Epidemiology, diagnosis, and treatment of pericapsular pyomyositis of the hip in children.

Background: The yield of synovial fluid cultures in patients meeting clinical criteria for septic hip arthritis remains low. In the presence of positive blood cultures, these patients are diagnosed and treated as "presumed septic arthritis." We hypothesized that some of these patients may instead have an extra-articular infection, such as pericapsular pyomyositis.

Methods: An IRB-approved prospective study of children with suspected septic hip arthritis at a tertiary care children's hospital over a 2-year time period was conducted. Children were evaluated with a previously published clinical algorithm with the addition of magnetic resonance imaging (MRI).

Results: Of the 53 patients presenting with an acutely irritable hip, 32% were found to have pericapsular pyomyositis, whereas 15% were diagnosed with septic arthritis. Although C-reactive protein (CRP, ≥33.1 mg/L) performed well at predicting infection, there were no significant differences in CRP, erythrocyte sedimentation rate, white blood cell count, temperature, or weight-bearing status in children with septic arthritis compared with pericapsular pyomyositis. In addition to MRI, there was a difference in the size of hip effusion on ultrasound, which was significantly smaller in cases of pericapsular pyomyositis. CRP (≥74.3 mg/L) was found to be predictive of need for surgical intervention in children with pericapsular pyomyositis.

Conclusions: Correct anatomic diagnosis of the site of infection is essential for the efficient care of the child. Herein, we found that pericapsular pyomyositis is twice as common as septic arthritis in children presenting with an acutely irritable hip. Clinical algorithms are incapable of differentiating these pathologies suggesting that both be considered under the current diagnosis previously referred to as "presumed septic arthritis." Incorrect diagnosis of a septic arthritis in the presence of a pericapsular pyomyositis could potentially lead to unnecessary debridement of the joint in the presence of extra-articular infection, thus contaminating the joint. Conversely, debriding the joint instead of the epicenter of the infection can prolong the infectious process. For these reasons, we conclude that MRI has the potential to improve the clinical care of children by providing a more precise diagnosis.

Level Of Evidence: Level II-"Diagnostic" [Development of diagnostic criteria on the basis of consecutive patients (with universally applied reference "gold" standard)].