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.
The exact etiology of TS is unknown. The literature demonstrates multiple proposed etiologic theories but none of these postulated hypotheses have been conclusively substantiated. Proposed risk factors include but are not limited to:
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. An earlier study reported that patients with TS demonstrated higher serum interferon concentration values. Seasonal variation in association with TS diagnoses remains controversial. One study reported a seasonal variation in the incidence of TS, with more cases presenting in October and fewer cases in February . Studies investigating possible viral pathogen candidates, including parvovirus B-19 and human herpes simplex virus-6, have not been conclusive .
Other hypothesized risk factors include postvaccine or drug-mediated hypersensitivity reactions or certain allergic predispositions. Another potential clinical association has been proposed for Legg-Calvé-Perthes disease (LCPD) and TS. While this relationship remains controversial, some studies have reported increased incidence rates of LCDP following TS (up to 3%) compared to the relative LCPD incidence rate reporting in the general population (0.9 per 100,000 patients).
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 complete resolution of symptoms within 1 to 2 weeks, although recurrence rates can be as high as 20% .
TS most commonly presents as acute unilateral limb disuse ranging from nonspecific hip pain or subtle limp to 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.
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 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.
Comprehensive evaluation and diagnostic workup should include a white blood cell count (WBC), C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), and hip radiography and ultrasonography.
A 2017 systematic review and meta-analysis highlighted demographic, clinical, and laboratory variables seen in pediatric patients presenting with TS, septic arthritis (SA), or lyme arthritis (LA) of the hip. The authors noted several key findings that can aid in the clinical differentiation: 
TS remains a diagnosis of exclusion, although these studies have highlighted the diagnostic utility of a synovial fluid aspiration and analysis.
Additional laboratory workup includes a CRP greater than 2 mg/dl, which has been shown to be an independent risk factor for septic arthritis. A urinalysis and culture are typically normal. Because procalcitonin levels remain low during bouts of inflammatory disease, an increase should raise suspicion of septic arthritis. Depending on the history, consider antinuclear antibody, rheumatoid factor, HLA-B27, and tuberculosis skin testing 
In a Lyme endemic area, only 5% of children with acute, nontraumatic hip pain had a Lyme infection, so routine serology is not necessary. It should be performed if an alternative diagnosis such as septic/pyogenic arthritis is being considered and in those with an atypical clinical course .
Although plain films may be normal for months after onset of symptoms, the medial joint space is typically slightly wider in the affected hip indicating the presence of fluid. One-half to two-thirds of patients with transient synovitis may have an accentuated pericapsular shadow 
Ultrasound is extremely accurate for detecting an intracapsular effusion. Ultrasound-guided hip aspiration not only relieves pain and limitation of movement but it often provides a rapid distinction from septic arthritis. Ultrasound-guided hip aspiration should be done in all individuals in whom ultrasonography has exhibited evidence of an effusion, and any of the following predictive criteria are present:
If the aspirate has a positive gram stain, more than 90% polymorphonuclear cells, or a glucose less than 40 mg/dL or markedly different from the serum glucose, the patient is more likely to have septic arthritis and not transient synovitis.
In settings in which routine aspirations of effusions is not performed, a dynamic contrast-enhanced MRI may help differentiate transient synovitis from septic arthritis.
Bone scintigraphy demonstrates mildly elevated uptake; however, it does not help differentiate etiologies.
The Kocher criteria remain a helpful set of clinical risk factors differentiating SA and TS in pediatric patients presenting with hip pain. The criteria include the increasing diagnostic probability in favor of the former, yielding a 99.6% probability favoring SA as a diagnosis when all four criteria are met:
When none of the above risk factors are present upon presentation, the probability of the patient having SA of the hip drops below 0.2%. A subsequent study incorporated CRP measurements into the clinical workup. Caird et al. performed a Level I study that concluded that a temperature above 38.5 was the best predictor of septic arthritis followed in decreasing order by CRP (>1mg/dL), ESR, refusal to bear weight, and serum WBC count .
Following the appropriate diagnosis of TS made following a thorough, comprehensive diagnostic workup, management involves supportive care and rest from activity. NSAIDs can be used for pain control. Other modalities include the application of heat and/or massage modalities. In the setting of clinical concern or when the diagnosis is unclear, admitting the patient for observation can allow for serial observation following an initial period of supportive management.
Symptoms generally improve after 24 to 48 hours. Complete resolution of symptoms often takes up to 1 to 2 weeks in up to 75% of patients. The remainder may have less severe symptoms for several weeks. If significant symptoms persist for seven to 10 days after the initial presentation, consider other diagnoses. Patients with symptoms for more than a month have been found to have a different pathology.
Patients presenting with acute hip pain should be ruled out for alternative diagnoses, especially those that could potentially lead to devastating sequelae if not diagnosed relatively quickly. These conditions include osteomyelitis, septic arthritis, primary or metastatic lesions, Legg–Calve–Perthes disease (LCPD), and slipped capital femoral epiphysis (SCFE). Other diagnoses include Lyme arthritis, pyogenic sacroiliitis, and juvenile rheumatoid arthritis.
In total, TS of the hip recurs in up to 20% to 25% of patients. Patient should be educated regarding the increased risk of recurrence in the setting of a previously documented diagnosis of TS. One study reported the subsequent recurrence rates in patients with a previously documented diagnosis of TS were 69%, 13%, and 18% at one-, two-year, and long-term follow-up, respectively .
The major complication associated with TS is recurrence of symptoms.
Sequelae include coxa magna and mild degenerative changes of the femoral neck. Coxa magna is observed radiographically as an overgrowth of the femoral head and broadening of the femoral neck. Coxa magna leads to dysplasia of the acetabular roof and subluxation. An incidence rate of coxa magna of 32.1% has been reported in the first year following transient synovitis.
Legg-Calve-Perthes disease develops in 1% to 3% of individuals.
The recurrence rate of transient synovitis is 4% to 17%; most recurrences develop within six months