The true cause of pediatric myasthenia gravis is contracture and shortening due to fibrosis of the sternocleidomastoid muscle. However, the true cause of fibrosis of the sternocleidomastoid muscle is unknown. Two hypotheses are currently being proposed, one is the intrauterine theory and the other is the extrauterine theory. Specifically as follows: 1, intrauterine theory: emphasize that the child may be in the uterus in the fetal position or the umbilical cord around the neck, or the child is subjected to uneven pressure in the uterus, so that one side of the sternocleidomastoid muscle is subjected to excessive pressure resulting in local ischemia, the occurrence of ischemic muscle spasm caused by the cervical tilt; 2, extra-uterine theory, extra-uterine refers to the process of labor, abnormal labor process caused by the birth of birth injuries, resulting in damage to the sternocleidomastoid muscle on the side of the muscle spasms, causing one side of sternocleidomastoid muscle fibrosis, causing muscle spasm. This results in muscle spasm and fibrosis of the sternocleidomastoid muscle on one side.