Congenital torticollis caused by unilateral absence of the sternocleidomastoid muscle

Congenital torticollis is most commonly caused by benign fibrosis of the sternocleidomastoid muscle. Absence of the sternocleidomastoid muscle is a rare cause of congenital torticollis. There have been fewer than a dozen reported cases of agenesis of the sternocleidomastoid muscle. We describe a case of congenital absence of the sternocleidomastoid diagnosed by US and confirmed on MRI.