Neonatal psoas pyomyositis simulating pyarthrosis of the hip.

Osteomyelitis with concomitant pyarthrosis or isolated pyarthrosis heads the differential list for a newborn with limb disuse and a flexion deformity of the hip. The diagnosis of psoas abscess, a primary suppurative myositis, should additionally be entertained. Stronger consideration should be given to this clinical entity after pyarthrosis of the hip has been excluded by an arthrocentesis performed under fluoroscopy. Radiologic imaging may facilitate the diagnosis. A blood culture may yield the organism responsible for the skeletal muscle abscess, even in nontoxic patients.