Congenital absence of the sternocleidomastoid muscle
is a rare condition where a person is born without the neck's most prominent muscle (the sternocleidomastoid). This muscle normally comes in a pair, one on the right and left side of the neck. It allows the head to turn to the other side (rotate) as well as the neck to bend forward and backward (flexure). It is also involved in breathing. There have been twelve reported cases of individuals being found to have one of their sternocleidomastoid muscles missing (unilateral absence). This absence may limit the range of head and neck motion, however in other cases the individuals did not have any physical limitations (and thus the condition may be more common than thought). Several cases have been reported to cause congentital torticollis which is a condition in which the individuals head is held tilted to one side, but it is not the most common cause of congenital torticollis (for more information see Congenital torticollis.
). It is unknown what causes congenital absence of the sternocleidomastoid muscle. When range of motion of the neck or head is limited, physical therapy may help. Talk with your doctor if you or a family member has been diagnosed with congenital absence of the sternocleidomastoid muscle.