Patient Selection

Lumbar somatic block is often used in a pain clinic setting. In addition, some surgical patients, such as those undergoing herniorrhaphy, benefit from appropriate use of the block. Also, although the frequency of flank incision for renal surgical procedures has decreased since the advent of lithotripsy, patients undergoing flank incisions are well managed with a combination of lower intercostal and lumbar somatic block and light general anesthesia.

Pharmacologic Selection

The local anesthetic choice for lumbar somatic block is limited only by the extent of additional blockade and concerns over systemic toxicity. If pinpoint diagnostic accuracy is essential for chronic pain syndromes, local anesthetic volumes as small as 1 to 2 mL are appropriate; if surgical anesthesia is desired, volumes of 5 to 7 mL per lumbar root are appropriate.

Anatomy

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 position. Lumbar somatic nerves leave the vertebral foramina slightly caudad and ventral to the transverse process of its respective vertebral level (Fig. 35-1).

Figure 35-1. Lumbar somatic block: anatomy.

As Figure 35-2 illustrates, from the intervertebral foramina the lumbar somatic nerves angle caudad and anteriorly and hence 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.

Figure 35-2. Lumbar somatic block: anatomy.

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 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.

Figure 35-3. Lumbar somatic block: cross-sectional anatomy. Ao, aorta; IVC, inferior vena cava.

Figure 35-4. Lumbar somatic block: dermatomal anatomy.

Position

The conceptual similarity of this block to an intercostal nerve block carries through to the actual 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 (i.e., 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 (paravertebral in location). The anatomic rationale 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 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 will contact the transverse process at a depth of 3 to 5 cm in the average adult (needle position 1). Failure to contact the transverse process at that depth implies that the needle has passed between the two transverse processes.

Figure 35-5. Lumbar somatic block: placement of skin markings.

Figure 35-6. Lumbar somatic block: technique.

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 in order 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 approximately 1 to 2 cm, after contact with bone is lost (needle position 2). This will place the tip in the plane immediately anterior to the transverse process. When 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.