What is the cause of transient synovitis?

Cause of Transient Synovitis

The exact cause of transient synovitis remains unclear, but it is largely believed to be viral in nature, occurring as a post-infectious inflammatory response following an upper respiratory infection. 1

Clinical Characteristics and Etiology

• Transient synovitis is the most common cause of acute hip pain in children between 3-10 years of age 2
• The condition is self-limiting and benign, with symptoms typically resolving within 1 week 1
• While the exact etiology remains uncertain, several factors have been implicated:
  • Recent viral upper respiratory infections are commonly associated with transient synovitis 2
  • Recent COVID-19 infection has been reported as a potential trigger in some cases 3, 4
  • Bacterial infections and trauma have also been suggested as possible precipitating factors 2

Pathophysiology

• The condition involves inflammation of the synovial lining of the joint, particularly the hip joint 5
• It is characterized by a sterile effusion within the joint space that can be detected on ultrasound 6
• The inflammatory response is temporary, which distinguishes it from persistent synovitis (lasting >3 months) that may lead to joint damage 5

Differential Diagnosis

• Transient synovitis is a diagnosis of exclusion, with septic arthritis being the most important condition to rule out 2
• Key differentiating features from septic arthritis include:
  • Absence of fever (temperature <37.5°C/99.5°F) 2
  • Normal or only mildly elevated inflammatory markers (ESR <20 mm/hr, normal WBC, normal CRP) 1
  • Negative synovial fluid analysis and cultures 3
  • Less severe pain and spasm on hip movement 2

Diagnostic Approach

• Ultrasound is highly sensitive for detecting joint effusion, which is characteristic of transient synovitis 6
• In confirmed cases of transient synovitis, ultrasound-guided joint aspiration can lead to shorter hospital stays and reduced duration of limping 6
• MRI can help differentiate transient synovitis from septic arthritis, with decreased femoral head enhancement on early post-contrast imaging being a reliable feature of septic arthritis not seen in transient synovitis 6

Clinical Pitfalls

• Failure to exclude septic arthritis can lead to serious complications including femoral head destruction and permanent deformity 2
• Persistent symptoms beyond 7-10 days warrant reevaluation as this may indicate a different underlying condition 2
• Transient synovitis may be confused with early manifestations of more serious conditions such as Lyme arthritis in endemic areas 6

Understanding that transient synovitis is primarily a post-infectious inflammatory response helps guide appropriate management, which focuses on symptomatic treatment while monitoring for any signs that might suggest a more serious condition.