There are several possible causes for left shoulder numbness: first, cervical spondylosis, when cervical spondylosis compresses the nerve roots, numbness in the left shoulder can occur, cervical spine X-ray, nuclear magnetic examination for diagnosis and treatment, if necessary, cervical traction and oral medication. Second, nerve compression. When the suprascapular nerve is compressed, such as tumor compression or inflammation, numbness of the skin in the innervated area of the left shoulder can occur, and electromyography and physical examination should be performed for diagnosis. Thirdly, myofasciitis, myofasciitis can cause pain in the medial side of the left scapula, which can also radiate to the left shoulder and upper extremity, resulting in numbness and weakness, and the diagnosis should be made by physical examination to exclude other diseases.