There are several possible causes for left arm pain and numbness: First, cervical spondylosis. When cervical spondylosis compresses the nerve roots on the left side, the symptoms of left arm pain and numbness can occur, which can be accompanied by finger weakness, and then the diagnosis and treatment should be carried out by X-ray and MRI of the cervical spine. Second, head disease, when there is a tumor in the head, it can compress the nerve, resulting in left arm pain and numbness, often accompanied by weakness of the left upper limb, then the relevant head MRI and CT examination should be performed for early diagnosis and treatment. Thirdly, myofasciitis, when myofasciitis occurs, it can radiate to one upper limb, resulting in pain, numbness and weakness, and it is necessary to conduct relevant physical examination and examination to exclude other diseases for diagnosis. Fourth, cervical spine tumor, when the cervical vertebral body or spinal nerve tumor, can also compress the nerve and appear the symptoms of left arm pain and numbness, then we should conduct cervical spine nuclear magnetic examination for diagnosis and treatment.