Transient synovitis, which is often referred to as toxic synovitis, is a temporary type of arthritis. This condition only affects children, more commonly boys than girls, usually between the ages of three to 10. The arthritis usually affects one hip, however the knee or thigh may also be stricken. It typically causes pain and swelling, and the parent may also notice the child walking with a limp.

Arthritis is usually chronic; however, transient synovitis both appears and disappears suddenly, usually within seven to 10 days. The exact cause of this temporary condition is unknown, but it does often appear after the child has suffered from a viral infection. This post-infectious arthritis may follow diseases such as chickenpox, mumps, and Epstein-Barr virus, as well as hepatitis B, herpes, and smallpox. Symptoms of transient synovitis may mimic other conditions, such as septic arthritis, so parents should bring the child to the doctor to rule out other possible conditions.

In addition to pain and swelling, a child with transient synovitis may suffer from a low fever. The child may complain of pain in the front and middle of the thigh. He will typically display a limp that cannot be explained by physical causes, such as an injury. Pain in the hip or knee joint may increase with walking, other movements, and standing. To relieve pain, a child with this condition may lie on his back with the knee on the affected side bent.

Although transient synovitis is temporary, the child should be tested to ensure that the source of the pain is not a more serious condition, such as septic arthritis or Legg-Calve-Perthes disease. The doctor will likely take imaging tests of the hip, such as x-rays, an ultrasound, and a bone scan. He may order a complete blood cell (CBC) count. To confirm the diagnosis, the doctor may sometimes aspirate, or draw out, fluid from the joint with a needle and send it to the laboratory for testing.

There is little treatment needed or available for transient synovitis, other than measures taken to alleviate the child's discomfort. The parent should keep the child home from school and encourage him to remain in bed and limit movement. When the child suffers from a fever, the parent should take his temperature periodically and report any increase in temperature. A nonsteroidal anti-inflammatory drug (NSAID), like ibuprofen, may be administered to alleviate inflammation, pain, and swelling. Children who do not recover within 10 days should be re-examined by the doctor.