Primary bacterial intercostal pyomyositis diagnosis: A case report

Pyomyositis is a microbial infection of the muscles and contributes to local abscess formation. Staphylococcus aureus frequently causes pyomyositis; however, transient bacteremia hinders positive blood cultures and needle aspiration does not yield pus, especially at the early disease stage. Therefore, identifying the pathogen is challenging, even if bacterial pyomyositis is suspected. Herein, we report a case of primary pyomyositis in an immunocompetent individual, with the identification of S aureus by repeated blood cultures. PATIENT CONCERNS: A 21-year-old healthy man presented with fever and pain from the left chest to the shoulder during motion. Physical examination revealed tenderness in the left chest wall that was focused on the subclavicular area. Ultrasonography showed soft tissue thickening around the intercostal muscles, and magnetic resonance imaging with short-tau inversion recovery showed hyperintensity at the same site. Oral nonsteroidal anti-inflammatory drugs for suspected virus-induced epidemic myalgia did not improve the patient’s symptoms. Repeated blood cultures on days 0 and 8 were sterile. In contrast, inflammation of the soft tissue around the intercostal muscle was extended on ultrasonography. DIAGNOSES: The blood culture on day 15 was positive, revealing methicillin-susceptible S aureus JARB-OU2579 isolates, and the patient was treated with intravenous cefazolin. INTERVENTIONS: Computed tomography-guided needle aspiration from the soft tissue around the intercostal muscle without abscess formation was performed on day 17, and the culture revealed the same clone of S aureus. OUTCOMES: The patient was diagnosed with S aureus-induced primary intercostal pyomyositis and was successfully treated with intravenous cefazolin for 2 weeks followed by oral cephalexin for 6 weeks. LESSONS: The pyomyositis-causing pathogen can be identified by repeated blood cultures even when pyomyositis is non-purulent but suspected based on physical examination, ultrasonography, and magnetic resonance imaging findings.