Selective Nerve Root Blocks: Interpretation

The archetypical positive response to a selective nerve root block is that once the root is anaesthetized the patient is relieved of their pain. Such a clear response indicates unequivocally that the root is either the source of the patient's pain or is the sole segmental nerve that is mediating the patient's pain. No relief of pain excludes the root as either the source of pain or the pathway by which the pain is mediated.

More commonly, incomplete or mixed patterns of response occur. The patient may be relieved of their radicular pain but continues to complain of low back pain or referred pain. Such a response implicates the anaesthetized root as the source of their radicular pain but not their somatic pain.

The patient may be partially relieved of their radicular pain, in which case another root may be contributing to their radicular pain. Blocking a root at a second level may succeed in totally abolishing their pain. Such a response implicates both roots.

Most vexatious is a response in which the patient is partially relieved of their somatic pain. Such a response indicates that one of the structures innervated by that segment is contributing to the patient's pain. Partial relief may occur either because other structures are also contributing to the patient's overall pain or because the one structure that is the sole cause of their pain has only partially been anaesthetized by the root block. These possibilities cannot be resolved by root blocks alone. Patients with this pattern of response should be reevaluated by other means. Discography can be used to evaluate the disc at the level in question; zygapophyseal joint blocks can be used to evaluate zygapophyseal joints.

In a patient with somatic pain and in whom discography and zygapophyseal joint blocks are both negative, a positive response to a selective nerve root block strongly suggests that the dural sleeve is the source of their pain. Unfortunately, there are no more definitive means of confirming this contention; selective nerve root blocks applied in this way are the only means by which some degree of confirmation may be obtained for a hypothesis that the patient's pain stems from the nerve root sleeve.