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 (see Figure 1).
When a nerve root becomes compressed and inflamed, it can produce back and/or leg pain. Occasionally, an imaging study (e.g. MRI) may not clearly show which nerve is causing the pain and an SNRB injection is performed to assist in isolating 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). Flouroscopy (live X-ray) is used to ensure the medication is delivered to the correct location. If the patient’s pain goes away after the injection, it can be inferred that the back pain generator is the specific nerve root that has just been injected. Following the injection, the steroid also helps reduce inflammation around the nerve root.
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.
Additional Injections Info:
Technically, SNRB injections are more difficult to perform than epidural steroid injections and should be performed by experienced physicians. Since the injection is outside the spine, there is no risk of a wet tap (cerebrospinal fluid leak). However, since the injection is right next to the nerve root, sometimes an SNRB will temporarily worsen the patient's leg pain.
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Selective Nerve Root Blocks (SNRB) and Facet Joint Injections
In cases where the facet joint itself is the pain generator, a facet block injection can be performed to alleviate the pain. Similar to SNRBs, facet block injections are a diagnostic tool used to isolate and confirm the specific source of back pain for the patient. Additionally, facet blocks have a therapeutic effect as they numb the source of pain and soothe the inflammation for the patient.
The facet joints are paired joints in the back that have apposing surfaces of cartilage (cushioning tissue between the bones) and a surrounding capsule. Twisting injuries can cause damage to one or both facet joints, and cartilage degeneration associated with aging may also cause pain.
In a facet block procedure, a physician uses fluoroscopy (live X-ray) to guide the needle into the facet joint capsule to inject lidocaine (a numbing agent) and/or a steroid (an anti-inflammatory medication). If the patient's pain goes away after the injection, it can be inferred that the pain generator is the specific facet joint capsule that has just been injected.
If the facet block procedure is effective in alleviating the patient's low back pain, it is often considered reasonable for the procedure to be done up to three times per year. There are very few risks associated with this technique.
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