SNRB is traditionally performed with fluoroscopic guidance to facilitate accurate needle placement. The basic technique is to position the patient in an appropriate position to allow visualization of the neural foramen en face. Using C-arm fluoroscopy, final adjustments can be made to best view the foramen. After marking the skin entry site and prepping, local anesthetic is used at the needle entry site. Using a "down the barrel" technique, the needle is advanced into the foramen (Fig. 5). The target area within the foramen is a perineural placement of the needle, avoiding puncture of the nerve root itself. The acceptable location of the needle in or adjacent to the foramen will vary slightly, depending on the anatomic level and the need for diagnostic specificity. The ideal needle placement will vary for cervical, thoracic, lumbar, and sacral roots.
Fig. 6. Prominent spread to the adjacent epidural space during a cervical selective nerve root block.
As the needle is advanced, its depth is monitored with fluoroscopy in a perpendicular projection or anteroposterior and lateral fluoroscopy. At all levels, a bony landmark can provide a very accurate depth gauge to avoid excessive advancement into the foramen. On entering the neural foramen, care should be exercised to not pierce the nerve root itself. In the lumbar levels, the needle is advanced into the area immediately inferior to the pedicle forming the superior border of the foramen. Because of the downward slope of the exiting nerve root, this target area is less likely to contain the nerve root itself. A small contrast injection (1-2 cc) is made to identify the nerve root (4), and confirm the absence of epidural spread and lack of intravascular flow of contrast (Fig. 6). Corticoster-oid and anesthetic solution is then injected around the nerve root sleeve. A typical injected solution would include 80 mg of methylprednisolone (1 cc) (Depo-Medrol, Pharmacia Upjohn), 1 cc of 0.5% lidocaine, and 1 cc of 0.5% bupivacaine. Partial epidural spread of contrast can be seen with a medial position of the needle. If the injection is to have diagnostic value, an epidural injection should be avoided because of the risk of extension to adjacent roots. Most operators will use a 22-gauge needle with a beveled tip, which allows some steering. The needle will tend to track away from the beveled face on the needle tip. This allows for minor adjustments in position as the needle is advanced into the foramen. The 22-/25-gauge blunt-tipped needle (Whitacre Needle,
Becton Dickinson & Co.) is also used. The blunt tip may also reduce the risk of piercing the nerve.
SNRB of the cervical segments has unique anatomic considerations. The nerve root has a more horizontal course and a close proximity to the vertebral artery (Fig. 7). Both computed tomography (CT) and fluoroscopic approaches have been described (2,5,6). When fluoroscopy is used, the patient is placed supine on the table with the head turned toward the contralateral shoulder (Fig. 8). Sometimes an angled wedge sponge can be used to elevate the ipsilateral side to better visualize the neural foramen en face without needing steep angulation of the C-arm. After the site is prepped and infiltrated with a short-acting local anesthetic, a 22- or 25-gauge needle is advanced toward the base of the superior articulating facet at approx the 6-7 o'clock position in the neural foramen. This bony landmark can function as a depth gauge for needle placement. The needle can then be redirected slightly anterior into the lateral aspect of the neural foramen adjacent to the base of the superior process. Care should be taken to avoid the anterior aspect of the foramen and to remain in the lateral aspect of the foramen because of the close proximity of the vertebral artery. A contrast injection (1-2 cc) is used to confirm the location of the nerve root, which will be outlined with contrast, and to avoid an intravascular injection.
Fig. 9. Cervical SNRB using CT guidance.
Fig. 8. Cervical SNRB using the fluoroscopic technique. The nerve is outlined by an epineurogram. Note also the epidural extension of contrast in spite of the lateral position of the needle.
When CT guidance is utilized, the landmarks for needle place-ment will remain the same but must be appreciated in the axial plane (Fig. 9). Although the procedure may be more time consuming, the axial visualization will clearly define the relationship of the vertebral artery to the exiting nerve root. It is important to review magnetic resonance (MR) images of the cervical spine prior to performing a selective nerve root block to avoid puncturing an aberrant or tortuous vertebral artery.
The least common level to block is the thoracic spine. The approach used for the thoracic spine is similar to that for the lumbar spine, but the incident angle used must be reduced to avoid the pleural reflection. The target area is slightly more lateral than in the lumbar spine (Fig. 10). Paravertebral thoracic blocks are described using a palpation technique (7). Fluoro-scopy can be used and is helpful if specific levels are to be blocked. As the nerve exits the neural foramen, it will enter its subcostal location. The target area is inferior to the costovertebral articulation.
The sacral roots also have unique consideration for access. With the patient prone, craniocaudal angulation of the C-arm will usually allow for visualization of the sacral foramen en face (Fig. 11). After identifying the appropriate level, a 22-gauge needle can be advanced to the posterior superior margin of the neural foramen. This provides a depth gauge to the nerve root. The needle can then be advanced slowly into the foramen in close proximity to the nerve root. A contrast injection can identify
Fig. 9. Cervical SNRB using CT guidance.
Structures Innervated by the Nerve Root
Ventral ramus Intervertebral disc
Longitudinal ligament Anterior dura
Dorsal ramus (medial branch)
Posterior longitudinal ligament Posterior outer annulus Anterior dura the foraminal position and most likely also outline the nerve root. The block can then be performed with the usual medications.
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