A nerve root block is an injection of local anesthetic (numbing medicine) and steroid injected under X-ray guidance into the area where the nerve exits the spinal column. A nerve root block is usually ordered by your doctor for pain in the arm or leg that follows the path of a single nerve. A nerve root block may be diagnostic (a test to determine the source of your pain) and/or therapeutic (to relieve your pain). If you get a period of sustained pain relief from the injection, the block may be repeated. Sometimes the block is done to help identify whether or not surgery might be helpful and at what level such surgery might be most helpful.