Selective Nerve Root Blocks: Introduction

Nerve root blocks were developed as a means by which to define the source, if not the cause, of nerve root pain (1). Their value was purported to lie in patients in whom imaging studies were equivocal or in whom imaging studies suggested possible compression of several lumbar nerve roots (1, 2, 6, 9). By provoking and anaesthetizing a putatively symptomatic nerve root, a physician could determine whether or not that root was the source of the patient's symptoms. Once the symptomatic root was identified in this way, surgical therapy could be directed selectively at that nerve and not at others which were not symptomatic despite appearing compromised on imaging studies. However, what is referred to as the lumbar "nerve root" is not a simple structure.

Within each lumbar intervertebral foramen the rootlets of the dorsal and ventral roots converge to form the spinal nerve, with the dorsal root ganglion lying just proximal to the spinal nerve (Fig. 1). The nerve roots approach the intervertebral foramen by circumventing the medial aspect of the pedicle immediately above the foramen, lying in the entrance zone of the root canal, otherwise known as the lateral recess. The nerve roots and their accompanying radicular veins and arteries are enclosed in a tapering sleeve of dura mater which peripherally blends with the epineurium of the spinal nerve (Fig. 1).

Figure 1.

Fig. 1 Right lumbar spinal nerve viewed from the rear with the dural sleeve opened, showing the relationship of the spinal, its roots and its ventral ramus to the pedicle. The 'safe triangle' is the region where a needle may be introduced without striking the neural elements or entering the dural sleeve.

Within the vertebral canal and the intervertebral foramen the dural sleeve is surrounded by loose, fibrous tissue and fat continuous with that surrounding the thecal sac. Ventral to the nerve root sleeve run the sinuvertebral nerves. Typically these are represented by a major filament that passes transversely just below the pedicle to enter the vertebral canal in company with the anterior spinal canal artery." Additionally or alternatively several filaments enter the vertebral canal ventral to the nerve root sleeve. The sinuvertebral nerves innervate the ventral aspect of the thecal sac and nerve root sleeve and furnish branches to the intervertebral discs and posterior longitudinal ligament forming the floor of the vertebral canal.

Viewed against this background of anatomy the effect and interpretation of nerve root blocks are not as simple as originally portrayed. Local anaesthetics injected around the nerve root will anaesthetize not only the roots themselves and the spinal nerve but also the dural sleeve and the sinuvertebral nerves. Consequently, although segment specific a nerve root block cannot be tissue specific.