Another common injection, a selective nerve root block (SNRB), is primarily used to diagnose the specific source of nerve root pain and, secondarily, for therapeutic relief of low back pain and/or leg pain.
If a nerve root becomes compressed and inflamed, it may produce back and/or leg pain. Occasionally, an imaging study (e.g. MRI) may not show which nerve is causing the pain and an SNRB injection is performed to isolate the source of pain. In addition to its diagnostic function, this type of injection for pain management can also be used as a treatment for a far lateral disc herniation (a disc that ruptures outside the spinal canal).
In an SNRB, the nerve is approached at the level where it exits the foramen (the hole between the vertebral bodies). The injection is done both with a steroid (an anti-inflammatory medication) and lidocaine (a numbing agent). To ensure the medication is delivered to the correct location, fluoroscopy (live X-ray) is used. If the patient’s pain goes away after the injection, it can be concluded that the back pain is caused by the specific nerve root that has just been injected.
Success rates vary depending on the primary diagnosis and whether or not the injections are being used primarily for diagnosis. While there is no definitive research to dictate the frequency of SNRBs, it is generally considered reasonable to limit SNRBs to three times per year.