Differential MRI findings of transient synovitis of the hip in children when septic arthritis is suspected according to symptom duration.

We investigated the differential MRI findings in children with transient synovitis of the hip in whom septic arthritis was suspected. Under the hypothesis that disease progression can alter representative MRI findings, we stratified these findings in accordance with symptom duration as this can correlate with disease progression. We analyzed 65 children who underwent MRI for acute hip pain and who were suspected of having a septic condition (i.e. presented with fever or increased inflammatory markers) when the imaging was performed. Symptom duration was defined as the interval from the first presentation of hip pain to the MRI scan. We divided the patients into two subgroups according to symptom duration: patients with short symptom duration (≤2 days, short-term subgroup, n = 30) and those with long symptom duration (≥3 days, long-term subgroup, n = 35). Twenty-eight (43.1%) of the study subjects were diagnosed with septic arthritis. Whereas only a high-grade joint effusion was a significant MRI finding differentiating septic arthritis from transient synovitis in the whole cohort, the presence of contralateral joint effusion in the short-term subgroup (P = 0.024) and the absence of a change/enhancement of the signal intensity of soft tissue in the long-term subgroup (P < 0.001) were significant predictors of transient synovitis. The significances of differential MRI findings for septic arthritis and transient synovitis seem to change according to symptom duration. We suggest that symptom duration, which may correlate with disease progression, should also be considered when interpreting MRIs of children under suspicion of septic arthritis.