Transient synovitis

Transient synovitis also called toxic synovitis, is an acute, non-specific, inflammatory process affecting the joint synovium ¹. Transient synovitis is called “transient” because it lasts only a short time. Transient synovitis of the hip is the most common cause of acute hip pain in children aged 3-10 years. Sometimes it occurs in children younger than 3 years of age. Transient synovitis is more common in boys than in girls ² and about 1% to 4% of the time a child may demonstrate bilateral involvement ³. While transient synovitis is a benign, self-limiting process, healthcare providers must recognize the critical importance of differentiating transient synovitis from an acute infectious process such as septic arthritis. Septic arthritis in the children’s hip is an emergency surgical condition that if not rapidly treated, can lead to the rapid destruction of the hip, sepsis and even death ⁴.

Transient synovitis causes

Doctors don’t know the exact cause of transient synovitis of the hip. It might be caused by a virus or it might be from an allergic reaction to an infection somewhere else in the body. 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 ⁵:

• preceding upper respiratory infection
• preceding bacterial infection – poststreptococcal toxic synovitis
• preceding trauma

Many children will present with a history of preceding upper respiratory infection symptoms, or in the setting of recent trauma. According to Kastrissianakis and Beattie ⁶, patients diagnosed with transient synovitis are more likely to have experienced preceding viral symptoms including vomiting, diarrhea, or common cold symptoms. An earlier study reported that patients with transient synovitis demonstrated higher serum interferon concentration values ⁷. 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 ⁸. Studies investigating possible viral pathogen candidates, including parvovirus B-19 and human herpes simplex virus-6, have not been conclusive ⁹.

Other hypothesized risk factors include post-vaccine or drug-mediated hypersensitivity reactions or certain allergic predispositions. Another potential clinical association has been proposed for Legg-Calvé-Perthes disease and transient synovitis. While this relationship remains controversial, some studies have reported increased incidence rates of Legg-Calve-Perthes disease following transient synovitis (up to 3%) compared to the relative Legg-Calve-Perthes disease incidence rate reporting in the general population (0.9 per 100,000 patients) ¹⁰.

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 is estimated to be at 0.2% and 3%, respectively ⁸. A 2010 study from the Netherlands reported the mean age at presentation was 4.7 years ¹¹. 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 ¹². The incidence rate in males is twice that of females, and about 1% to 4% of the time a patient may demonstrate bilateral involvement ³.

Transient synovitis symptoms

The main symptom of transient synovitis is pain in the hip. In some children, the hip pain gets worse very quickly. In other children, the hip pain gets worse slowly. At first, the hip pain may be so mild that they don’t know there is something wrong.

When the pain gets bad enough, children who have transient synovitis have a hard time walking. If your child has transient synovitis of the hip, he or she may have pain whenever the hip is moved. Your child may walk with a limp. Because of the pain, your child may have trouble standing. Some children may have pain of the inner thigh or knee area, instead of around the hip. Many children who have this condition want to lie on their back with the knee on the side that hurts bent and turned out with their foot pointed away from their body. This position may lessen the pain.

Examination of the child with unilateral hip pain usually reveals mild restrictions to range of motion, especially to the abduction and internal rotation position. The child 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 ¹³. In some reports one-third of children presented with normal range of motion on physical exam.

While transient synovitis 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.

Transient synovitis duration

The acute inflammatory phase clinically manifests as a pain that is self-limiting and resolves within 24 to 48 hours ¹. Symptoms generally improve after 24 to 48 hours. The natural history favors complete resolution of symptoms within 1 to 2 weeks in up to 75% of patients, although recurrence rates can be as high as 20% ¹⁴. 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.

Transient synovitis diagnosis

Your doctor will look at your child’s hip to find out what kind of movement makes the pain worse. Transient synovitis remains a diagnosis of exclusion. Your doctor may order blood tests, X-rays, and an ultrasound. These tests will help your doctor make sure that the cause of hip pain isn’t caused by something more serious.

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 ¹⁵, highlighted demographic, clinical, and laboratory variables seen in pediatric patients presenting with transient synovitis, septic arthritis or Lyme arthritis of the hip. The authors noted several key findings that can aid in the clinical differentiation:

• Febrile at presentation
  • Over 50% of patients with septic arthritis
  • 30% of patients with transient synovitis
  • 23% of patients with Lyme arthritis
• Refusal to bear weight
  • Over 60% in patients diagnosed with either transient synovitis or septic arthritis
  • Only 33% of patients diagnosed with Lyme arthritis
• Inflammatory markers
  • ESR range for septic arthritis patients was 44 – 64 mm/hr
  • ESR range for transient synovitis patients was 21 – 33 mm/hr
  • ESR range for Lyme arthritis patients was 37 – 46 mm/hr
• Synovial fluid aspiration results
  • Synovial white blood cell counts (cells/mm3) demonstrated a similar trend as noted with measured ESR levels at presentation
    transient synovitis (5,644 – 15,388)
  • Lyme arthritis (47,533 – 64,242)
  • Septic arthritis (105,432 – 260,214)
• Peripheral white blood cell count was similar between each of the diagnostic groups

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 ¹⁶.

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 ¹⁷.

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 ¹².

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:

• Temperature greater than 99.5 °F (37.5 °C)
• ESR greater than or equal to 20 mm/hr
• Severe hip pain and spasm 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 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.

Multiple algorithms and previously reported step-by-step guidelines are available in the literature ¹².

The Kocher criteria remain a helpful set of clinical risk factors differentiating septic arthritis 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 septic arthritis as a diagnosis when all four criteria are met:

• White blood cell count > 12,000 cells per microliter of serum
• Inability or refusal to bear weight
• Febrile (> 101.3 degrees Fahrenheit or 38.5 degrees Celsius)
• ESR > 40 mm/hr

When none of the above risk factors are present upon presentation, the probability of the patient having septic arthritis 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 white blood cell count ¹⁸.

Transient synovitis treatment

Diagnosis of transient synovitis is made following a thorough and comprehensive diagnostic workup. In the setting of clinical concern or when the diagnosis is unclear, admitting the child for observation can allow for serial observation following an initial period of supportive management.

Rest at home is the most important way to help your child’s hip get better. Your child may need to take a nonsteroidal anti-inflammatory medicine (NSAID), such as ibuprofen (brand names: Advil, Motrin), to reduce the swelling and inflammation around the hip joint. Also, your doctor may advise you to apply heat to your child’s hip. Massage may make the area to make it feel better, too.

Your child’s doctor will probably ask you to take your child’s temperature regularly and to report any temperature higher than 99.5°F (37.5 °C). A fever may mean that your child has a problem other than hip synovitis. To make sure that your child is doing well, your doctor may want to recheck your child 12 to 24 hours after the first visit.

With rest and medicine, your child’s hip will probably get better in 3 or 4 days. If the pain is still bad after 7 to 10 days, your child should be rechecked by your doctor. Your doctor may order some tests to make sure there isn’t something else wrong with your child’s hip.

After the pain leaves, your child can resume his or her usual activities. In most children, there are no complications from transient synovitis of the hip. They recover completely. To make sure everything is all right, your doctor may want to take another X-ray of your child’s hip in about 6 months to exclude Legg-Calvé-Perthes disease.

Transient synovitis prognosis

In total, transient synovitis of the hip recurs in up to 20% to 25% of patients ¹. Patient 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 one-, two-year, and long-term follow-up, respectively ¹⁹.

A retrospective study that included 198 children with the diagnosis of a transient synovitis found that between the diagnosis of transient synovitis and a 3-month follow-up, 20 children did not remain symptom-free (10.1%). 4 (2%) of these patients were diagnosed with Perthes disease and the other 16 had a normal radiological follow-up. All children who were symptom-free had negative follow-up X-rays ²⁰.

References

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