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Abstract
Background The aim of this article was to provide a detailed description of the neonatal anatomy related to the erector spinae plane block and to report the spread of the dye within the fascial planes and potential dermatomal coverage.
Methods Using ultrasound guidance, the bony landmarks and anatomy of the erector spinae fascial plane space were identified. The erector spinae plane block was then replicated unilaterally in two fresh unembalmed neonatal cadavers. Using methylene blue dye, the block was performed at vertebral levels T5—using 0.5 mL in cadaver 1—and T8—using 0.2 mL in cadaver 2. The craniocaudal spread of dye was tracked within the space on the ultrasound screen and further confirmed on dissection.
Results Craniocaudal spread was noted from vertebral levels T3 to T6 when the dye was introduced at vertebral level T5 and from vertebral levels T7 to T11 when the dye was introduced at vertebral level T8. Furthermore, the methylene blue spread was found anteriorly in the paravertebral and epidural spaces, staining both the dorsal and ventral rami of the spinal nerves T2 to T12. Small amounts of dye were also found in the intercostal spaces.
Conclusion In two neonatal fresh cadavers, the dye was found to spread to multiple levels and key anatomic locations.
- truncal blocks
- pediatric pain
- ultrasound in pain medicine
- pediatrics
- anatomy
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Introduction
The erector spinae plane (ESP) block is a novel ultrasound-guided interfascial technique serendipitously discovered while treating thoracic neuropathic pain in an adult.1 2 ESP has further been reported to successfully manage acute and chronic pain for truncal procedures.3 4 This ‘happily accidental’ block has been compared with other neuraxial techniques such as the epidural and paravertebral block and may prove to be a safer alternative.4 Although the ESP block has been successful in the adult population, there are only 42 documented cases (as of December 2019) in neonates, infant, and children.5–14 Despite the increasing number of indications for the ESP block, the anatomy, mechanism of action, concentration, and volume of anesthetic is yet to be determined especially in neonates and children.3 5
Objectives
The aim of this study was to provide a detailed description of the anatomy related to the ESP block in a small neonatal sample. The extent of the spread of a fluid—in this case, methylene blue dye—was also determined.
Materials and methods
This study was approved by the PhD and Research Ethics Committee, University of Pretoria, South Africa. Two fresh unembalmed neonatal cadavers subject to cryopreservation were obtained through the National Tissue Bank, University of Pretoria. We performed a unilateral ESP block at the T5 vertebral level in cadaver 1 and at the T8 vertebral level in cadaver 2. Both cadavers were preterm stillborn. Cadaver 1 weighed 1.6 kg and was 370 mm long, cadaver 2 weighed 0.6 kg and was only 350 mm long. An EdgeTM Ultrasound machine (ref: P15000-11, SN-03P55Z) with a 6-13MHz linear array probe (footprint size of 2.5 cm) was used in both procedures. The methylene blue mixture consisted of 10 mL of iodinated contrast material, diluted in 85 mL normal saline.
Cadavers were placed in the prone position, the spinous process and its corresponding transverse processes were identified with the aid of ultrasound. Subsequently, an ultrasound transducer was repositioned in both a transverse and parasagittal alignment to identify anatomical structures.
For the procedure, the transducer was placed parasagittally over the transverse process of T5 and T8, respectively, about 1 cm lateral to the spinous process. Using an in-plane approach a 21 mm needle was then directed in a cephalad to the caudal direction towards the transverse process. Once the tip of the needle contacted the transverse process, 0.05 mL of saline solution was injected to confirm the position of the needle tip. The erector spinae fascial plane space was further confirmed as the erector spinae muscle bundle was hydro dissected away from the transverse process. Methylene blue dye (0.5 mL for the first cadaver and 0.2 mL for the second cadaver) was then injected while observing the spread of the dye within the plane between the erector spinae muscle group and the transverse processes on the ultrasound screen. Dissections were then performed 30 min after the dye injection.
Results
After exposure to the relevant tissue planes extensive dye spread was noted in a craniocaudal direction, both superficial and deep to the erector spinae fascial plane (figure 1). In the first cadaver, the dye was injected unilaterally (left side) at vertebral level T5 to determine the spread in the thoracic region. Based on surface staining, the dye spread from vertebral levels T2 to T12. Superficially, the trapezius, rhomboids, latissimus dorsi, and erector spinae muscles were stained by the methylene blue dye. Deeper, the dye was located over the posterior aspect of the transverse process, near the costotransverse ligament (foramen) (figure 2). Staining spread from T3 to T6 at the ventral and dorsal roots/ganglion level (figures 3 and 4). In the second cadaver, the dye was also injected on the left side but at the T8 level to determine the spread over the abdominal dermatomes. Based on superficial staining, the dye spread from vertebral levels T7 to L1, that is, superficial to the trapezius, latissimus dorsi, and erector spinae muscles. The ventral and dorsal roots/ganglion were stained from T7 to T11. (figures 3 and 4). In both cases, methylene blue dye was seen in the paravertebral and epidural spaces. The dura mater surrounding the spinal cord was found to be stained before the spinal cord was then cut and removed to expose the ventral and dorsal rami.
Photographic image showing the superficial staining of the erector spinae muscle as well as the spread of dye deep to it.
Photographic image displaying the cleaned lamina, transverse process, and parts of the ribs.
Photographic image revealing the staining of the rami of spinal nerves.
Photographic image showing the dye staining of the intercostal nerves as the dye partially spread into the intercostal spaces.
Discussion
The ESP block is a novel technique that can be used as an alternative block for truncal procedures. We found that in fresh neonatal cadavers, injecting 0.2 and 0.5 mL of dye into the erector spinae fascial plane space at vertebral level T5 and T8 spreads 4–5 levels for the thoracic and abdominal regions, respectively. Although the exact dose for neonates, infants, or children has yet to be established, the spread in this small study suggests that 0.1 mL/kg per dermatome may be a useful guide.15
As dye was injected in close proximity to the costotransverse foramen, we found that the dye penetrates anteriorly through the intertransverse connective tissue, into the paravertebral, epidural, and intercostal spaces to stain the ventral and dorsal rami of the spinal nerves.16
Hamilton and Manickam suggested that the erector sheath is the reason for the success of the ESP block.17 They suggested that the sheath is intermittently tethered anteromedially to the bony structures along its course, resulting in multiple varied apertures or perforations.17
Supporting both Hamilton and Manikam, we believe that the erector sheath and the thoracolumbar fascia combine to form a continuous tissue plane over multiple vertebral levels, allowing for the craniocaudal spread of dye. Furthermore, the anterior perforations within the sheath explain the mechanism of anterior spread into the paravertebral, epidural, and intercostal spaces as seen in this study. We hypothesize that the anterior perforations may be attributed to the porous tissue found around the superior costotransverse ligament. Additionally, the spread could have been further facilitated through the costotransverse foramen that acts as a bony gap. The findings in this study are similar to that of other authors who suggest that the success of this block is due to the diffusion of local anesthetic through soft tissue gaps.3 18
At dissection, some staining was also noted in the overlying muscular structures—the trapezius and rhomboid muscles—posterior to the erector spinae muscle at vertebral level T5. Barker and Briggs, concluded in their study, that a layer of the thoracolumbar fascia fuses with the muscular fascia of the trapezius and rhomboid muscles, which may account for the posterior staining seen in this study.19
Technical difficulties in the application of this block could result from the thinner muscle layers, sliding fascial planes and loose connective tissues in neonates and children.18 20 Therefore, a finer needle, for example, 27 G facilitates needle tip placement within these thin layers.
The spread of ESP block has been determined in adults. Ivanusic and co-workers reported on the lack of spread to the ventral rami.1 Adhikary and others, not only reported a ventral and dorsal rami spread but also an epidural and intercostal space spread.3 Takata and colleagues reported that although the ESP block provided tolerable anesthesia for thoracoscopic lobectomy, it also provided weak dermatomal spread towards the anterior cutaneous region.3 Several authors, including Chin and co-workers, confirmed analgesic spread for the thoracic and abdominal regions when performing the block at vertebral levels T4/T5 and T7/T8, respectively.21 22
The question of anatomical differences between age groups arises because of the contradicting results from various cadaveric studies performed predominantly in adults.16 17 19–21 Apart from the demographics such as weight, height and body shape, anatomical differences between neonates, infants children, and adults do exist. Factors such as the developmental formation of the vertebral curvature may contribute to the differences in paravertebral tissue and muscle thickness seen between age groups. Furthermore, given the more elastic pediatric spine,18 together with the less dense ligaments and cartilaginous laminae could allow for a more favorable spread in neonates and infants.23 This would also affect the depth at which the ESP block is performed. Incomplete myelination of nerve fibers in neonates and infants allows lower concentration and volume of anesthetic required to perform the block.14
Conclusions
In two fresh neonatal cadavers, small amounts of dye injected into the ESP spread to the paravertebral, epidural, and intercostal spaces. Additionally, the dye was found to spread over multiple dermatomal levels while targeting the ventral and dorsal rami from T2 to T12. Clinical studies will be needed to identify if ESP blocks offer advantages over paravertebral or epidural injections in children and neonates.
Acknowledgments
The authors gratefully acknowledge the donated cadaveric specimens, without which this research would not have been possible.
Footnotes
Contributors SG: conception and design of the study, acquisition of data, analysis and interpretation of data, drafting the article, revising it critically for important intellectual content, final approval of the version to be submitted. DM: acquisition of data and revising it critically for important intellectual content. AB: revising it critically for important intellectual content. ANVS: conception and design of the study, revising it critically for important intellectual content and final approval of the version to be submitted.
Funding This study was funded by National Research Foundation (NRF).
Competing interests None declared.
Patient consent for publication Not required.
Provenance and peer review Not commissioned; externally peer reviewed.