Pain in the area of sternocleidomastoid muscle may be caused by the following reasons: First, it may be caused by trauma, such as accidental sprain, or local contusion, which is usually manifested as localized swelling, and in severe cases, subcutaneous petechiae may also appear, and there is a clear history of trauma. Secondly, the pain may be caused by inflammation of the tissues near the sternocleidomastoid muscle, for example, the lymph nodes near the sternocleidomastoid muscle are enlarged by inflammation, or the inflammation of the thyroid gland induces the pain of the sternocleidomastoid muscle. Generally, a hard granular substance is felt near the sternocleidomastoid muscle and is painful to press, which is caused by enlarged lymph nodes. In the case of thyroiditis, the localized enlargement is usually in the middle of the upper part of the neck, and the enlargement goes up and down with swallowing activities.