Edition 4 - peripheral nerve injury

The aim of the research roundup is to provide vet professionals with an easy-to-read digest on three items of research in pain management which focus on one common subject area. 

These research roundups provide a single point of reference for the reader and are also incorporated into our accredited courses.

Subject

Avoiding peripheral nerve injury (PNI) during locoregional anaesthesia

https://www.nysora.com/topics/complications/complications-prevention-neurologic-injury-peripheral-nerve-blocks/

Title of the publication

Complications and Prevention of Neurologic Injury with Peripheral Nerve Blocks

Aim of the publication

This review addresses various factors that may contribute to neurologic complications after peripheral nerve blocks and suggests principles of practice and implications of monitoring modalities to mitigate the risk of neurologic complications.

Review of methodology

As a narrative review this work provides a beautiful description with illustrations of the anatomy of nerve fibres. This gives us a perfect reminder of our anatomy and that helps our understanding of why there is concern around damage of nerve fibres. Research in this area is challenging as all work looks at animal models rather than human models – which is complicated by the fact that animal models vary due to species used. Extrapolation of such findings to clinical practice is difficult, although the view by some that intraneural injection does not cause problems, is considered outdated. 

Summary of information provided

Peripheral nerve injury (PNI) can be classified according to severity, which also helps understand prognosis.

Neuropraxia – myelin damaged, conduction is slowed – prognosis is good

Axonotmesis – axonal continuity is lost, endoneurium is intact – no conduction, prognosis fair

Neurotmesis – loss of both axonal & endoneurial continuity, no conduction . Poor prognosis. 

The anatomical site of injury is important – proximal axonal lesions, which are closer to the cell body, are thought to be more severe than distal axonal lesions. This could be relevant to our cases, for example with a cervical paravertebral block our injection site is more proximal on the nerve path compared to a saphenous nerve block. 

Nerve injury can be caused by physical trauma to the nerve, as well as chemical injury from substances injected. Such injury can lead to a range of complications such as ischaemia and inflammation. Blood vessels within the nerve bundle can be damaged, leading to haematoma formation. 

If direct intraneural injection occurs, this causes rupture of the perineurium and loss of the protective environment within the fascicle, leading to axonal degeneration.

Most studies on chemical damage to nerves have been conducted in vitro. The injection of ropivacaine either intrafascicularly or extrafascicularly caused demyelination, axonal degeneration and Wallerian degeneration – which was more severe with the intrafascicular route.  Any substance (including saline) injected perineurally has the potential to cause nerve damage however extrapolation of these in vitro work to the clinic floor is difficult. 

PNI not only occurs following regional anaesthesia – nerves can also be damaged due to patient positioning under anaesthesia – either due to stretch or compression of the nerve.

Key points to aid reflection 

·      Intraneural injection does not always lead to nerve injury

·      Unintentional intraneural injection is probably more common than previously recognised

·      Most postoperative neurologic injury is classified as neuropraxic and recovery to function can be expected

Our assessment of this publication

So how do we stay safe? When performing blind nerve blocks, avoid injecting if any increase in resistance to injection is detected. For electrical nerve location (ENL), presence of an evoked motor response at a current of less than 0.5 (0.1 ms) indicates intimate needle-nerve relationship, needle-nerve contact, or intraneural needle placement. Histological nerve injury occurred in 50% of the pigs when motor response was obtained at less than 0.2 mA, compared to no histological changes at 0.3–0.5 mA. It remains to be proven whether ultrasound visualisation of the needle tip prior to injection can aid in preventing PNI. It is most likely that a combination of each of these represents the safest approach to avoid intraneural injury. 

We found this review an interesting read and it supports our approach of ‘first of all, do no harm’. 

In this section you will find our ‘everyone should read this’ recommendation and a review that we have chosen to support your next steps in integrating this with your clinical practice. 

Everyone should read this 

Intraneural Injection in Regional Anesthesia: What Does the Literature Tell Us?

https://link.springer.com/article/10.1007/s40140-013-0028-3

Key points from this work

In the past, careful needle guidance in conjunction with observation of the patient’s response was used as a method to detect inadvertent in intraneural injection. This relies on a conscious patient! 

Needle to nerve contact does not always result in a motor response with ENL. In some studies, intraneural needle placement required a current of >0.5mA to generate a motor response. 

For a successful nerve block a sufficient concentration of local anaesthetic must diffuse intraneurally and also be spread along a myelinated nerve to block the critical number of adjacent nodes of Ranvier. 

Given the success of extraneural local anaesthetic deposition, intentional intraneural needle placement is to be avoided. 

Further resources to link to 

Getting started with nerve location webinar https://www.zeropainphilosophy.com/members-webinars