Transient Synovitis

Etiology

The exact etiology of transient synovitis 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:

• Preceding upper respiratory infection 
• Preceding bacterial infection
  • Poststreptococcal toxic synovitis
• Preceding trauma [1][2]

Many pediatric patients will present with a history of preceding upper respiratory infection symptoms, or in the setting of recent trauma. According to Kastrissianakis and Beattie, patients diagnosed with transient synovitis are more likely to have experienced preceding viral symptoms, including vomiting, diarrhea, or common cold symptoms.[2] 

An earlier study reported that patients with transient synovitis demonstrated higher serum interferon concentration values.[3] Seasonal variation in association with transient synovitis diagnoses remains controversial. One study reported a seasonal variation in the incidence of transient synovitis, with more cases presenting in October and fewer cases in February.[4] Studies investigating possible viral pathogen candidates, including parvovirus B-19 and human herpes simplex virus-6, have not been conclusive.[5]

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 transient synovitis. Although this relationship remains controversial, some studies have reported increased incidence rates of LCPD following transient synovitis (up to 3%) compared with the relative LCPD incidence rate reported in the general population (0.9 per 100,000 patients).[6]

Epidemiology

Transient synovitis of the hip most frequently occurs in children ages 3 to 10 years old. The average annual incidence of transient synovitis and the total lifetime risk are estimated to be 0.2% and 3%, respectively.[4] A 2010 study from the Netherlands reported the mean age at presentation was 4.7 years.[7] Although 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.[8][9][10] The incidence rate in males is twice that of females, and about 1% to 4% of the time, a patient may demonstrate bilateral involvement.[11]   

Pathophysiology

The pathoanatomy underlying transient synovitisis relatively nonspecific. The proposed pathologic cascade entails nonspecific inflammation targeting the synovial joint lining causing hypertrophic changes. Typically, 1 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%.[12][13][14][15]

History and Physical

History

Transient synovitis most commonly presents as acute unilateral limb disuse ranging from nonspecific hip pain or a 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 evaluate 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 transient synovitis. Given the recurrence rates, a previous diagnosis of TS in the patient history can also increase the likelihood of a transient synovitis diagnosis.[14][15]

Physical Examination 

Examination of the patient with unilateral hip pain usually reveals mild restrictions to the 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 decreased intra-articular pressure. In some reports, one-third of patients presented with a normal range of motion on physical exam. 

Although transient synovitis remains a diagnosis of exclusion, provocative maneuvers such as the basic log roll or performing the Patrick test can provide valuable information if the patient can tolerate them. 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.[16][17][18][19][20][19]

Evaluation

Comprehensive evaluation and diagnostic workup should include a white blood cell count (WBC), C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), hip radiography, and ultrasonography.  

A 2017 systematic review and meta-analysis highlighted demographic, clinical, and laboratory variables seen in pediatric patients presenting with transient synovitis, septic arthritis (SA), or Lyme arthritis (LA) of the hip. The authors noted several key findings that can aid in clinical differentiation:[21]

• Febrile at presentation
  • Patients with SA: >50% 
  • Patients with TS: 30% 
  • Patients with LA: 23%  
• Refusal to bear weight
  • Patients with TS or SA: Over 60% 
  • Patients with LA: 33%
• Inflammatory markers (ESR range)
  • Patients with SA: 44-64 mm/hr
  • Patients with TS: 21-33 mm/hr
  • Patients with LA: 37-46 mm/hr
• Synovial fluid aspiration results
  • Synovial WBC counts (cells/mm³) demonstrated a similar trend as noted with measured ESR levels at presentation
    • TS: 5644-15,388
    • LA: 47,533-64,242
    • SA: 105,432-260,214 
• Peripheral WBC count was similar between each of the diagnostic groups

Transient synovitis 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 is 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.[22]

In a Lyme endemic area, only 5% of children with acute, nontraumatic hip pain had a Lyme infection, so routine serology is not necessary. However, it should be performed if an alternative diagnosis, such as septic or pyogenic arthritis, is being considered, and in those with an atypical clinical course.[23]

Although plain films may be normal for months after the 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.[9]

Ultrasound is extremely accurate for detecting an intracapsular effusion. Ultrasound-guided hip aspiration not only relieves pain and limitation of movement but 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 effusion and any of the following predictive criteria are present:

• Temperature greater than 99.5 ° F
• ESR greater than or equal to 20 mm/hr
• Severe hip pain and spasms with movement

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 are not performed, a dynamic contrast-enhanced MRI may help differentiate transient synovitis from septic arthritis. Bone marrow changes are noted to be the most specific finding for septic arthritis, while synovial enhancement was found to be the most sensitive MRI finding for septic arthritis.[24]

Bone scintigraphy demonstrates mildly elevated uptake; however, it does not help differentiate etiologies.

Multiple algorithms and previously reported step-by-step guidelines are available in the literature.[9][25][26][27]

The Kocher criteria remain a helpful set of clinical risk factors differentiating SA and transient synovitis 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:

• WBC greater than 12,000 cells per microliter of serum
• Inability or refusal to bear weight
• Febrile (>101.3 ° F or 38.5 ° C)
• ESR greater than 40 mm/hr

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 ° C was the best predictor of septic arthritis, followed in decreasing order by CRP (>1 mg/dL), ESR, refusal to bear weight, and serum WBC count.[16]

Treatment / Management

Following the appropriate diagnosis of transient synovitis made following a comprehensive diagnostic workup, management involves supportive care and rest from activity. Nonsteroidal anti-inflammatory drugs can be used for pain control. Other modalities include the application of heat 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 takes up to 1 to 2 weeks in about 75% of patients. The remainder may have less severe symptoms for several weeks. If significant symptoms persist for 7 to 10 days after the initial presentation, consider other diagnoses. Patients with symptoms for more than 1 month have been found to have a different pathology.

Differential Diagnosis

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 quickly. These conditions include:

• Osteomyelitis
• Septic arthritis
• Primary or metastatic lesions
• LCPD
• Slipped capital femoral epiphysis (SCFE).

Other diagnoses include:

• Lyme arthritis
• Pyogenic sacroiliitis
• Juvenile rheumatoid arthritis

Prognosis

Transient synovitis of the hip recurs in up to 20% to 25% of patients. Patients should be educated regarding the increased risk of recurrence in the setting of a previously documented diagnosis of transient synovitis. One study reported the subsequent recurrence rates in patients with a previously documented diagnosis of transient synovitis were 69%, 13%, and 18% at 1-year, 2-year, and long-term follow-up, respectively.[28] 

Complications

The major complication associated with transient synovytis is a recurrence of symptoms.