In this post I’m going to link to and review two resources;
- Tips & Tricks: Ultrasound in the Diagnosis of a Pediatric Hip Effusion. April, 2018 by Kathryn H Pade, MD, Viveta Lobo MD and Laleh Gharahbaghian, MD, FACEP
- Transient Synovitis. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Christine C. Whitelaw; Matthew Varacallo.
Here are excerpts from Resource (1):
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.
[The authors then describe the technique for POCUS evaluation of pediatric hip effusion – See Technique in the article for details.]
And here are excerpts from resource (2):
Transient synovitis (TS) is an acute, non-specific, inflammatory process affecting the joint synovium. TS of the hip is a common cause of hip pain in the pediatric patient population. While the condition is a benign, self-limiting process, providers must recognize the critical importance of differentiating TS from an acute infectious process [septic arthritis].
The exact etiology of TS is unknown. The literature demonstrates multiple proposed etiologic theories but none of these postulated hypotheses have been conclusively substantiated.
Many pediatric patients will present with a history of preceding URI symptoms, or in the setting of recent trauma. According to Kastrissianakis and Beattie, patients diagnosed with TS are more likely to have experienced preceding viral symptoms including vomiting, diarrhea, or common cold symptoms.
TS of the hip most frequently occurs in children ages 3 to 10 years old. The average annual incidence of TS and the total lifetime risk is estimated to be at 0.2% and 3%, respectively . A 2010 study from the Netherlands reported the mean age at presentation was 4.7 years. While the majority of cases occur in pediatric patients between the ages of 3 and 10 years of age, the literature does demonstrate rare case presentations in both younger infants and the adult population . The incidence rate in males is twice that of females, and about 1% to 4% of the time a patient may demonstrate bilateral involvement .
The pathoanatomy underlying TS is relatively nonspecific. The proposed pathologic cascade entails nonspecific inflammation targeting the synovial joint lining causing hypertrophic changes. Typically one or multiple aforementioned risk factors can be elicited from the clinical history upon presentation. The acute inflammatory phase clinically manifests as a pain that is self-limiting and resolves within 24 to 48 hours. The natural history favors a complete resolution of symptoms within 1 to 2 weeks, although recurrence rates can be as high as 20% .
History and Physical
TS most commonly presents as acute unilateral limb disuse ranging from nonspecific hip pain or subtle limp to a refusal to bear weight. Depending on the age of the patient, the history may only be significant for the child or infant becoming increasingly agitated or crying more often than at baseline. Therefore, heightened clinical suspicion is warranted in younger pediatric patients and infants. In addition, examiners should elicit for any pain or discomfort localized or radiating to or from the lower back. Oftentimes the clinician may only be able to rely on the history obtained from the parents or guardians. Direct observation of the child in the emergency room or clinic can often yield valuable information. A recent history of an upper respiratory tract infection, pharyngitis, bronchitis, or otitis media is often elicited and favors a diagnosis of TS.
Physical examination 
Examination of the patient with unilateral hip pain usually reveals mild restrictions to range of motion, especially to the abduction and internal rotation position. The patient may present with the hip in the flexed, abducted, and externally rotated position as this relaxes the hip joint capsule to decrease intra-articular pressure . In some reports one-third of patients presented with a normal range of motion on physical exam.
While TS remains a diagnosis of exclusion, provocative maneuvers such as the basic log roll* or performing the Patrick test if the patient is able to tolerate. The latter is also known as the FABER test for flexion, abduction and external rotation and this maneuver is performed by having the patient flex the leg with the thigh abducted and externally rotated. Pain on the ipsilateral anterior side is indicative of a hip disorder on that side. If the pain is elicited on the contralateral side posteriorly around the sacroiliac joint, it suggests pain mediated by dysfunction in that joint.
* “The log roll test is the single most specific test for hip pathology. With the patient supine, gently rolling the thigh internally (A) and externally (B) moves the articular surface of the femoral head in relation to the acetabulum, but does not stress any of the surrounding extra-articular structures.” From Evaluation of the Hip: History and Physical Examination. N Am J Sports Phys Ther. 2007 Nov; 2(4): 231–240.