Transient Synovitis Workup

The following studies may be indicated in transient synovitis (TS):

• CBC count: The white blood cell (WBC) count may be slightly elevated.
• Erythrocyte sedimentation rate (ESR)

  • The erythrocyte sedimentation rate (ESR) may be slightly elevated. One study found that the combination of an ESR greater than 20 mm/h and/or a temperature greater than 37.5°C identified 97% of individuals with septic hip.^[6]
  • Another study by Kocher et al used 4 independent predictors of septic arthritis to distinguish it from transient synovitis and the need for further workup.^[7] They concluded that patients who were nonweightbearing and had history of fever, an ESR greater than 40 mm/h, and a WBC count greater than 12,000 cells/mm had a 99.6% high predicted probability of septic arthritis.
  • Luhmann et al applied these 4 criteria to their patient population and discovered a 59% predicted probability.^[8] However, when they applied the 3 criteria of history of fever, a serum WBC count of greater than 12,000 cells/mm, and a previous health-care visit, they found a predicted probability of 71% that the patient had septic arthritis.
• C-reactive protein

  • C-reactive protein (CRP) level rises within 6 hours after the onset of septic arthritis of the hip and peaks at 2 days.^[9]
  • A CRP >2 mg/dL (>20 mg/L) has been found to be an independent risk factor strongly associated with septic hip arthritis.^[10]
  • Adding in the CRP as a predictive factor, Jung et al found that patients with 4 of 5 predictors (body temperature >37 º C, ESR >20 mm/h, CRP >1 mg/dL, WBC >11,000/mL, and an increased hip joint space of >2 mm) had a high probability of having septic arthritis and were candidates for further study by MRI or joint aspiration.^[11]
• Urinalysis and culture: Both of these tests should be normal.
• Urine glycosaminoglycans: One study found a decreased level of urine glycosaminoglycans in patients who were diagnosed with Perthe disease, compared with those with transient synovitis and a control group.
• Procalcitonin levels: These may be helpful in distinguishing between bacterial infections and inflammatory processes. Procalcitonin levels remain low during bouts of inflammatory disease but increase in septic arthritis and may be even more useful in distinguishing septic arthritis from osteomyelitis.^[12]

Radiographs exclude bony lesions (eg, occult fracture, osteoid osteoma) unless the child had onset of symptoms within 3 days, has no fever, appears well, and has only mildly restricted abduction without guarding against movement in other planes. Plain films may be normal for months after onset of symptoms. Medial joint space may be slightly wider in the affected hip (see the image below).

If excess fluid is present or the patient has early Legg-Calvé-Perthes (LCP) disease, plain radiography may reveal an increase in the teardrop distance (ie, distance between the medial acetabulum and ossified part of the femoral head). Compared with the other side, this distance should be the same or within 1 mm. One half to two thirds of patients with transient synovitis may have an accentuated pericapsular shadow.

In one study, as many as 58% of patients with transient synovitis had the Waldenström sign (ie, lateral displacement of the femoral epiphyses with surface flattening). Other studies have reported a positive obturator sign in established incidents of transient synovitis. This is a prominent shadow caused by the soft tissues that overlie the interpelvic aspect of the acetabulum. Radiography may reveal diminution of the definition of soft tissue planes around the hip joint or slight demineralization of the bone of the proximal femur, particularly in the metaphyseal region.

Although extremely accurate for detecting an intracapsular effusion, ultrasonography does not assist in determining the cause and is used best to guide hip aspiration. An effusion is present if ultrasound demonstrates capsular distension greater than 2 mm. Occasionally, the radiologist can differentiate between transient synovitis and early LCP on the basis of effusion rather than synovial membrane thickening. However, ultrasonography cannot rule out osteomyelitis or soft tissue infection.

A study by Lee et al proposed that physicians may differentiate transient synovitis from septic arthritis by considering the results of an MRI.^[13] This study found that septic arthritis demonstrated signal intensity alterations in the bone marrow of the affected hip. Yang et al confirmed this finding in a study on 49 patients with transient synovitis and 18 patients with septic arthritis.^[14] He demonstrated not only the statistically significant finding of signal intensity in the bone marrow, but also found signal intensity alterations and contrast enhancement of the soft tissue in patients with septic arthritis. Furthermore, the statistically significant findings in the patients with transient synovitis included contralateral (asymptomatic) joint effusions and the absence of signal intensities in the bone marrow. Both diseases showed ipsilateral effusions with synovial thickening and enhancement.

Bone scintigraphy demonstrates mildly elevated uptake; however, bone scintigraphy may also reveal a transient decrease in uptake of technetium 99m phosphate. Bone scintigraphy does not help the physician differentiate etiologies.

Perform aspiration with ultrasonographic guidance 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
• ESR greater than or equal to 20 mm/h
• Severe hip pain and spasm with movement

The aspirate should assist the physician in differentiating transient synovitis from septic arthritis. The physician can confirm 30-50% of septic arthritis incidents with Gram stain. In individuals with septic arthritis, the WBC count varies (25,000-250,000/mcL); however, in these individuals, the WBC count consistently demonstrates 90% polymorphonuclear cells. Also, in persons with septic arthritis, the glucose is often less than 40 mg/dL or is markedly different from the serum glucose.

In one study, 36 children with an effusion underwent aspiration with ultrasonographic guidance. The Gram stain identified 1 child with an acute infection. The 35 children with a negative Gram stain were sent home with no further complications.

In another study published by Skinner et al, 25 children with a clinical diagnosis of transient synovitis were observed.^[15] They all had a joint effusion by ultrasound, but no aspiration was performed. The mean age of the patient population was 6 years, the average size of the effusions was 9 mm, and the distribution between the sides affected was equal. They found that by 2 weeks postdiagnosis, all patients were pain and limp free. The effusions, although still present in some, were decreasing in size. They concluded that transient synovitis is benign and can be treated with supportive therapy.