LAUSR

A previously healthy 10yearold girl presented with 3 days of fever, right groin pain and a limp. She had been active on a new bicycle in the last few days without a definite history of trauma. She had mild perineal tenderness; musculoskeletal and gynaecological examinations were otherwise normal. Blood tests showed a C reactive protein of 162 mg/L without leucocytosis. Pelvic ultrasound was unremarkable and blood culture was sterile. She was treated with intravenous ceftriaxone, but her fever and perineal pain continued. An unenhanced MRI of the pelvis revealed a 1.4×1.2×1.7 cm collection in the right obturator internus muscle (figure 1). Surrounding oedema extended into the right obturator externus. Her antibiotic was changed to flucloxacillin given intravenously for 1 week followed by 5 weeks orally. Obturator pyomyositis is a rare condition in immunocompetent children, and the diagnosis is often missed or delayed. Minor trauma or intense exercise such as cycling are sometimes described in the history. Discrete perineal tenderness and preservation of hip movement on examination are clues which can distinguish obturator pyomyositis from conditions such as septic arthritis or psoas abscess. MRI is the investigation of choice and is highly sensitive to diagnose obturator abscesses, but investigation may be delayed if these clinical clues to the diagnosis have been missed. Common causative organisms include Staphylococcus aureus and Streptococcus pyogenes. Treatment comprises analgesia, bed rest and intravenous antibiotics until there is clinical improvement and reduction in fever and inflammatory markers. Surgical drainage is occasionally needed when conservative treatment fails.