This case represents a benign infantile sternocleidomastoid muscle fibromatosis. The patient's history of vacuum assisted vaginal delivery is a known risk factor.

The proposed mechanism for fibromatosis colli is birth trauma leading to intramuscular hemorrhage, and subsequent fibrosis.

Ultrasound is the imaging modality of choice and performed for diagnostic confirmation as well as the exclusion of other pathology in the neck.

Courtesy of Dr. Maxim Pougatchev.