Transient synovitis is the most common cause of acute hip pain in children. This condition has been known by many other names, including transient epiphysitis, coxitis serosa, irritable hip, intermittent hydarthosis, phantom or observation hip, and toxic, reactive or nonspecific synovitis.
While transient synovitis may cause considerable pain, it is a self-limited, usually benign condition. Joints other than the hip may be involved. Rarely, adults are affected.
The etiology of transient synovitis remains unknown. Some studies associate up to 6 percent of cases with trauma. A causal relationship with an allergic reaction to an infectious agent has been suggested but not proven. One study demonstrated a fourfold rise in viral titers in 45 percent of patients with transient synovitis. The study also revealed that 43 percent of these patients had elevated serum interferon levels, consistent with a concurrent viral infection. To date, this is the best evidence to support a viral etiology for transient synovitis.
The diagnosis of transient synovitis is one of exclusion, and this condition must be quickly and accurately distinguished from several disorders that require prompt definitive treatment (Table 1). The initial hip radiographs are quite helpful in diagnosing Legg-Calve-Perthes disease and slipped capital femoral epiphysis, but the task of differentiating septic arthritis from transient synovitis often requires tests and procedures that are not available in the office setting. It is particularly important to rule out septic arthritis, a disorder with potentially devastating effects.
Ultrasound examination can help distinguish transient synovitis from early Legg-Calve-Perthes disease. Synovial membrane thickening is often present in patients with Legg-Calve-Perthes disease but is usually absent in those with transient synovitis. However, since overlap occurs, synovial membrane thickening only serves as a clue in differentiating between the two entities.
Ultrasonography is best used to detect joint effusion and to guide hip joint aspiration. It cannot identify the cause of a joint effusion.
ASPIRATION OF JOINT EFFUSION
Hip joint aspiration should be performed when septic arthritis is suspected. Gram staining of the aspirated fluid will confirm the diagnosis of septic arthritis in 30 to 50 percent of cases. White blood cell counts in the joint fluid vary from 25,000 to 250,000 per [mm.sup.3] (25 to 250 x [10.sup.9] per L) but consistently show 90 percent polymorphonuclear leukocytes. The aspirate glucose concentration is often less then 40 mg per dL (2.2 mmol per L) or is markedly different from the serum glucose level.
Aspirate cultures are positive in 66 to 75 percent of cases of septic arthritis. In culture-negative cases, the diagnosis of septic arthritis is made on the basis of purulent joint fluid. In most culture-negative cases, children have received antibiotics for other reasons, such as an ear infection, which may confuse the diagnosis and may make cultures less likely to grow a pathogen.
The diagnosis of suspected transient synovitis is enhanced by a thorough medical history and a careful physical examination, with attention to the temperature, the presence or absence of hip tenderness with palpation and the amount of pain and spasm noted with hip rotation. In particular, children with transient synovitis have a pain-free range of passive motion, whereas those with septic arthritis have pain with any motion.
The laboratory evaluation should focus on the erythrocyte sedimentation rate. Initial radiographs include an anteroposterior view of the pelvis and a "frog-leg" lateral view. At this point, the physician must decide whether or not to aspirate the joint. As an adjunctive step, a sonogram can be obtained to document the presence or absence of an effusion. Recent studies[2,4,9] indicate that, if laboratory and clinical suspicion is extremely low, patients can be followed closely on an outpatient basis.
To rule out septic arthritis, hip aspiration should be performed in patients with sonographic evidence of a hip effusion and any of the predictive criteria, including a temperature of 37.5[degrees]C (99.5[degrees]F) or more, an erythrocyte sedimentation rate of 20 mm per hour or greater, and the presence of hip pain with palpation or the presence of severe hip pain and spasm with movement (particularly pain with movement in all directions). A joint aspiration will not affect the results of future bone scans (if they become necessary).
Aspiration may be performed with fluoroscopic or ultrasound guidance. Ultrasound guidance is preferred, because there is less radiation exposure. If aspiration is performed under fluoroscopy and the initial attempts are "dry," dye should be injected to confirm that the joint capsule has truly been entered. Some authorities believe that a dry tap suggests the presence of an infection and is an indication for surgical drainage. When ultrasound-guided aspiration is performed, it is evident when the joint capsule is entered.
The management of transient synovitis consists of home care with bed rest. Nonsteroidal anti-inflammatory drugs (NSAIDs) may be used. Accurate temperatures must be obtained on a regular basis, and any fever must be reported to the physician. With rest and use of NSAIDs, transient synovitis usually resolves rapidly.
A repeat examination after 12 to 24 hours will usually help affirm an initial diagnosis of transient synovitis. Patients should be reevaluated if significant residual symptoms persist seven to 10 days after the initial presentation. In some cases, however, low-grade symptoms can last up to several weeks.
Sequelae and Follow-up
Sequelae or conditions associated with transient synovitis include coxa magna, Legg-Calve-Perthes disease and mild degenerative cystic changes of the femoral neck. Coxa magna is the enlargement and deformity of the femoral head and neck. This condition is caused by the hypertrophy of cartilage due to inflammation.
The reported incidence of Legg-Calve-Perthes disease following transient synovitis is about 2.5 percent (range: 1 to 20 percent).[12,18] Consequently, it has been recommended that follow-up radiographs be obtained six months after an episode of transient synovitis. However, some experts reserve radiographs for children who have stiffness or other hip symptoms.
Patients who develop Legg-Calve-Perthes disease usually have recurrent and/or persistent symptoms after the initial onset of transient synovitis. Ultrasound evidence of persistent joint effusion four to six weeks after transient synovitis is cause for suspicion that the patient will eventually develop Legg-Calve-Perthes disease. Legg-Calve-Perthes may be a true sequela of transient synovitis, but some cases undoubtedly represent an initial misdiagnosis of transient synovitis, when the true diagnosis was early Legg-Calve-Perthes disease.
[Figure 1 ILLUSTRATION OMITTED]
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The Author JOHN J. HART, M.D. is in private practice in Wichita, Kan. Previously, he was associate director of the Wesley family practice residency program and clinical assistant professor in the Department of Family and Community Medicine at the University of Kansas School of Medicine-Wichita. Dr. Hart is a graduate of the University of Kansas School of Medicine and the Wesley family practice residency program
Address correspondence to John J. Hart, M.D., Hillside Medical Office, 855 N. Hillside, Wichita, KS 67214.
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