PEM POCUS Series: Hip Effusion

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PATIENT CASE: Child with a Limp

Sarah is a 4-year-old girl who comes into the emergency department complaining of a limp for the last day. She had an upper respiratory infection which started a week ago for which she had been taking acetaminophen and ibuprofen with her last dose of either being 2 days ago. Those symptoms have improved. Yesterday, she started complaining of diffuse right leg pain primarily at her hip, thigh, and knee. Today, her parents noted she was walking with a limp.

On arrival, her vital signs are:

Vital Sign	Finding
Temperature	100.1F
Heart rate	100 bpm
Blood pressure	97/50
Respiratory rate	19
Oxygen saturation (room air)	100%

She is well appearing and walks with an antalgic gait favoring the left leg. She has a normal HEENT, neck, cardiac, respiratory, abdominal, and back examination. She points to her right anterior thigh when you ask her where her pain is. She has limited range of motion with internal and external rotation of her right hip and complains of pain. She cries when you palpate any part of her leg, but is able to range her knee, ankle, and foot fully. She has 2+ dorsalis pedis and posterior tibialis pulses and intact sensation to light touch throughout. 

Given her pain with range of motion at her hip, you order a hip radiograph, but while waiting for it, decide to perform a hip point of care ultrasound (POCUS) examination.

PEDIATRIC HIP EFFUSION: Ultrasound Technique

It can be difficult for children to locate the exact area of pain and often hip pain can present with referred knee and thigh pain. A thorough physical examination along with hip POCUS can help localize the area of discomfort. When we use POCUS to evaluate the hip, we are primarily looking for a joint effusion in the synovial space. This is best visualized anterior to the femoral neck.  

Technique

1. The patient should be positioned supine.
   • Depending on the age of the child, the child can be positioned supine in the guardian’s lap while undergoing the examination. 
   • Offering the child a toy, book, or phone/tablet for distraction during the examination can also help ease anxiety.
2. Use a linear high frequency transducer with a wide footprint.
3. Place the transducer along the anterior hip.
   • Use the greater trochanter as a lateral landmark and place the probe on top of the femoral head and neck (Figure 3)
4. Aim the probe marker towards the patient’s umbilicus.

5. Identify the anatomical landmarks on ultrasound (figure 4) 
   • Hip muscles: Sartorius, quadriceps, and iliopsoas
   • Bones: Femoral head, femoral neck
6. The area of interest in looking for a hip effusion is the synovial space anterior to the femoral neck and NOT anterior to the femoral head. This area is also referred to as the anterior synovial recess.

• Tips:
  • It is often helpful to ultrasound the unaffected side as a comparison.
  • Be aware of the patient’s comfort throughout the examination.

ABNORMAL ULTRASOUND FINDINGS

A hip effusion will appear anechoic (black) in the synovial space anterior to the femoral neck (anterior synovial recess). Measure the distance between the anterior surface of the femoral neck and the posterior surface of the iliopsoas muscle. Examples are illustrated in figures 5 and 6.

There have been different methods used to assess if an effusion is present including:^​1,2​

• Measured effusion is >0.5 cm 
• Measured effusion with >0.2 cm difference compared to the contralateral hip
• Gestalt view with the anterior synovial recess areas appearing asymmetric compared to the other hip

Limitations of the Hip POCUS

Note that hip POCUS does not tell you the cause of an effusion but rather only whether an effusion is present or not. Effusions can result from infectious, inflammatory, and traumatic etiologies, and thus it is important to take into account the context of the patient’s presentation. 

References

1. Vieira R, Levy J. Bedside ultrasonography to identify hip effusions in pediatric patients. Ann Emerg Med. 2010;55(3):284-289. PMID 19695738
2. Cruz C, Vieira R, Mannix R, Monuteaux M, Levy J. Point-of-care hip ultrasound in a pediatric emergency department. Am J Emerg Med. 2018;36(7):1174-7. PMID 29223689 

FACTS and LITERATURE REVIEW

There have been multiple case reports of hip POCUS identifying hip effusions in children with fractures, septic joints, and in the work up of children with limps.^​1–3​ There are limited studies examining the accuracy of hip POCUS compared to radiology-performed ultrasound in children (table 1).

Study	N	Sensitivity	Specificity	Comments
Vieira et al., Ann Emerg Med, 2010​4​	28	85%	93%	If the pediatric emergency physician had a high confidence in ultrasound accuracy, the sensitivity increased to 90% and specificity to 100%
Cruz et al., Am J Emerg Med, 2018​5​	516	85%	98%	For each additional hip POCUS performed, the odds of an accurate scan increased by 2.4%.

 

CASE RESOLUTION

The patient’s hip plain film radiographs are unremarkable. You decide to incorporate hip POCUS to your evaluation. You place a linear, high-frequency transducer and visualize the patient’s bilateral hips. You observe the following:

Right Hip (Affected Side) POCUS Video and Key View

Though the child complained of hip, knee, and thigh pain, your POCUS evaluation identifies a right hip effusion which helps you narrow the location of her pain.

 

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Left Hip (Normal Side) POCUS Video and Key View

 

ED Course

The patient’s labs result, and she has reassuring labs, which include a serum WBC 8.3 x10⁹/L and ESR 34 mm/hr. The patient receives ibuprofen during her visit and within 1 hour is able to walk without a significant limp and states she feels better. The patient’s family notes they have spent the last few weeks of summer in a Lyme-endemic region, and so you send off Lyme titers.

Given the resolution of limp, lack of fever, and reassuring lab values, you do not believe the patient has a septic joint. Her effusion is more likely the result of a transient tenosynovitis. You recommend close pediatrician follow-up.

Pediatrician Clinic Follow-Up

At her pediatrician clinic visit 1 week later, her Lyme titers return negative, and the patient continues to be limp-free.  

 

The PEM POCUS series was created by the UCSF Division of Pediatric Emergency Medicine to help advance pediatric care by the thoughtful use of bedside ultrasonography.

Read other PEM POCUS tutorials. Learn more about bedside ultrasonography on the ALiEM Ultrasound for the Win series.

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