How is lumbar selective nerve root blocks (SNRBs) administered for pain management?

Updated: Jun 19, 2018
  • Author: Anthony H Wheeler, MD; Chief Editor: Meda Raghavendra (Raghu), MD  more...
  • Print


The technique described by Bonica begins with the patient in a prone position. The C-arm (image intensifier of patient) is rotated in a ipsilateral oblique angle with respect to the targeted nerve root, thereby bringing the "Scotty dog" appearance to view. Rotation of the C-arm or patient is continued until the ventral aspect of the superior articulating process (ear of the Scotty dog) has the same vertebral number as the nerve root to be blocked. The nerve root to be injected should be located between the anterior and posterior aspects of the vertebral body superior end plate. See image below.

Oblique "Scotty Dog" view is shown in the bottom r Oblique "Scotty Dog" view is shown in the bottom radiographic image of the lumbar spine.

The superior end plates should appear superimposed on fluoroscopy, thereby providing a bony limit to the depth of needle penetration. The nerve root normally passes a few millimeters inferior to the pedicle (eye of the Scotty dog) and 1-2 mm superficial to vertebral body. The lower thoracic and upper lumbar SNRBs should be blocked slightly more inferolaterally. The artery of Adamkiewicz is the main supply of arterial blood to the lowe rtwo thirds of the spinal canal and enters the canal anywhere from T7-L4.

The L5 nerve root is set up fluoroscopically in a similar fashion. However, standard positioning may cause the iliac crest to obstruct the proceduralist's approach. In this situation the needle is passed through an upside down triangular window formed by the inferior margin of the transverse process of L5, the superior articulating process of S1, and the iliac crest. The vertebral body limits the depth of needle penetration as it does in more cephalad SNRBs of the lumbar spine.

When performing a SNRB of the S1 sacral nerve root, the C-arm can be placed in a straight and the projection, unless an additional 5-10° of ipsilateral angulation allows better visualization of the S1 foramen, which cares as a circular lucency. There is no bony back trauma and to limit needle penetration, therefore, repeated visualization of the needle in AP and lateral planes using fluoroscopy must be performed more frequently.

Technically, when performing lumbar SNRBs a skin wheal is placed 1.5 cm lateral to the upper portion of the spinous process. A 5-cm, 25-gauge needle is directed vertically downward, while tissues along the way are infiltrated with 5-7 cc of a dilute LA solution (eg, 0.5% lidocaine or 0.125% bupivacaine) until the needle impinges upon the lamina of the vertebra. An 8-cm, 22-gauge needle is inserted perpendicular to the skin in the parasagittal plane through the anesthetized area, until the second needle reaches the uppermost part of the lateral edge of the lamina.

After contact with the lamina, the needle is marked 1.5 cm above the skin. The needle is then withdrawn until it is subcutaneous in location, then moved laterally approximately 1.5 cm and advanced past the lamina to a depth of 1.5 cm, where it should make contact with the nerve, eliciting paresthesia.

Needle placement can be verified by radiography, eliciting paresthesia, or using a nerve stimulator. See the images below.

Position of the patient for lumbar paravertebral s Position of the patient for lumbar paravertebral somatic block technique.
Lumbar paravertebral somatic block technique. See Lumbar paravertebral somatic block technique. See text for details.
Lumbar paravertebral somatic block technique. See Lumbar paravertebral somatic block technique. See text for details.
Lateral view showing needle position of lumbar par Lateral view showing needle position of lumbar paravertebral somatic block technique.

For diagnostic or prognostic purposes, 2 cc of a potent LA solution (eg, 0.5% bupivacaine with epinephrine) is typically injected after radiographic verification of the position of the needle bevel. This volume is sufficient to block the nerve as it exits from the intervertebral foramen, provided the needle tip is within 1-2 mm of the nerve. For therapeutic purposes, 5 mL of solution can be used to prolong analgesia but this is likely to spread to one or more adjacent segments. Multiple nerve roots can be addressed by injecting 25-30 cc of LA solution into the psoas compartment, which contains the lumbar plexus. This spreads sufficiently to block sympathetic nerves, the lumbar plexus, and lumbosacral trunks.

Did this answer your question?
Additional feedback? (Optional)
Thank you for your feedback!