Pediatric Psoas Abscess

A febrile, toxic child holding the hip flexed and resisting extension with refusal to weight-bear is the classic picture of hip septic arthritis, but if the hip ultrasound shows no joint effusion I would strongly consider a psoas, or iliopsoas, abscess, which closely mimics a septic hip. A psoas abscess is pus within the psoas or iliacus muscle; it may be primary, arising haematogenously and most often due to Staphylococcus aureus including MRSA in an otherwise healthy child, or secondary, spreading from the spine - osteomyelitis or discitis - or from bowel or renal infection. The clue is the positive psoas sign, pain on hip extension, because the inflamed psoas is stretched, together with flank, back or groin pain. I would check inflammatory markers, which are raised, take blood cultures, and obtain an MRI, which is the key diagnostic tool: it defines the abscess and its extent and detects any spinal source, with ultrasound and contrast CT as alternatives. The differential also includes transient synovitis, osteomyelitis, discitis, appendicitis and pyomyositis. Management is antibiotics plus drainage: empirical antibiotics covering S. aureus and MRSA, then culture-directed, and drainage of the abscess - image-guided aspiration for a small accessible collection or open retroperitoneal drainage for a larger or loculated one - and I would identify and treat any secondary source. With prompt imaging and treatment the prognosis is good. The key error to avoid is dismissing the child as transient synovitis, which would delay treatment of a true abscess.