Transient Synovitis

Transient synovitis is a benign, self-limiting inflammatory condition of the hip joint. It is the commonest cause of acute hip pain and limp in young children. It typically occurs between three and eight years of age, frequently following a preceding viral upper respiratory tract infection.

The critical clinical priority is confident, careful distinction from septic arthritis. The management implications differ substantially, and missing the alternative diagnosis can have serious consequences.

Structured clinical prediction rules support, though do not entirely replace, careful clinical judgement in making this distinction.

Pathophysiology

• A transient, self-limiting synovial inflammatory reaction within the hip joint produces a sterile joint effusion.
• The trigger is believed in many cases to relate to a recent or concurrent viral infection. The precise mechanism linking the antecedent viral illness to the hip synovitis remains incompletely understood.
• The condition is characteristically self-limiting. Synovial inflammation and effusion resolve spontaneously over one to two weeks in the great majority of affected children.
• No specific, targeted treatment is required beyond symptomatic management. There are no recognised long-term sequelae in the substantial majority of cases.

• The characteristic presentation is acute onset of hip pain with a limp or refusal to bear weight, developing over hours to a day or two. Pain may present in the groin, thigh or, given the referred pain pattern, knee. There is frequently a recent or concurrent, generally mild, viral upper respiratory tract infection.
• Affected children are characteristically well or only mildly unwell, without significant fever or marked systemic upset. A low-grade temperature may occasionally be present, which is important given the potential for diagnostic overlap with septic arthritis if fever is present.
• Absence of marked systemic upset is an important, though not entirely absolute, distinguishing feature from septic arthritis.
• Examination shows reduced range of hip motion, particularly internal rotation. The child frequently holds the hip in flexion, abduction and external rotation, the position that maximises intracapsular volume and minimises discomfort from the effusion.
• The child generally remains able to bear at least some weight, or shows only partial reluctance. This contrasts with the more typically complete inability or refusal to bear any weight in septic arthritis.
• The condition is generally unilateral. Bilateral presentation is uncommon and warrants careful consideration of an alternative underlying cause.

• Diagnosis combines the characteristic clinical presentation with structured application of validated clinical prediction rules.
• The most notable is the Kocher criteria: fever, inability to bear weight, elevated erythrocyte sedimentation rate, and elevated white cell count. The more criteria present, the greater the probability of septic arthritis rather than transient synovitis.
• These rules support risk stratification but do not provide absolute diagnostic certainty in isolation. Careful, ongoing clinical judgement remains essential alongside them.
• Full blood count and inflammatory markers (erythrocyte sedimentation rate and C-reactive protein) are generally mildly elevated or normal in transient synovitis. Septic arthritis typically, though not invariably, shows more substantial elevation. This is useful but imperfect discriminating information.
• Hip ultrasound demonstrates a joint effusion in the great majority of children with transient synovitis. It does not distinguish transient synovitis from septic arthritis, as both characteristically produce an effusion. It confirms an effusion warranting further diagnostic consideration rather than providing definitive distinction.
• Where uncertainty persists after full clinical and laboratory assessment, joint aspiration (generally ultrasound guided) gives definitive clarification. This applies particularly where any specific risk factor is present or the clinical picture remains ambiguous. Microscopy, culture and cell count of the aspirated synovial fluid are performed.
• Maintain a low threshold for aspiration, given the serious consequences of missing septic arthritis.

• Symptomatic management is the standard, appropriate approach once septic arthritis has been reasonably and confidently excluded. It comprises rest, simple analgesia (paracetamol or a nonsteroidal anti-inflammatory drug), and gradual, progressive return to normal activity as symptoms allow.
• The condition is generally self-limiting. Symptoms typically resolve substantially within a few days to one to two weeks, and complete resolution is generally anticipated within this timeframe.
• Give the family clear, explicit safety-netting advice on the features that warrant prompt reassessment: persistent or worsening pain, new fever, or continued inability to bear weight beyond the expected resolution timeframe.
• Where diagnostic uncertainty exists, or any relevant red flag feature is present, an appropriately cautious approach is a period of close, structured observation. This may be in hospital or through arranged, prompt outpatient reassessment.
• Keep a low threshold for repeat clinical assessment or, where the clinical picture evolves or fails to improve as expected, joint aspiration. Missing an underlying septic arthritis is the essential concern.
• Recurrence occurs in a proportion of affected children and is generally managed with the same symptomatic approach as the initial episode.
• The overall prognosis is excellent. There is no recognised association with long-term hip joint damage or dysfunction in the great majority of affected children.