A Rare Cause of Neck Swelling: A Case Report and Review of Primary Pyomyositis of the Sternocleidomastoid.

Primary pyomyositis is a rare bacterial infection of skeletal muscle occurring without a contiguous source of infection. Involvement of the sternocleidomastoid muscle (SCM) is exceptionally uncommon in temperate regions and may result in serious complications if diagnosis is delayed. We report a case of primary pyomyositis of the left SCM in a previously healthy 19-year-old male who presented with rapidly progressive neck pain and swelling nine days after minor blunt neck trauma. Contrast-enhanced computed tomography demonstrated an intramuscular SCM abscess with marked muscle oedema. Due to worsening pain and concern for evolving compartment syndrome, urgent surgical drainage and fasciotomy were performed, followed by delayed primary closure using a vessel-loop technique. The patient made a full functional recovery following antimicrobial therapy. To place this presentation in the clinical context, previously reported cases of primary SCM pyomyositis were examined. These reports highlight a consistent pattern of acute presentation, frequent diagnostic delay, and the potential for serious complications, including mediastinitis, when treatment is delayed. This case emphasises the importance of early imaging and timely surgical intervention in young, immunocompetent patients presenting with unexplained lateral neck swelling.