Fibromatosis colli leading to positional plagiocephaly with gross anatomical and sonographic correlation.

Fibromatosis colli, also known as 'sternocleidomastoid tumour of infancy' or 'pseudotumour of infancy', is a rare condition involving fibrosis and swelling, or 'tumour' of the sternocleidomastoid muscle in newborns that typically occurs after a traumatic delivery. Although usually self-limited, fibromatosis colli can lead to congenital muscular torticollis and positional plagiocephaly due to uneven forces on the neonatal skull. Ultrasound is the diagnostic imaging modality of choice and can prevent additional imaging and unnecessary intervention.