Selective nerve root blockade has received attention as a diagnostic and therapeutic tool in the management of referred pain, presumably of radicular origin. From the surgical point of view, the potential utility
of this test lies in diagnostic specificity: not in its ability to identify a radicular etiology as the source of referred pain, but to localize a symptomatic level. In certain instances with clinical evidence of radiculopathy and no underlying structural cause, nerve root blockade has been used to guide surgical intervention such as laminectomy or fu-sion.54 This is particularly distressing, since selective nerve root blockade has been found, in randomized prospective studies, to be neither sensitive nor specific.55,56 Based on these data, it would appear that these blocks may have a therapeutic role, but the role as a definitive diagnostic maneuver is minimal.
A particularly unfortunate clinical situation occurs when a patient who has been diagnosed with radiculopathy is informed that surgery is required for neural compression even though, from a strictly anatomical point of view, no surgical lesion exists. Again, the potential dichotomy between the diagnostician and the surgeon bears scrutiny: although the patient may in fact have a radiculopathy that is helped by selective nerve root blockade, this may not be amenable to surgical treatment.
Selective nerve root blockade has been used in the diagnosis of radicular syndromes.55-58 However, recent reports have called attention to temporary pain relief by reversible anesthetic blocks that failed to yield reliable long-term predictions about interventional results. There have been disappointing results from neuroablation procedures including dorsal rhizotomy59 as well as ganglionectomy60 when these procedures were selected on the basis of response to selective nerve root blockade. Wetzel et al.59 reported a 19% success rate on patients who underwent selective lumbar sensory rhizotomy, with levels being selected on the basis of response to selective nerve root blockade. In this study, the decision to perform rhizotomy was based on the response to selective neural blockade that required reproduction of familiar pain, the disappearance of root tension signs after infiltration of anesthetic, and correlation between clinical and radiographic findings. These criteria were met in 90% of the cases, but satisfactory relief was not reliably obtained by selective sensory rhizotomy of the appropriate root. In addition, results of selective blockade may be confounded by systemic effects of lidocaine. When this is viewed in conjunction with the results of anesthesia of cutaneous nerves in the area of referred pain (i.e., pain relief), a notable lack of anatomical specificity becomes quite evident.60,61
North et al.56 performed diagnostic nerve blocks in a randomized prospective manner. In this study, 33 patients underwent a battery of local anesthetic blocks in an attempt to evaluate sciatica. The specificity of sciatic nerve block was 24% immediately and 36% at 1 hour. The sensitivity of selective nerve root blockade was 91% immediately and 88% at 1 hour. When analyzed in the context of blocks (from proximal to distal), the root block alone yielded significant pain relief in 9% immediately and 21% at 1 hour. The root block yielded greater relief of pain than any other block in 30% of patients immediately and 42% at 1 hour. In all other cases the sciatic block or facet block yielded equal or better results.
To date, there has been no convincing study demonstrating the ability of conventional surgery (i.e., lateral recess decompression or foraminotomy) to reliably treat referred pain diagnosed on the basis of response to selective nerve root blockade. It is possible that selective blockade may be of therapeutic value in the ongoing treatment of chronic radicular pain. However, reliance on this technique as the sole or determining diagnostic maneuver from which surgery is planned is only to be condemned.
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