/wider footprint to allow better visualization of anatomy.
3. Technique: With the patient lying supine, identify the greater trochanter on the symptomatic hip of the patient. Place the linear probe in the sagittal oblique plane parallel to the long axis of the femoral neck (with the indicator toward the patient’s head). The femoral neck is a hyperechoic line lateral to the femoral head slanting downward. Move the probe superior until you identify the femoral head, which can be seen as a curved hyperechoic line. In the normal hip, the joint capsule appears as a hyperechoic band above the femoral head and proximal femoral neck.
a. Tip: In pediatric patients with open growth plates, identification of the
capital femoral epiphysis will help confirm you are in the correct location.
(Figure 1 and 2)
Figure 1: With the patient lying supine, place the linear probe over the long axis of the femoral neck.
Figure 2: Normal Sonographic Anatomy of the Pediatric Hip
4. Hip Anatomy: The hip joint is formed by the articulation of the femoral head and acetabulum of the pelvis. The joint is enclosed by a fibrous capsule and surrounded by extracapsular ligaments. Normally, a small amount of physiologic fluid is present within the joint space (<5mm). (Figure 3) (Clip 1)
Figure 3: Normal sonographic measurement of the pediatric hip joint space
5. Hip Effusion: Measure the maximal distance between the anterior surface of the femoral neck and the posterior surface of the iliopsoas muscle. Sonographic criteria for a pediatric hip effusion is:
i. Anterior Synovial Fluid collection greater than 5mm OR
ii. >2mm difference when compared to the asymptomatic contralateral hip
iii. Tip: Compare the asymptomatic hip for anatomy and joint space measurement. (Figure 4)
Figure 4: Sonographic measurement of a pediatric hip demonstrating a joint effusion.
Point-of-care ultrasound of the hip is more sensitive than plain radiographs and can be done by emergency physicians to accurately diagnosis a joint effusion. Further use by emergency physicians can improve diagnostic accuracy, quality of patient care and timely diagnosis.