49 Selective Nerve Root Block

Perspective

The term transforaminal injection is often used interchangeably with the term selective nerve root injection. The spinal nerves enter and exit the bony spinal canal through the intervertebral foramina. Just lateral to the foramen, a small volume of injectate can be placed directly adjacent to a single nerve. Blocking of a single spinal nerve with local anesthetic can be used diagnostically to clarify which nerve root is contributing to clinical symptoms in patients with pathology at multiple levels and a confusing pattern of symptoms. In this way, selective nerve root injection can be used to assist the surgeon’s decision making when pondering the proper operative approach. The results must be interpreted cautiously because the potential space surrounding the spinal nerves in the paravertebral region is contiguous with the epidural space. Indeed, as the volume of injectate is increased, the material spreads laterally along the spinal nerve and proximally through the intervertebral foramen to the epidural space directly surrounding the spinal nerve within the dural cuff. Although some physicians conduct selective nerve root injection just outside the intervertebral foramen and transforaminal injection by advancing the needle tip a few millimeters farther to enter the foramen, this distinction likely carries little practical meaning. Even a small volume of material injected at either location often enters the epidural space by contiguous spread.

The most common application of transforaminal injection is to inject steroids. The rationale for injecting steroids is that they suppress inflammation of the nerve, which in many instances is believed to be the basis for radicular pain. A transforaminal route of injection rather than an interlaminar route is used so that the injectate is delivered directly onto the target nerve, which ensures that the medication reaches the site of the suspected lesion in maximum concentration.

Cervical Transforaminal Injection

Placement

Anatomy

At typical cervical levels, the ventral and dorsal roots of the spinal nerves traverse laterally and caudally in the vertebral canal to form the spinal nerve in the intervertebral foramen. The foramen is oriented obliquely anteriorly and laterally. Its roof and floor are formed by the pedicles of consecutive vertebrae. Its posterolateral wall is formed largely by the superior articular process of the lower vertebra and in part by the inferior articular process of the upper vertebra and the capsule of the zygapophyseal joint formed between the two articular processes. The anteromedial wall is formed by the caudad portion of the upper vertebral body, the uncinate process of the lower vertebra, and the posterolateral corner of the intervertebral disk. Immediately lateral to the external opening of the foramen, the vertebral artery rises closely anterior to the articular pillars of the zygapophyseal joint.

The spinal nerve, in its dural sleeve, lies in the caudad half of the foramen. The cephalad half is occupied by epiradicular veins. The ventral ramus of the spinal nerve arises just lateral to the intervertebral foramen and passes anteriorly and laterally onto the transverse process. Radicular and spinal medullary arteries arise from the vertebral artery and the ascending cervical artery; radicular arteries supply the spinal nerve itself, whereas spinal medullary arteries continue medially to join the anterior and or posterior spinal arteries, which provide critical perfusion to the spinal cord itself.

The procedure can be performed with the patient lying in a supine, oblique, or lateral decubitus position, depending on the operator’s preference and the patient’s comfort. The position must allow adequate visualization of the cervical intervertebral foramina in the anteroposterior (AP), lateral, and oblique planes (Fig. 49-1A). The important first step is to obtain a correct oblique view of the target foramen (Fig. 49-1B). In this view the foramen is maximally wide transversely, and the anterior wall of the superior articular process projects onto the silhouette of the lamina. If these criteria are not satisfied, the inclination of the fluoroscope must be adjusted until they are. The correct oblique view is essential because in less oblique views, which may nevertheless show a foramen, the vertebral artery lies along the course of the needle. Older C-arm fluoroscopy units often restrict the degree of rotation of the side opposite the unit to less than 45 degrees, which can prevent adequate visualization of the cervical intervertebral foramina on the patient’s right side when the C-arm is positioned from the patient’s left. The 60 degrees of anterior oblique angulation often needed for good visualization can be achieved simply by placing a foam cushion under the patient’s right side, thereby tilting him or her to the left, or by tilting the surface of the table to the patient’s left.