It is useful to conceptualize paravertebral lumbar somatic block as an intercostal block in miniature. Using this concept, the short vertebral transverse process (a “rudimentary rib”) becomes the principal focus and landmark for needle positioning. Each lumbar somatic nerve leaves the vertebral foramina slightly caudad and ventral to the transverse process of its respective vertebral level (
It is useful to conceptualize paravertebral lumbar somatic block as an intercostal block in miniature. Using this concept, the short vertebral transverse process (a “rudimentary rib”) becomes the principal focus and landmark for needle positioning. Each lumbar somatic nerve leaves the vertebral foramina slightly caudad and ventral to the transverse process of its respective vertebral level ( Fig. 35-1 ).
As Figure 35-2 illustrates, from the intervertebral foramina the lumbar somatic nerves angle caudad and anteriorly and, in this process, pass anterior to the lateral extent of the transverse process of the next lower vertebral body (see Fig. 35-1 ). For example, as the L1 somatic root leaves its intervertebral foramen, its route places it immediately anterior at the lateral border of the L2 transverse process. Similarly, the T12 somatic root (a subcostal nerve) is found immediately anterior at the lateral extent of the L1 transverse process.
Returning to the intercostal nerve analogy, each lumbar nerve gives off an immediate posterior branch to the paravertebral muscles and skin of the back. Again, as with intercostal nerve anatomy, the lumbar somatic nerve also receives white rami communicantes from the upper two or three lumbar nerves and gives rise to gray rami communicantes to all lumbar somatic nerves. After these connections to the sympathetic nervous system, the main somatic nerve then passes directly into the psoas major muscle or comes to lie in a plane between the psoas and the quadratus lumborum muscles. Here the nerves intertwine to form the lumbar plexus. Figure 35-3 highlights this cross-sectional anatomy. Figure 35-4 illustrates the cutaneous distribution of the lumbar somatic nerves.
The concept that the lumbar somatic block is similar to an intercostal nerve block carries through to the performance of the technique. The most advantageous position is to have the patient prone, with a pillow under the lower abdomen to reduce lumbar lordosis. Skin markings are made as illustrated in Figure 35-5 ; that is, the lumbar spinous process of each vertebra corresponding to the roots to be blocked is identified and marked. Then, from the cephalad edge of each of these lumbar posterior spines, lines are drawn horizontally, and marks are placed on the lines 2.5 to 3 cm from the midline. The anatomic concept behind these markings is that the cephalad edge of each lumbar posterior spine is approximately on the same horizontal plane as its own vertebral transverse process. Skin wheals are then made at the site 2.5 to 3 cm from the midline on the lines overlying the lower edge of the transverse process. Through the skin wheals an 8-cm, 22-gauge needle is inserted in a vertical plane without a syringe attached ( Fig. 35-6 ). As the needle is advanced, it contacts the transverse process at a depth of 3 to 5 cm in the average adult (needle position 1). Failure to contact the transverse processes at that depth implies that the needle has passed between the two transverse processes.
To contact bone, a repeat insertion is made through the same skin wheal but with a slight cephalad angulation of the needle. Once the transverse process has been identified, the needle tip is withdrawn to a subcutaneous location before being reinserted to pass just caudad to the previously identified transverse process. This allows block of the lumbar root corresponding to the same lumbar vertebra. The needle is reinserted just cephalad to the corresponding transverse process to block the lumbar root one segment more cephalad. As the needle “slides” off and past the transverse process, it should be advanced approximately the thickness of the transverse process, or about 1 to 2 cm, after contact with bone is lost (needle position 2). This places the tip in the plane immediately anterior to the transverse process. When the final needle position has been established, approximately 5 mL of local anesthetic solution is injected. The process should be repeated at each site at which local anesthetic block is desired.Potential Problems.
Because the lumbar roots are in close proximity to other neuraxial structures, it should be kept in mind that epidural and subarachnoid anesthesia has been produced after attempts at lumbar somatic block. It is most likely that in these cases the needle was angled medially during insertion rather than being maintained in a parasagittal plane. Likewise, because of the proximity of the sympathetic ganglion to the lumbar roots, if the needle is inserted too deeply the volume of local anesthetic solution injected is often enough to cause lumbar sympathetic blockade. If this happens, a decrease in blood pressure similar to that seen during low spinal anesthesia may result.