Fibromatosis Colli Presenting as a Left-Sided Cervical Mass: A Case Report

Fibromatosis colli is a rare, benign condition of infancy characterized by fibrous enlargement of the sternocleidomastoid (SCM) muscle. It typically presents during the first several weeks of life as a painless cervical mass and may be associated with congenital muscular torticollis (CMT). Ultrasonography is the preferred initial imaging modality; however, variable sonographic appearances may complicate recognition. A two-week-old female patient, with an uncomplicated neonatal course, presented for a routine well-child examination and was found to have a new, painless, left-sided cervical mass. Initial ultrasonography demonstrated a 2.1 × 1.5 × 2.5 cm circumscribed structure with internal Doppler vascularity and no visible fatty hilum, raising concern for a solid lesion or abnormal lymph node. Contrast-enhanced computed tomography demonstrated fusiform enlargement of the left SCM without a discrete cervical mass or lymphadenopathy, consistent with fibromatosis colli. Subsequent review of the initial ultrasound at a tertiary pediatric center was also considered consistent with the diagnosis. At approximately seven weeks of age, rightward head rotation and leftward head tilt consistent with CMT were documented, and the patient was managed conservatively with physical and occupational therapy. At approximately 4.5 months following diagnosis, parental report indicated equal bilateral head rotation and achievement of multiple age-appropriate developmental goals. This case highlights several clinically relevant features of fibromatosis colli. The condition may present after an initially normal newborn examination, and its variable sonographic appearance may create diagnostic uncertainty. Recognition of the relationship between the sonographic abnormality and the SCM may improve diagnostic confidence and potentially avoid additional imaging with ionizing radiation. Continued clinical surveillance is important because CMT may become apparent after the initial presentation and can be managed conservatively.