The 'irritable hip' continues to pose a challenge for clinicians. Even with predictive clinical algorithms, decision making can be difficult. Emergency treatment is required if septic arthritis is suspected. Other differential diagnoses such as transient synovitis, pyomyositis of the pelvic girdle muscles and osteomyelitis must be considered in order to help guide appropriate investigations and allow early treatment. We report the case of a 13-year-old boy presenting to our institution with an acutely painful left hip but still able to weight bear. Despite a fever and raised inflammatory markers, the clinical examination did not correspond to that of an infected hip joint. Urgent magnetic resonance imaging (MRI) confirmed pyomyositis of the pectineus muscle. To our knowledge, this is the first reported case in the literature. The child was treated with seven days of intravenous antibiotics. There was a good clinical response as well as normalisation of the C-reactive protein level and white cell count. The patient was discharged home with a further week of oral antibiotics. Follow-up MRI at two weeks demonstrated a dramatic reduction in the inflammation of the pectineus. At the clinic follow-up appointment, the child was asymptomatic and back to normal function. Pyomyositis is typically found in tropical areas but its rates in temperate climates have been rising. It usually affects large groups of muscles such as the quadriceps and gluteal muscles. MRI is the gold standard investigation. If diagnosed early, the condition can be treated successfully with intravenous antibiotics alone. Given the widespread availability of MRI, we recommend its increased use to distinguish between pyomyositis and other paediatric hip pathologies.

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