Nerve (Target) Localization
If the L4 transverse process is difficult to visualize it is recommended to focus on the transversus abdominis muscle. Follow the course of the muscle more posterolatteral until it becomes aponeurotic. This aponeurotic structure will normally attach to the posterior side of the QL muscle (Figures 5 and 6) and thus blend with the anterior layer of the thoracolumbar fascia (according to the two-layer model).2
Local Anesthetic Injection
Optimal cranial local anesthetic spread can be monitored and confirmed by turning the transducer 90 degrees away from the transverse axial position to the longitudinal (parasagittal) position to visualize separation of the QL and PM muscles and local anesthetic spread from the iliac crest towards the ribs and the diaphragm (Figure 9).
Local Anesthetic Spread
Comparison with TAP Block
Many transversus abdominis plane (TAP) block techniques, ultrasound or landmark based, have been described over the years. Their analgesic effect, however, may not be consistent. The site and technique of TAP block has been found to influence the extent and pattern of local anesthetic spread.
In a MRI study by Carney,7 subcostal and mid axillary TAP blocks performed anteriorly resulted in contrast local anesthetic spread predominantly in the TAPs of the upper and lower abdominal wall with minimal posterior spread. Similar results were confirmed by Børglum.8 On the other hand, posterior TAP block approaches, i.e., posterior to the mid axillary line and next to the antero-lateral border of the quadratus lumborum (QL) muscle, result in local anesthetic spread not only posteriorly around the QL muscle but far medially to the paravertebral spaces extending from the T5 to L1 vertebral levels.7
Blanco originally described ultrasound-guided (USG) TAP block using a "no-pops" technique in 2007.9 This is the first quadratus lumborum block (QLB1) which aims to deposit local anesthetic at the anterolateral aspect of the QL muscle (Figure 10). The site of needle insertion is very similar to the landmark based Triangle of Petit approach described by McDonnell; i.e. posterior to the mid axillary line.10 Apparently, it also results in a similar spread pattern.7
Figure 10. Schematic diagram of Blanco's quadratus lumborum block (QLB) 1 and 2. (Reference: Blanco et al.)11
A subsequent study by Børglum demonstrated redundant local anesthetic spread with the QLB1; i.e. lateral and a spread away from the site of injection (Figure 11).12
Figure 11. MRI scan showing dye and local anesthetic spread laterally and anteriorly after a QLB1 injection (Reference: Børglum et al)12
PM = Psoas muscle
QL = Quadratus lumborum muscle
Blanco later abandoned the original QLB1 technique and described the QLB2 which aims to deposit local anesthetic at the posterior aspect of the QL muscle between the QL and the latissimus dorsi muscles.11,13
In contrast with QLB1 and QLB2 blocks, the TQL block aims to inject local anesthetic deep to the transversalis fascia (TF) with a posterior approach into the interfascial space between the QL and psoas major (PM) muscles and contain the local anesthetic spread around and anterior to the QL muscle. Local anesthetic injected anterior to the QL muscle was found to promote cephalad spread within the fascial plane between the QL muscle and PM muscle to the thoracic paravertebral spaces (Figure 12). The spread pattern is posterior to the transversalis fascia (Figure 13) and less "redundant" spread laterally as seen with the original QLB1 injection.
Figure 12. MRI coronal plane view showing cranial dye spread into the thoracic paravertebral space following the bilateral TQL injection (most clearly visualized on the left side)
Arrow = Local anesthetic cephalad spread from the site of lumbar administration at the level of L4.
Figure 13. MRI axial plane view showing dye spread between the psoas major (PM) and QL muscles following a bilateral transmuscular quadratus lumborum block (TQL) injection. The local anesthetic lies deep to the transversalis fascia. ESM = erector spinae muscles.
Mechanism of Action (Anatomical Study)
Below is a parasagittal view of the QL muscle, the psoas muscle and the diaphragm (Figure 14). Note that the cephalad parts of the QL muscle and the PM next to the diaphragm create a shape of a funnel which enables the injected local anesthetic trapped in the fascial plane between the two muscle to spread further into the thoracic paravertebral space below the diaphragm. This spread pattern is in accordance with the previous description by Saito.6
This is possible since the QL and PM muscles insert within the thoracic cage (i.e. 12th rib and vertebral body of T12 and disc, respectively) and since the transversalis fascia is continuous with the endothoracic fascia.
Figure 14. A parasagittal view of the quadratus lumborum muscle (green), the psoas muscle (PM) (red) and the diaphragm (blue).