There are several possible causes for left arm and shoulder pain: First, cervical spondylosis. When cervical spondylosis compresses the nerve roots, it can cause pain and numbness in one shoulder and arm, and then an X-ray, CT or MRI of the cervical spine should be performed for early diagnosis and treatment. Second, shoulder joint disease, when the shoulder joint has frozen shoulder or rotator cuff tear, it can also appear one side of the arm and shoulder pain, then we should carry out relevant cervical spine X-ray and MRI of the shoulder joint for early diagnosis and treatment. Thirdly, myofasciitis, patients with myofasciitis can have early pain in the medial scapula, which can radiate to the ipsilateral shoulder and arm, resulting in numbness, weakness and pain.