The causes of numbness in the left back include the following: 1. cervical spondylosis, after cervical spondylosis there will be herniated discs, which cause pain and numbness in the left back and shoulder. 2. it may be that the herniated disc occurs in the thoracic spine or the thoracolumbar segment, which can also cause numbness, swelling and pain in the left back at this time. 3. it may be localized fasciitis in the left back, which is often This kind of fasciitis often causes numbness, pain or limited movement. In addition, we should also consider tumors of bones (such as tumors of the spine and ribs) and soft tissues, which can cause numbness. Also note that rheumatism, ankylosing spondylitis, and rheumatoid arthritis can also cause localized numbness.