Sternocleidomastoid muscle enlargement may be caused by local aseptic inflammation, the harm is that there will be swelling, pain or fever discomfort; may also be caused by malignant tumors of the transverse muscle, the harm is that in addition to the pain of the serious cases will be life-threatening. 1. Local aseptic inflammation: due to the stimulation of inflammatory mediators, local swelling, will affect the surrounding blood vessels, thus causing blood circulation is impeded, thus causing local swelling, the harm is that there will be a sense of swelling, pain or fever, with the disappearance of the inflammation will relieve the symptoms. 2. Transverse muscle malignant tumor: malignant tumors caused by abnormal development of muscle cells, the danger is that in addition to pain, serious cases will be life-threatening. If the symptom of enlarged sternocleidomastoid muscle persists without relief, you need to consult a doctor in time to find out the cause and actively treat it.