The thickness of the left and right sternocleidomastoid muscles may not be the same, which may be caused by physiological reasons, or local scar tissue hyperplasia, congenital muscular strabismus and other reasons. 1. Physiological reasons: the human body is not completely symmetrical, it may be due to the different labor intensity of the upper limbs, improper posture, etc., resulting in the thickness of the left and right sternocleidomastoid muscles are not the same, but generally physiological thickness of the left and right sternocleidomastoid muscles are not the same, can be restored by changing the bad posture, exercise and other ways. 2. Local scar tissue hyperplasia: As one side of the sternocleidomastoid muscle has been traumatized to form a scar, which will lead to the thickness of the left and right sternocleidomastoid muscles are not the same, early detection may also be corrected through rehabilitation training, or surgical resection of the scar tissue. 3. Congenital myotonic cervix: Congenital myotonic cervix can also lead to the thickness of the left and right sternocleidomastoid muscles are not the same, which can cause the patient’s neck to be skewed like one side, the symptoms of the face is short and wide, should be early in the hot compresses, massages, and other conservative treatments, conservative treatments do not have significant relief should be early in sternocleidomastoid muscle amputation or relaxation surgery treatment. The thickness of the sternocleidomastoid muscle is not the same as the left and right may also be other reasons, should go to the hospital in a timely manner to improve the examination, clear cause to give targeted treatment.