Causes / typical injury mechanism: Transient synovitis of the hip, also called toxic synovitis, presents with acute hip pain often following a recent upper respiratory infection (URI) or viral illness.

Classic history and presentation: Acute onset of unilateral hip pain (bilateral in 2%-5% of cases), characterized by pain, limping, joint effusion, and limited range of motion (ROM) commonly. This is seen in young children (ages 3-8 years) following a viral infection. Patients often have a history of antecedent URI symptoms, acute otitis media, vomiting, and/or diarrhea. Pain may be referred from the hip to the groin, thigh, and knee with or without low-grade fever. Affected patients generally appear well and are still able to bear weight but ambulate with a limp; however, some will refuse to bear weight. The hip pain is often vague in location, and the patient will usually hold the hip in mild flexion, abduction, and external rotation.

Prevalence:

• Age – Usually 3-8 years old.
• Sex / gender – Prevalence in males is twice that of females.
• Lifetime incidence – 0.2%-3%.

Risk factors: Preceding recent viral illness, which can present as a URI or as gastroenteritis (with vomiting and diarrhea).

Pathophysiology: Transient synovitis is caused by inflammation of the synovial lining of the hip.