Dx: Small Effusion in the Hip

The ultrasound does show a small effusion, but this is not necessarily abnormal. Before you call anyone, you should measure the width of the effusion and compare it to the other side (Images 2 & 3). It turns out that though you didn’t exactly take your measurement at the recommended site, the effusion is symmetric to the one on the asymptomatic hip, and is small enough to be considered normal.

After 10mg/kg of ibuprofen and 15mg/kg of acetaminophen both PO and two hours of waiting, he is still not really “walking” but can do a straight leg raise and limp around a bit. Rather than admit this patient like your colleague did, you call the patient’s pediatrician and together formulate a reasonable plan for outpatient care involving ibuprofen, careful aftercare instructions and a recheck the next morning if he is not improving.

The next day you call the family at home and find out that the young lad is playing outside and is almost back to normal self. There is no one there to pat you on the back, so you do it yourself.

Tips & Tricks for Imaging the Pediatric Hip

01 Benefits of Ultrasound: Using ultrasound to rule out a significant hip effusion avoids radiation and can be done at the bedside. EP-performed bedside ultrasound for this indication has been shown to have a sensitivity of 85% and a specificity of 93%. This is better than plain films, but you should be aware of its limitations.

02 Look before You Leap: Ultrasound can be used to help you delineate whether a patient has a joint effusion or not.  It is more sensitive than X-ray, can tell you if there is a joint effusion, and help you localize the best area to tap during an arthrocentesis.

03 Go Linear: For most joints use a 5-7.5 MHz linear array transducer.  Apply a large amount of ultrasound gel to improve your acoustic interface. If the patient is thin and devoid of much subcutaneous fat, you may need to utilize an acoustic standoff pad or water submersion to improve your sonographic window. If the patient is larger and has more subcutaneous tissue, use a lower frequency probe (e.g. 2-5MHz curvilinear transducer).

04 Take Multiple Views: Always obtain images in multiple planes (longitudinal, transverse, oblique) to help define the borders of the structure you are looking at and avoid mistaking vascular structures for a joint effusion. Use color or spectral Doppler if there is any question as to whether the hypoechoic structure is a vessel or not.

05 Compare Sides: Utilize contra-lateral limbs for comparison views, especially when you are unsure of what you see.  If you see a similar structure on the asymptomatic limb, it may be normal. To take a measurement of the hip effusion width, align your probe parallel to the femoral neck and measure at the concavity of the femoral neck. A positive result is an effusion >0.50 cm width or >0.20 cm more than asymptomatic side.

06 Don’t Move: Avoid changing the position of the joint between performing the bedside ultrasound and performing an arthrocentesis.  Fluid may move with joint repositioning.

07 Practice Makes Perfect: With bedside ultrasound there is no substitute for experience. The more ultrasounds you do, the better you will be able to differentiate abnormal from normal, even when you may not be sure exactly what the abnormality is.

Brady Pregerson (@TheSafetyDoc) manages a free online EM Ultrasound Image Library and is the editor of the Emergency Medicine Pocketbook series. For more info visit EMresource.org.

Teresa S. Wu (@TeresaWuMD) is the Associate Residency Director, and Director of Ultrasound and Simulation Programs and Fellowships, for the Maricopa EM Program in Phoenix.

STEP 3: TAKE THE QUIZ

STEP 4: PROCESS PAYMENT