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A Study of the Paravertebral Anatomy for Ultrasound-Guided Posterior Lumbar Plexus Block

Kirchmair, Lukas, MB; Entner, Tanja, MD; Wissel, Jörg, MD; Moriggl, Bernhard, MD; Kapral, Stephan, MD; Mitterschiffthaler, Gottfried, MD

doi: 10.1213/00000539-200108000-00047
TECHNOLOGY, COMPUTING, AND SIMULATION: Society for Technology in Anesthesia: Technical Communication
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*Institute of Anatomy and Histology, †Department of Neurology, and ‖‖Department of Anaesthesiology and Critical Care Medicine, Leopold-Franzens University of Innsbruck, Innsbruck, Austria; ‡Anatomische Anstalt, Ludwig-Maximilian University Munich, Munich, Germany; and §Department of Anaesthesiology and Intensive Care Medicine, University of Vienna, Vienna, Austria

Presented in part at the 19th annual meeting of the European Society of Regional Anesthesia (ESRA), Rome, Italy, September, 2000. Published in part in the International Monitor (2000;12:197) (Special Abstract Issue, 19th Annual ESRA Congress).

April 16, 2001.

Address correspondence and reprint requests to Lukas Kirchmair, MB, Institute of Anatomy and Histology, University of Innsbruck, Muellerstrasse 59, A-6010 Innsbruck, Austria. Address e-mail to lukas.kirchmair@tirol.com.

The benefits of applying real-time ultrasound (US) guidance to achieve successful and safe peripheral nerve blocks have been demonstrated (1–4). The exact delineation of injection sites and the monitoring of needle insertion and spread of local anesthetics established US as an useful adjunct during the performance of supraclavicular (1), femoral (2,3), and stellate ganglion blocks (4). The use of US for posterior lumbar plexus blocks has not been studied. Several approaches to the psoas compartment block have been described (5–7). The advantages of an approach at L2-3 were mentioned (8), but cases of renal hematoma (9) made them precarious. Several different techniques to locate the lumbar plexus, including loss of resistance (5), elicitation of paresthesias (6), and nerve stimulation (7) have been described. None of these techniques provides information on the exact relationship between the needle and the plexus.

To overcome these disadvantages, we investigated the feasibility of posterior paravertebral sonography as a basis for a US-guided posterior approach to the lumbar plexus. This study was conducted in two stages: a pilot study to establish the detailed US anatomy of this region and a volunteer study to examine the feasibility of posterior paravertebral sonography in individuals of varying body types.

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Methods

All volunteers gave informed consent, and institutional approval was obtained. Spinal deformities and pregnancy were criteria for exclusion. A standard US device (Sonoline VersaPlus; Siemens, Vienna, Austria) was used. Four different US transducers (two curved array transducers, 4 and 5 MHz; two linear array transducers, 7.5 and 5 MHz; Siemens) were evaluated, and the best imaging was achieved by the curved array transducers.

The normal sonographic appearance of the relevant paravertebral structures (psoas muscle, quadratus lumborum muscle, and erector spinae) was investigated at L2-3, L3-4, and L4-5 by means of two corresponding posterior sonograms (longitudinal and transverse;Fig. 1). The normal cross-sectional anatomy of L2-5 levels was also demonstrated by means of cross-sectional preparations derived from an embalmed cadaver (Fig. 2). The transverse sonograms were compared with these preparations to establish the sonographic anatomy of the paravertebral region. For longitudinal sonograms, the transducer was placed approximately 3 cm parallel to the lumbar spinous processes to localize the corresponding transverse processes (Fig. 1). The latter were used as landmarks, as was the cephalad portion of the sacrum, by caudad movement of the transducer. Those bony structures produced bright reflections followed by distal sound extinction (Fig. 3). To ensure that the echoes did not result from the articular processes, the transducer was moved further laterally to depict just the tips of the processes. Exact localization of levels L2-3, L3-4, and L4-5 was achieved by counting the transverse process echoes from the sacrum upward. At each level, half the distance between two adjacent transverse processes was set in the center of the longitudinal sonogram (Fig. 3), and the transducer was rotated approximately 90 degrees into a transverse plane (Fig. 1). It was here that the number of bony components disturbing the spread of the US beam is least; this is necessary to gain optimal sonograms. The lateral aspect of the vertebral arch and the articular process occurred as landmarks at the medial border of the sonogram (Fig. 4).

Figure 1

Figure 1

Figure 2

Figure 2

Figure 3

Figure 3

Figure 4

Figure 4

Twenty healthy volunteers were seated prone with a cushion placed under the abdomen to minimize lumbar lordosis. A curved array transducer (3,5C40+; Siemens; operated with 4 MHz) was applied, and sonograms were obtained as described. The psoas muscle was traced in the transverse sonograms of L3-4 and L4-5 levels on both sides, and measurements of its cross-sectional areas were computed and compared between examiners.

Twenty-one healthy volunteers were examined at L2-3, L3-4, and L4-5. The volunteers were allocated to three groups (Table 1): normal weight (n = 13; body mass index [BMI] 18.5–24.9 kg/m2), overweight (n = 5; BMI 25.0–29.9 kg/m2), and obese (n = 3; BMI >30.0 kg/m2). Posterior paravertebral sonography was considered feasible when the psoas muscle, the quadratus lumborum muscle, and the erector spinae could be clearly delineated in transverse sonograms at each level. Skin-plexus distance measurements were performed by one of the examiners at L2-3, L3-4, and L4-5. Skin-plexus distances were measured between the junction of the posterior third and the anterior two-thirds of the psoas muscle and the skin surface (estimated position of the lumbar plexus) (10). The measurements were computed in the center of the transverse sonograms parallel to the axis of the US beam to minimize measurement errors (11).

Table 1

Table 1

Finally, skin-plexus distance measurements were performed equally by means of computed tomography (CT) in 10 embalmed cadavers to obtain reference values. The cadavers were also seated in the prone position to avoid a decrease of the skin-plexus distances caused by body-weight pressure.

Interobserver reliability in the pilot study was determined by using Kendall’s coefficient of concordance, W, to compare the psoas muscle cross-sectional areas measured by the two examiners. The Kruskal-Wallis test was used to reveal significant skin-plexus distance differences among the L2-3, L3-4, and L4-5 levels, as well as skin-plexus differences among the three BMI groups. The Mann-Whitney U-test was performed in case of significant skin-plexus distance differences between the analyzed levels as well as BMI groups (revealed by the Kruskal-Wallis test). To analyze correlations between the measured skin-plexus distances and BMIs, Spearman’s coefficient of correlation was applied. P values <0.05 were considered statistically significant. Skin-plexus distances are presented as median ± sd.

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Results

In all 20 volunteers (10 men, 10 women), median age 24 yr (range, 19–43 yr), height 177 cm (range, 164–192 cm), weight 71 kg (range, 50–100 kg), and BMI 22 kg/m2 (range, 18–27kg/m2), posterior paravertebral sonography was performed successfully. The psoas muscle, the quadratus lumborum muscle, and the erector spinae could be traced reproducibly at all examined levels (n = 80). Kendall’s W was 0.9 (P < 0.001).

Twenty-one volunteers (13 men, 8 women), divided into three groups (Table 1), had 126 lumbar levels (L2-3, L3-4, and L4-5, right and left side) examined. Posterior paravertebral sonography was feasible at 112 of 126 lumbar levels (Table 2). In the Overweight and Obese groups, sonography was impossible in one volunteer at all levels (left and right side) and in another at the level of L4-5 on one side (Table 2). For both examiners, sonography was unfeasible in the same volunteers at equal levels.

Table 2

Table 2

Although there was an increase of the skin-plexus distances from L2-3 to L4-5, the Kruskal-Wallis test revealed no significant differences among L2-3, L3-4, and L4-5 (P = 0.42). The skin-plexus distances were 5.5 ± 1.4 cm at L2-3, 5.5 ± 1.4 cm at L3-4, and 5.8 ± 1.3 cm at L4-5. However, skin-plexus distance differences among the Normal Weight, Overweight, and Obese groups (Table 2) were significant (P < 0.001). Further, Spearman’s coefficient of correlation showed a significant positive correlation of 0.9 (P = 0.01) between the skin-plexus distances and the BMIs.

In 10 embalmed cadavers (3 male, 7 female) skin-plexus distance measurements were computed at 56 lumbar levels (L2-3, n = 18; L3-4, n = 20; L4-5, n = 18) by means of CT. Their median age at death was 81 yr (range, 51–88 yr) and height, 164 cm (range, 148–178 cm). Seven of 10 cadavers had normal body habitus, and 3 of 10 were obese. Skin-plexus distances increased from L2-3 to L4-5, similar to the skin-plexus distances obtained by sonography, but they showed higher median values: 7.0 ± 1.6 cm at L2-3, 7.3 ± 1.5 cm at L3-4, and 7.9 ± 1.3 cm at L4-5. The Kruskal-Wallis test showed no significant skin-plexus distance differences among the three levels (P = 0.1).

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Discussion

This investigation is the first dealing with a detailed and reliable description of the sonographic appearance of the lumbar paravertebral region. Posterior paravertebral sonography is reliable and accurate, as shown by the results of the interobserver reliability test.

The main study revealed that in 19 of 21 examined volunteers, posterior paravertebral sonography was feasible for both examiners. In 2 of 21 volunteers, posterior paravertebral sonography failed at all examined levels because of obesity. The quality of the obtained sonograms did not allow a sharp delineation of the psoas muscle and the adjacent structures. There are two reasons that sonography may fail in obese subjects: first, the applied transducer provides too low tissue penetration, and second, thick subcutaneous fat tissue causes heavy reflections because of US dispersion in dense tissues. Further, the spread of US may be disturbed in men at L4-5 by tall iliac crests, which are an anthropometric feature of male pelvises (Fig. 1). For that reason, both examiners were unable to perform sonography at L4-5 in 2 of 13 male volunteers, each on one side. These results indicate that posterior paravertebral sonography can be reliably performed in normal weight and in the majority of overweight and obese individuals. Occasionally, a reliable sonographic examination at L4-5 may be unfeasible in men because of the obstructing iliac crests.

Usually, the lower poles of the kidneys reach the level of L3, but during deep inspiration they may descend to reach the level of L3-4, appearing as hypoechoic, oval-shaped structures in the posterior transverse sonograms of L3-4. Distinguishing between the kidneys and the typical echotexture of the psoas muscle [hyperechoic striations on an echo-poor background (12)] was feasible in all successfully examined volunteers. Aida et al. (9) reported two cases of renal subcapsular hematoma caused by lumbar plexus blockade at L3 and stated that a posterior approach to the lumbar plexus must be performed at L4-5 to avoid renal injury. The use of real-time US guidance for approaches at L2-3 and L3-4 should help to avoid such complications by visualizing the structures at risk.

This study revealed that it is necessary to apply curved-array transducers operating at lower frequencies (4–5 MHz) because they provide appropriate tissue penetration and image size but less spatial resolution. Therefore, it was not possible to distinguish between peripheral nerves (13,14) as parts of the lumbar plexus and tendon fibers (which appear as hyperechoic striations, similar to peripheral nerves) within the psoas muscle (12). For a reliable and accurate delineation of the latter, the application of linear array transducers (>7.5 MHz) is recommended (15). Nevertheless, Koyama et al. (16) reported the depiction of parts of the lumbar plexus within the psoas muscle by using a 3.5-MHz curved array transducer.

Consequently, skin-plexus distance-measurements were made indirectly with the use of a reference point (junction of the anterior two-thirds and the posterior third of the psoas muscle in its anteroposterior diameter) that was estimated to be the approximate position of the lumbar plexus (10). The lumbar plexus is situated within the posterior part of the psoas muscle at all lumbar levels (Fig. 2) (8,10,17–20). The median skin-plexus distances measured with US are smaller than those measured with CT at all examined levels. The most likely explanation for this decrease is the pressure of the transducer against the skin (compression of the subcutaneous tissue and paraspinal muscles) that is mandatory for obtaining optimal sonograms. We estimated this reduction to be approximately 1–2 cm (depending on the dimension of subcutaneous tissue). Although the obtained skin-plexus distances did not exactly represent the real values for that reason, we considered them as guidelines for further applications of posterior paravertebral sonography. The increase of the skin-plexus distances from L2-3 to L4-5 (not significant) revealed by both CT and US measurements can be explained with the topographical position of the psoas muscle (as the muscle courses caudad, it moves anteriorly) (21). Nevertheless, skin-plexus distances showed significant differences among the Normal Weight, Overweight, and Obese groups (see Table 1 for median values); this could be confirmed by the significant positive correlation between the BMIs and skin-plexus distances.

In conclusion, we demonstrated detailed and reliable visualization of the lumbar paravertebral region by means of posterior paravertebral sonography. Nevertheless, we were unable to delineate the lumbar plexus. With the use of US, particularly approaches at L2-3 and L3-4 should be feasible without any complications that occur because of “blind” approaches.

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