Title: Pyomyositis of the Sternocleidomastoid Muscle Presenting With Torticollis in an Infant

Background Pyomyositis is a bacterial infection of skeletal muscle that is uncommon in children and is frequently misdiagnosed due to its nonspecific presentation. The involvement of the sternocleidomastoid muscle is rare in infants. Since early-stage pyomyositis presents without abscess formation or nonspecific symptoms, it is particularly prone to delayed or incorrect diagnosis in infants. We report a case of sternocleidomastoid pyomyositis in a previously healthy 6-month-old male infant. There was no relevant family history of immunodeficiency or recurrent infections. Case presentation The patient developed fever, followed by progressive left cervical swelling, which was initially suspected to be due to cervical lymphadenitis. Despite oral antibiotic therapy, the symptoms worsened, and torticollis developed. Laboratory evaluations revealed leukocytosis and elevated C-reactive protein levels. Contrast-enhanced computed tomography revealed enlargement of the left sternocleidomastoid muscle with reactive cervical lymphadenopathy, raising suspicion of pyomyositis. Subsequent magnetic resonance imaging revealed diffuse muscle swelling and T2 hyperintensity without abscess formation, which was consistent with invasive-stage pyomyositis. The patient was successfully treated with intravenous cefazolin followed by oral cefaclor, with complete clinical and radiological resolution after 21 days of antibiotic therapy. Blood cultures were negative, immunological evaluation revealed no underlying abnormalities, and no recurrence was observed during the more than two years of follow-up. Conclusion This case highlights the importance of considering sternocleidomastoid pyomyositis in infants presenting with fever, cervical swelling, or torticollis. Early CT or MRI is critical for distinguishing pyomyositis from cervical lymphadenitis and for initiating timely medical treatment before abscess formation.