Introduction A unique feature of most abdominal wall (truncal) nerve blocks is that the nerve per se does not need to be visualized to achieve success. Local anesthetic injected into the correct tissue plane will spread to reach the nerve. the path taken by the nerve and its branches combined with the location of local anesthetic placement determine the distribution of the block and possibly the duration of the block.

The advent of portable ultrasound technology has made it possible to place peripheral nerve blocks within tissue planes. It has facilitated introduction of new innovative approaches to deeper nerves not previously attempted. Each approach has subtle differences and the resulting block depends on the site and volume of the local anesthetic agent injected. It goes without saying that the complex blocks should not be attempted without the use of ultrasound guidance. the risks are simply too high.

Anatomy Important considerations when performing truncal nerve blocks include understanding the path of the nerve(s) and its anatomical relationships along that path as well as the extent of the sensory, motor and autonomic distribution of that nerve. Spinal nerves in general divide into dorsal and ventral primary rami. the dorsal rami pass through the erector spinae muscle and branches into medial and lateral posterior cutaneous branches. the ventral rami in the thorax pass deep to the intercostal muscles to branch into lateral and anterior cutaneous branches in the chest wall, and, in the abdominal wall pass between the internal oblique and transversus abdominis muscles. the rami communicantes branch off the ventral rami to join the sympathetic chain. the thoracolumbar fascia plays an important role in confining the local anesthetic spread.

Erector spinae block The erector spinae plane block (ESPB) is perhaps one of the most exciting tissue plane block described recently. the muscles of the back, i.e. the trapezius, rhomboids (above T7) and the erector spinae form tissue planes that can be visualised with ultrasound and utilized for block placement. ESB is relatively easy to perform and not very time consuming. Local anesthetic placed deep to the erector spinae muscle adjacent to the transverse processes of specific vertebra spreads cephalad over more dermatomes than a paravertebral block. Local anesthetic also spreads from the ES plane into the paravertebral space where it blocks the sympathetic chain to provide visceral analgesia. Bilateral ESB behaves like an epidural, and, initial reports suggest that the duration of the ESB may be longer than a paravertebral block.

Quadratus lumborum block A variety of approaches have been described for the quadratus lumborum (QL) block, which is essentially a more proximal approach to the spinal or intercostal nerve than the transversus abdominis plane (TAP) block. Irrespective of the volume used spread of local anesthetic is limited and unreliable after a conventional mid-axillary TAP block (6). the QL spread differs with the approach used. With the lateral approach the local anesthetic agent is placed lateral to the QL and posterior to the thoracolumbar fascia; with the anterior approach the local anesthetic is placed in the plane between the QL and psoas major muscle with spread into the paravertebral space; and the posterior approach places the local anesthetic posterior to the QL and the erector spinae muscle while the intramuscular approach places the local anesthetic within the QL muscle and blocks the nerve as it traverses the muscle. Each of these, as well as others described, have subtle differences in block distribution.

Advantages Are there advantages to the different approaches? Based on studies in adults, and limited series in children, it seems that the closer the local anesthetic is placed to the origin of the nerve (i.e. in the paravertebral space) the spread is wider covering more dermatomes. the duration also seems to be longer perhaps because the paravertebral space may act as a depot.

Advantages over an epidural block include less risk of spinal cord damage or epidural hematoma; unilateral blocks may be preferred, less potential for urinary retention and patients could potentially be discharged earlier with a continuous peripheral nerve catheters in situ.

References

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