The sternocleidomastoid (SCM) muscle runs down either side of the neck.
Wryneck affects the SCM muscle, usually on only one side of the neck, causing the neck to spasm painfully and twist.
Although this lump disappears by itself after about three months, the SCM muscle becomes tight, contracted, and fibrous.
One theory suggests that damage occurs during the birth causing a blood clot to form in the SCM muscle.
It is believed that either pressure on the SCM muscle due to position of the head in the uterus causes the muscle to become fibrous and shorten or that the blood supply to the muscle is disturbed and the muscle becomes scarred.