Accuracy of injection pressure measurement at peripheral nerves using high-resolution 40 MHz ultrasound in an anesthetized porcine model

Peripheral nerve microanatomy: new insights into possible mechanisms for block success

Postmortem histology and in vivo, animal-based ultra-high-definition microultrasound demonstrate a complex array of non-communicating adipose tissue compartments enclosed by fascia. Classic nerve block mechanisms and histology do not consider this tissue. Injected local anesthetic agents can occupy any of these adipose compartments, which may explain the significant differences in outcomes such as success rates, onset time, block density, duration of nerve block, and secondary continuous block failure. Furthermore, these adipose tissue compartments may influence injection pressures, making conclusions about needle tip location unreliable. This educational review will explain the neural anatomy associated with these fatty compartments in detail and suggest how they may affect block outcomes.

Optimizing peripheral regional anaesthesia: strategies for single shot and continuous blocks

PURPOSE OF REVIEW

Regional anaesthesia is increasingly prominent within anaesthesia, offering alternative analgesic options amidst concerns over opioid-based analgesia. Since Halsted's initial description, the field has burgeoned, with ultrasound visualization revolutionizing local anaesthetic spread assessment, leading to the development of numerous novel techniques. The benefits of regional anaesthesia have gained increasing evidence to support their application, leading to changes within training curricula. Consequently, regional anaesthesia is at a defining moment, embracing the development of core skills for the general anaesthesiologist, whilst also continuing the advancement of the specialty.

RECENT FINDINGS

Recent priority setting projects have focussed attention on key aspects of regional anaesthesia delivery, including pain management, conduct and efficacy, education, and technological innovation. Developments in our current understanding of anatomy and pharmacology, combined with strategies for optimizing the conduct and maximizing efficacy of techniques, minimizing complications, and enhancing outcomes are explored. In addition, advancements in education and training methodologies and the integration of progress in novel technologies will be reviewed.

SUMMARY

This review highlights recent scientific advances in optimizing both single-shot and continuous peripheral regional anaesthesia techniques. By synthesizing these developments, this review offers valuable insights into the evolving landscape of regional anaesthesia, aiming to improve clinical practice and patient care.

Identification of spread after deliberate intraneural injection in five mammalian species

INTRODUCTION

This research endeavors to investigate the phenomenon of intraneural spread across distinct locations: subcircumneurium, extrafascicular intraneural, intrafascicular intraneural, and intraperineurium after deliberate intraneural injections across five mammalian species. The study also aims to propose determinants influencing this spread. Furthermore, the investigation strives to ascertain the optimal animal species and needle configuration for extrapolating intraneural injection outcomes to human contexts.

METHODS

This study examined 60 sciatic nerves from 30 fresh and untreated cadavers of rats, rabbits, dogs, pigs, and sheep. The specimens were organized into five groups, each comprising an equal number of nerves. Histological assessments were performed on 30 nerves, involving fascicle metrics. The remaining 30 nerves underwent intentional intraneural injections, facilitated by 19G and 23G needles under ultrasound and direct visualization guidance.Heparinized erythrocytes combined with a methylene blue solution were used as a marker to analyze the extent and patterns of intraneural spread. Needle orifice measurements were obtained, and these data were overlaid onto images of both nerves and needles. This enabled a comparative evaluation of sizes and an assessment of marker diffusion.

RESULTS

The findings indicated that sciatic nerves in rats, rabbits, and dogs were oligofascicular, characterized by larger fascicles, whereas pigs and sheep exhibited polyfascicular nerves comprised of numerous smaller fascicles. Fascicular diameters were variable across species, with dogs presenting the largest measurements. While intraneural spread was observed and documented, intrafascicular marker spreading was rare, occurring only in one rabbit specimen. Needle orifice attributes were scrutinized and visually depicted.

CONCLUSIONS

Despite the formidable challenges associated with the practical realization of intrafascicular injection, the utilization of animal models possessing monofascicular or oligofascicular nerves, such as rats, rabbits, and dogs, in conjunction with needles featuring aperture dimensions surpassing those of the fascicles, likely contributes to the compromised reliability of investigations into intraneural injection outcomes.

Fascicular injury is rare following needle transfixion: a study on median and ulnar isolated human nerves

BACKGROUND

Needle trauma has been associated with peripheral nerve injury and neurological dysfunction. However, inadvertent needle puncture is frequent while postblock dysfunction is rare. We conducted a cadaveric study to evaluate the association between needle puncture and fascicular injury.

METHODS

Five median and five ulnar (isolated) nerves were obtained from fresh human cadavers. Four different needles were used for the transfixing punctures: A 30° beveled (22G) nerve block needle, and 15° beveled (22G, 25G and 27G) Quincke spinal block needles. 10 transfixing punctures were made with each needle type on each nerve (40 punctures per nerve). Samples were then immersed in 5% formaldehyde solution for 30 days. Perpendicular cross-sections of the punctured segments were obtained. Samples were embedded in paraffin and analyzed under light microscopy with H&E staining. On each slice, the following variables were obtained: ratio of fascicular/epineurial tissue, the number of fascicles per nerve and the number of injured fascicles.

RESULTS

A total of 400 punctures were made (200 median and 200 ulnar) and 144 histological nerve sections analyzed (74 median and 70 ulnar). The median number of fascicles per section was 16 (range 7-23) and 17 (range 8-27) with a fascicular/epineural tissue ratio of 45% (range 35%-52%) and 44% (range 39%-54%) for median and ulnar, respectively. Three fascicular injuries were identified: one in ulnar and two in median. All injuries were caused by a 15° beveled needle, the ulnar with a 22G and the median with a 22G and a 27G.

CONCLUSIONS

Fascicular injury is rare following needle transfixion. Needle injury alone is unlikely to explain postblock neurological dysfunction.

Nerve block, nerve damage, and fluid injection pressure: overturning the myth

Histological and micro-ultrasound evidence rebuffs deep-rooted views on the nature of nerve block, nerve damage, and injection pressure monitoring. We propose that the ideal position of the needle tip for nerve block is between the innermost circumneural fascial layer and outer epineurium, with local anaesthetic passing circumferentially through adipose tissue. Thin, circumferential, subepineural expansion that is invisible to the naked eye was identified using micro-ultrasound, and could account for variability of outcomes in clinical practice. Pressure monitoring cannot differentiate between intrafascicular and extrafascicular injection. High injection pressure only indicates intraneural extrafascicular spread, not intrafascicular spread, because it is not possible to inject into the stiff endoneurium in most human nerves.

No evidence of fascicular injury following a low-volume intraneural injection of the median nerve: a cadaveric study

BACKGROUND

The test dose or hydrolocation technique allows rapid detection of spread location. Though its primary aim is to enhance safety in peripheral nerve blocks, evidence on the potential risks of an intraneural test aliquot is lacking. We conducted a cadaveric study to evaluate the risk of fascicular injury following a low-volume (<1 mL) intraneural injection of the median nerve.

METHODS

Ten upper limbs from fresh unembalmed human cadavers were studied. In-plane ultrasound-guided intraneural injections of the median nerve were performed at mid, proximal, and distal locations using 1 mL of methylene blue and heparinized blood solution. Nerves were extracted and samples immersed in 10% buffered formalin for 4 weeks. Perpendicular 3 mm slices were obtained for H&E staining and light microscopy analysis. Our main objective was to assess the number of injured fascicles. Secondarily, we evaluated the pattern of intraneural spread. Fascicular injury was defined as the presence perineurium or axonal disruption and/or the presence of erythrocytes inside a nerve fascicle.

RESULTS

Thirty injections were performed in 10 median nerves. Sonographic swelling was confirmed in 100% of the cases. 352 histological sections were analyzed to assess study outcomes. The mean number of fascicles on each section of median nerve was 20±6 covering 49%±7% of the nerve area. No evidence of axonal disruption nor intra-fascicular erythrocytes was found in any of the analyzed sections.

CONCLUSIONS

Low-volume intraneural injections do not result in evident fascicular injury. Our findings support the use of a test dose in ultrasound-guided regional anesthesia.