Application: Selective Nerve Root Blocks

Application

Selective nerve root blocks can be used to test various hypotheses. Since they anaesthetize the nerve roots and spinal nerve they can test whether or not a patient's pain stems from these structures or is mediated by them, but since they also anaesthetize the dura mater they can test the hypothesis that a patient's pain stems from the nerve root sleeve. Moreover, since they block the sinuvertebral nerves, nerve root blocks test whether or not a patient's pain stems from the discs or other tissues supplied by these nerves. Nerve root blocks, however, cannot discriminate between these various hypotheses. Since all of the above structures are blocked simultaneously, relief of pain does not implicate one source of pain ahead of any other. Any putative discrimination must be based on the patient's clinical features.

If the patient clearly suffers from radicular pain, namely shooting or lancinating pain in the lower limb along a narrow band (11). relief of that pain by a nerve root block reliably implicates the anaesthetized root as the source of that pain. If, on the other hand, the patient suffers from somatic pain - deep, dull, aching pain in the back or referred into the lower limb (11) - relief of that pain by a nerve root block implicates no particular structure. The pain could stem from any of the structures innervated by the sinuvertebral nerves of that level or from any of the structures innervated by the spinal nerve at that level. The former include the nerve root sleeve, the thecal sac and the intervertebral discs. The latter include discs, muscles and zygapophyseal joints.

Partial relief of somatic pain may occur because of the multi-segmental innervation of certain structures. For example, the sinuvertebral nerve and the spinal nerve in a particular intervertebral foramen innervate the disc at that level plus the disc of the segment above. Blocking these nerves, therefore, may partially relieve pain from either of these discs. Thus, in the context of somatic pain, a nerve root block is not source specific, and is of value only if it can be considered in the context of the results of discography and medial branch blocks or zygapophyseal joint blocks which might otherwise implicate or exclude the disc or posterior elements as the source of pain.