The causes of numbness and pain in the left leg and arm may be intracranial lesions, peripheral neuropathy, or simultaneous lesions of the left cervical spine and lumbar spine, cervical spinal stenosis, and so on. 1. Intracranial lesions: cerebral infarction, cerebral hemorrhage, etc. in the cranium, so that the nerve center innervating the left side of the limbs is damaged, and numbness and pain in the left leg and arm may occur, and may be accompanied by limb movement disorders, such as hemiplegia (one side of the limbs can not be freely moved) and so on. 2. Peripheral neuropathy: such as diabetic peripheral neuropathy or peripheral neuritis and other diseases, causing numbness in the innervated area, which can lead to numbness and pain in the left leg and arm. 3. Left cervical spine and lumbar spine at the same time: cervical spondylosis and lumbar disc herniation compress the nerve roots of the left side innervating the upper limbs and lower limbs respectively, which can lead to numbness and pain in the left leg and arm. 4. Cervical spinal stenosis: Simple cervical spinal stenosis can also cause numbness of the upper and lower limbs on one side. There may be other causes of numbness and pain in the left leg and arm. If the symptoms continue to be unrelieved, the patient should go to a regular hospital in time to complete the relevant examinations, so that the doctor can determine the specific cause of the disease according to the symptoms, signs and auxiliary examinations, and give the correct treatment for the cause of the disease in order to avoid delaying the disease.