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Transient synovitis (also called irritable hip) is the most common cause of acute hip pain in children, typically affecting boys aged 3–10 years.

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It is a self-limiting, benign inflammatory condition of the synovium, often following a viral illness or minor trauma.

Etiopathogenesis

Feature	Description
Cause	Unknown; likely post-viral inflammatory response or trauma-related
Pathology	Sterile effusion and synovial hypertrophy causing pain and limited motion
Laterality	Unilateral in >90% cases

Epidemiology

Factor	Detail
Age	Peak incidence: 3–8 years
Sex	Boys > girls (2–4:1 ratio)
Preceding illness	Often follows upper respiratory tract infection

Clinical presentation

Feature	Description
Limping	Most common presenting symptom (painless or antalgic)
Hip or groin pain	May radiate to thigh or knee
Reduced hip motion	Especially abduction and internal rotation
Afebrile or low-grade fever	Helps distinguish from septic arthritis
No systemic toxicity	Child appears well

Kocher Criteria (to differentiate from septic arthritis)

≤1 Kocher criterion → strongly suggests transient synovitis

Criterion	Transient Synovitis	Septic Arthritis
Non–weight-bearing	Possible	Common
Temperature >38.5°C	Rare	Common
ESR >40 mm/hr	Rare	Likely
WBC >12,000/mm³	Rare	Likely

Differentials

Condition	Key Differentiators
‣	High fever, ill child, ↑ CRP/ESR, joint destruction
‣	Gradual onset, femoral head necrosis on imaging
‣	Chronic symptoms, morning stiffness
Slipped capital femoral epiphysis (SCFE)	Older children (≥10), posterior slippage on X-ray

Laboratory Findings

Test	Result in Transient Synovitis
WBC count	Normal or mildly elevated
ESR/CRP	Usually normal to mildly raised
Blood cultures	Negative