First of all, numbness in the left arm may be the precursor of cerebrovascular disease, generally when cerebrovascular disease occurs ischemia or hemorrhage, the patient will have the corresponding symptoms of neurological deficit; cerebrovascular disease caused by the left arm numbness can be accompanied by other symptoms of neurological deficit, such as dizziness, headache, clumsy language, if necessary, you can check the head CT or nuclear magnetic examination to confirm the diagnosis; secondly, numbness in the left arm can also be seen in Cervical spondylosis, due to the protrusion of the cervical intervertebral discs leading to compression of the surrounding dural sac and nerve roots, the patient can have left arm numbness. Finally, numbness in the left arm can also be caused by localized lesions in the left arm, such as frozen shoulder, elbow tunnel syndrome or carpal tunnel syndrome.