Although the ESPB and RLPVB differ slightly in the site of injection, they share the same musculofascial plane within which the injected local anesthetic spreads. Under fluoroscopic observation, these two variants differ primarily in the lateral-medial spread of local anesthetics. The ESPB results in contrast spread
laterally to the ipsilateral facet line over the lung field, whereas the spread following RLPVB in more medial to the ipsilateral facet line in the paraspinal muscle region. However, these two variants share the similarity in the cephalocaudal spread of local anesthetics, although the extensiveness of spread was variable inter-individually, but relatively consistent individually, possibly affected by individual differences in local anatomy.

Regardless of the difference in lateral-medial distribution patterns of injectate between these two variants, we observed that the resultant analgesic distribution was identical within each individual patient. As has been previously documented by other researchers studying dye spread in cadaveric models [6,7], we observed substantial inter-individual variability in the dermatosensory blockade following ESPB/RPLVB. While all 3 cases demonstrated posterior and lateral chest wall analgesia, anterior chest wall analgesia was achieved in Cases 1 and 2 but not in Case 3. Additionally, all three patients report similarly satisfactory, significant, but partial (> 80%) pain relief from either ESPB or RLPVB. Comparing the injection site on the dorsal aspect of transverse process in ESPB the injection site on the dorsal aspect of lamina in RLPVB is a much larger surface to target, which can be translated into a potentially greater safety margin and ease of performance with equivalent clinical effect.

Despite the belief on the part of enthusiasts that RLPVB and ESPB are equivalent alternatives to the classical paravertebral block, our observation of variable anterior analgesic distribution of these blocks is notable and consistent with the variability documented in the extant literature on this topic. Clinically, the ESPB and RLPVB have been reported to be most efficacious in posterior-lateral chest wall pathology and surgical incisions [8-14]. Some studies have claimed the complete effectiveness of these blocks [15-19], whereas other studies have reported failed analgesia for chest well coverage, such as in radical mastectomies [3,14,20]. Anatomically, the absence of anterior dye staining of the ventral ramus following ESPB has been reported in two cadaveric studies [6,21] but one recent cadaveric study demonstrated limited and variable spread to the paravertebral area and spinal nerves after both ESPB and RLPVB [5,21,22].

Based on the post-procedural analgesia mapping in conjunction with the fluoroscopic observation of injectate spread, our data agrees that both ESPB and RLPVB primarily behave as interfacial plane blocks with variable clinical effects, which can be explained by the hypothesis that they exert their clinical effect via two potential pathways [3]. We hypothesize that, in the “lateral” pathway, local anesthetic deposited under the ESM flows laterally along the erector spinae plane bordering the external intercostal muscle anteriorly and serratus anterior muscles posteriorly (Figure 1 travel in arrows) and reliably and predictably blocks the dorsal ramus and lateral cutaneous branch of each spinal nerve.

However, in the “paravertebral” pathway, local anesthetic flows anteriorly, diffusing through the superior costotransverse ligament/foramen into paravertebral space to cover the ventral ramus (Figure1, the blue triangle). We further hypothesize that the “paravertebral” pathway may be dependent on the local anatomic variability surrounding the superior costovertebral ligament/foramen the potentially key anatomic barrier in anterior spread of local anesthetics into paravertebral space. Our findings suggest that the availability of the paravertebral pathway is not predictable at the time the block is placed. After a review of the recent literature, we believe that our study is the first in vivo comparison study on ESPB and RLPVB in the same patient. By conducting pre- and post-procedural dermatosensory assessments in nonsurgical patients, we avoided the complicating factors of surgery or superimposed multimodal analgesia. Similarly, our use of fluoroscopic guidance minimized the impact of technical variation in block placement and ensured appropriate interfacial local anesthetic spread. Our observations support the contention that significant but partial pain relief can be provided by these two block variants for trunk analgesia. In cases wherein anterior chest wall analgesia is essential, we advise caution about the utility of either ESPB or RLPVB. Additionally, given the clinical equivalence of these variants, RLPVB might have better clinical utility given easy technical performance and larger safety margin. Certainly, given the enthusiasm for these new blocks, and the variability in their observed analgesic distribution, we support the need for larger clinical trials to establish their appropriate application in clinical practice.

Declarations
Ethics approval and consent to participate: Patient’s consent was obtained.
Consent for publication: Consent for publication from both authors and patient were obtained.
Availability of data and material: Yes
Competing interests: The authors declare no conflicts of interest.
Funding: This study did not receive any funding in any form
Authors' contributions:
Jiang Wu: This author designed the study, provided clinical care, analyzed, and prepared the radiographs and drawings, conducted literature search, and finalize manuscript.
Nina Patel: This author conducted literature review, drafted manuscript, and prepared the drawings.
Wil Van Cleve: This author drafted manuscript and prepared the drawings. Ryu Komatsu: This author helped write manuscript.
Acknowledgements: None