Case 49

5 y/o M who came in with L leg pain.  He would not stand on the leg x 2 days.  He had a temporal temperature on arrival of 100.5 degrees F.  WBC was 10,800 cells/mm3, ESR 33, Normal CXR in our ED.

QUESTIONS

  1. What’s your differential diagnosis for a pediatric limp?
  2. What are the Kocher criteria and when do we apply the criteria?
  3. What probe will you use to evaluate the hip on US?
  4. How do you want to position your patient to optimize visualization of any effusion?
  5. What do you see in the images below?
  6. How do you diagnose a hip effusion?
     

The US images were as follows:

RIGHT hip

RIGHT hip

LEFT hip

LEFT hip

ANSWERS

  1. Septic arthritis, transient synovitis, SCFE, Legg-Calves Perthes, rheumatic fever, developmental dysplasia, arthritis, fracture, dislocation, cancer, osteomyelitis, appendicitis, myositis
  2. The Kocher criteria are ESR>40, WBC>12, difficulty weight bearing, fever. Apply the criteria when you want help differentiating between septic arthritis and transient synovitis.
  3. Linear probe or even possibly the small hockey stick probe for smaller anatomy
  4. Frog leg is ideal to evaluate the anterior hip, which is the ideal location to look for an effusion. Place the Pt supine with the hip in mild external rotation. Obtain an image in the sagittal oblique plane parallel to the long axis of the femoral neck to assess femoral head and neck and for any joint effusion.
  5. In the images above, the R hip is a normal hip without effusion and the L hip shows a small effusion, which is concerning for a septic arthritis. 

To evaluate for a hip effusion:

Place the patient supine in a frog leg position and place the probe in a sagittal oblique plane along the femoral neck.

from Pediatric Emergency Care 31(1):54-58 · January 2015

from Pediatric Emergency Care 31(1):54-58 · January 2015

Make sure to start with the unaffected side first.  The image you obtain should be similar to the following images:

The femoral neck needs to clearly identified with a visible cortical edge.  As you can see in the image below, the joint capsule measurement is performed in the space between the femoral neck and the iliopsoas muscle.

from ACEP

from ACEP

A joint capsule is considered abnormal if the measurement on the affected side is:

greater than 5mm
OR
the measurement is 2mm greater than the unaffected side

red line = 6mm

red line = 6mm

In addition, an effusion will change the shape of the space above the femoral neck from concave to convex.

RT = unaffected side - concave; LT = affected side - convex

RT = unaffected side - concave; LT = affected side - convex

In conclusion, ultrasound of the pediatric hip is a useful tool as a noninvasive method to aid in tapering a differential diagnosis.  Ultrasound can help assess emergent hip pathologies or identify those patients that require procedural intervention.  It is also important to remember that hip pain could represent pathologies outside of the hip itself including those in the abdomen, like psoas abscess or appendicitis, or those in the lower extremity, like knee pathologies.