Toxic synovitis, also known as transient synovitis, is the most common cause of hip pain in children. It is caused by a viral infection that sometimes (but not always) settles in the hip joint. The virus causes swelling at the hip joint, which makes walking painful. Usually just one hip is affected. But later on, the virus can pass to the other hip.
Toxic synovitis is more common in boys and affects preschool to early school-aged kids, but younger kids also can develop it.
Sometimes toxic synovitis is confused with septic arthritis, or infectious arthritis, a more serious condition caused by a bacterial infection that invades the joints and can cause long-term joint damage. Your doctor can rule out septic arthritis or other conditions through a physical exam and diagnostic tests.
Symptoms
When kids have toxic synovitis, the pain may come and go or shift from one hip to the other. Here are more signs to look for:
•recent history of a viral infection, such as a cold or stomach virus
•low-grade fever (up to 101°F)
•limping with toes turned outward, the knee bent or straightened
•walking on tiptoe
•hip discomfort that occurs after a long period of resting the joint (such as sitting in a car, at a desk, or watching TV)
•knee or thigh pain with no hip pain
•in younger children, crying
•in some cases, a refusal to walk
In babies, the most common signs of toxic synovitis are abnormal crawling and crying, and they may cry in situations where their hip joints are being moved, such as diaper changing.
Diagnosis
The first thing a doctor will do is examine your child, checking to see what kind of movement is painful by moving the knee, the hip, and other joints. This is to confirm that limping is caused by hip pain.
Next, the doctor may order an ultrasound of the hip, an imaging test that will determine whether there is fluid in the hip joint. Fluid indicates swelling and inflammation in the tissues. The doctor also might do a blood test to look for inflammatory markers, other signs that show the extent of swelling in the hip joint.
In cases where the inflammatory markers or fever seem high and the doctor is having difficulty ruling out septic arthritis, your child may need to have fluid removed for a culture (lab test). The doctor also may perform blood tests to rule out Lyme disease, as well as X-rays for Legg-Calve-Perthes disease, a progressive condition that causes a loss of bone tissue.
Treatment
Treatment for toxic synovitis usually includes anti-inflammatory drugs such as naproxyn or ibuprofen. Your child will take these medications for up to 4 weeks until the inflammation goes away. The doctor may also prescribe medicine for pain, such as acetaminophen.
Resting the hip joint is important and it's best if your child avoids putting weight on the hip while recovering. Usually within a day or two of taking the anti-inflammatory medicine, kids are able to walk again comfortably. However, participation in activities like gym class or contact sports may have to wait until your child fully recovers.