A left cervical mass in a one-month-old infant: fibromatosis colli case report

Fibromatosis colli (FC) is a rare, benign, and self-limiting fibroblastic proliferative lesion of the ster-nocleidomastoid muscle that typically presents as a neck mass during the first weeks of life and may be associated with torticollis. This case aims to emphasize the clinical and ultrasonographic features of FC and the favorable outcome of early conservative management. A female infant born at 40 weeks and 2 days of gestation via normal spontaneous vaginal delivery, weighing 2,960 g, presented on postnatal day 28 with a swelling in the neck region. Physical examination revealed a well-circumscribed, painless, non-fixed mass measuring approximately 1×1 cm, localized to the left sternocleidomastoid muscle. No significant torticollis was observed, and pregnancy and delivery history were unremarkable. Laboratory findings showed no evidence of infection or inflammation. Neck ultrasonography demonstrated fusi-form thickening of the left sternocleidomastoid muscle, loss of normal fibrillar muscle architecture, and increased vascularity on Doppler examination, consistent with FC. Conservative management with phy-sical therapy was recommended. During follow-up, the mass gradually regressed and completely resol-ved by the fourth month. This case shows that FC may occur even without traumatic delivery or evident torticollis. The left-sided localization is also notable, as right-sided involvement is more commonly reported. Ultrasonography is essential for diagnosis and helps avoid unnecessary advanced imaging or invasive procedures. FC should be considered in the differential diagnosis of infantile neck masses. Early ultrasonographic evaluation and conservative treatment are usually sufficient for favorable out-comes.