The Use of Nerve Locators for Regional Anaesthesia

Written by:
Joel Jay T. Martorillas, DVM, PGCert SAUA, MSc VAA, and
Dr Patrick Maguire, BVSc Hons1 DACVS-SA

Effective pain managementis essential for optimal patient care. Alleviating pain enhances a patient’s ability to resume normal function, improves quality of life and facilitates faster recovery times (Grubb and Lobprise, 2020b). In recent years, targeted nerve blocks have continued to gain popularity as adjuncts to systemic analgesics. Multimodal pain management strategies reduce individual medication dosages and in turn, reduce the incidence of adverse drug reactions. While numerous nerve blocks have been described, the efficacy and safety have historically been reliant on the practitioners’ knowledge of the anatomy and an element of luck when injecting locally. The more widespread use of nerve locators and other guidance techniques improves both safety and efficacy when delivering local analgesics.
 
The most widely used veterinary analgesics include NSAIDs and opioids. NSAIDs, while effective, can cause potentially significant gastrointestinal and/or renal side effects. Opioid boluses and infusions can result in nausea, postoperative ileus, and urine retention. Whilst young and healthy patients often tolerate these drugs well, elderly patients with decreased mobility, and underlying renal, and/or gastrointestinal disease may be at increased risk. This is especially relevant for patients undergoing more invasive and painful interventions.
 
Preventing pain is often more effective than treating it “after the fact”. The value of blocking pain at its source was recognised in the Ebers Papyrus, an ancient Egyptian medical document from 1550 BC. The use of opium poppy extracts to relieve surgical pain was described (Fascha, 2011). In the late 1800s, cocaine was found to have a localised numbing effect. Synthetic local anaesthetics such as bupivacaine and lidocaine have since been developed (Ruetsch et al., 2001). Bupivicaine and lidocaine inhibit the nociceptive inputs by selectively blocking the transmission of pain signals from a specific nerve group.

While nerve blocks are generally considered safe, some potential complications can arise from their administration. Nerve injury from direct trauma from the needle, injection of local anaesthetic into the nerve or nerve sheath, or pressure-induced ischemia represents risks. Nerve injuries may result in varying degrees of sensory or motor deficits, such as numbness, weakness, or paralysis in the affected limb. Using local anaesthetics in nerve blocks can sometimes result in systemic toxicity if the drug is inadvertently administered in excessive doses or directly into the bloodstream (Otero and Portela, 2019). In severe cases, this can lead to central nervous system (CNS) signs, cardiovascular effects, and seizure or cardiac arrest. Prompt recognition and appropriate management are crucial in preventing serious complications.

Femoral nerve block.

The following outlines the patient preparation and administration strategy we utilise at Beecroft when using the Stimuplex HNS 12 (B. Braun):
 
- The injection site is clipped and surgically prepped with aseptic solution (e.g., Chlorhexidine or iodine solution).
- A sterile drape is placed, just exposing the injection site and the necessary anatomical landmark.
- The positive electrode of the nerve stimulator is placed on the skin distal to the injection site (stifle or elbow).
- Prefill the extension line with local anaesthetics (Lidocaine or Bupivacaine HCl).
Set the nerve stimulator to the desired current ( 1mA, 0.1 ms., 1-2 Hz).
- Guided by the necessary anatomical landmarks, the needle (Stimuplex insulated needle) is slowly advance to the target nerve.
- Once the needle tip is in close proximity of the target nerve, the desired muscular response (e.g., tarsus flexion and extension or carpus extension) is observed.
- Extravascular and extraneural positioning of the needle tip is confirmed by drawing the syringe plunger and adjusting the current intensity to 0.3 mA or less.
- The desired dose of local anaesthetics is then delivered.
 
The following locoregional blocks are commonly performed at Beecroft Animal Specialist Services:
 
- Ulnar nerve block
- Ulnar, median, and musculocutaneous nerve block
- Sciatic nerve block
- Femoral nerve block

Radial Ulnar Median and Musculocutaneous (RUMM) block.

Like any intervention, other potential risks exist including delayed hair growth, infection, hypersensitivity reactions and/or hematoma formation. We feel strongly that the benefits of nerve blocks outweigh the potential complications associated with the procedure. We see enhanced post-procedure function, faster recovery, and decreased risk of complications and utilise peripheral nerve blocks extensively for both our soft tissue and orthopaedic patients.  
 
Delayed hair growth: 
In some regional blocks (i.e., epidural and spinal anaesthesia), Temporary hair loss or delayed hair growth may occur at the injection site (Troncy et al., 2002). This could be attributed to the trauma to the hair follicles during the procedure or the effects of local anaesthetics on the hair growth cycle. This hair loss is usually localized and self-limiting, with the hair typically regrowing within a few weeks to months. Monitoring the area for signs of infection or persistent hair loss is recommended.
 
Infection: 
Infection is a potential risk associated with any invasive procedure, including nerve blocks (although very rare). Sterile technique and proper disinfection of the injection site are essential to minimise the risk of introducing bacteria into the tissue. Signs of infection may include swelling, redness, discharge, or fever (Campoy and Read, 2013).
 
Local anaesthetic toxicity: 
Using local anaesthetics in nerve blocks can sometimes result in systemic toxicity if the drug is inadvertently administered in excessive doses or directly into the bloodstream (Otero and Portela, 2019). In severe cases, this can lead to central nervous system (CNS) signs, cardiovascular effects, and seizure or cardiac arrest. Prompt recognition and appropriate management are crucial in preventing serious complications.
 
Hematoma formation: 
Hematomas, a localised collection of blood, can occur at the site of the nerve block due to vascular injury during needle placement (Tanner and Hubbell, 2019). Hematomas can cause pain and swelling and potentially compromise nerve function if they exert pressure on the nerves. Close monitoring of the injection site and appropriate, including potential drainage or surgical intervention, may be necessary.
 
Nerve injury: 
Although rare, nerve injury can occur during nerve block administration. It can be caused by Proper technique, knowledge of anatomy, and careful needle placement are critical to minimise the risk (Campoy et al., 2019).
 
Allergic reactions: 
Dogs and cats may potentially develop hypersensitivity or allergic reactions to the local anaesthetics used in nerve blocks (Eggleston and Lush, 1996). Signs of an allergic reaction may include swelling, hives, itching, respiratory distress, or collapse. Immediate recognition and administration of appropriate medications, such as antihistamines or epinephrine, may be necessary to manage the allergic response.
 
It is important to note that complications associated with nerve blocks are relatively rare when performed by skilled and experienced veterinary professionals. Moreover, the use of specialised equipment such as ultrasound and nerve stimulators could mitigate the occurrence of the complications mentioned above.
 
Conclusion
The utilisation of nerve locators in veterinary medicine has brought about a significant transformation in the field of pain management. This approach has dramatically improved patient care, enhanced quality of life, and expedited recovery by facilitating targeted nerve blocks in conjunction with systemic analgesics. The administration of nerve blocks offers numerous advantages, including the ability to administer lower medication dosages, reduced risk of adverse reactions, and the prevention of pain at its source. This article emphasises the profound importance of locoregional blocks in augmenting pain management for surgical patients. Commonly employed locoregional blocks have become standard practice in providing effective pain relief for soft tissue and orthopaedic patients. Although potential complications may arise, their occurrence is infrequent when performed by skilled professionals, and any associated risks can be minimised by using proper techniques and specialised equipment. The benefits of nerve blocks far outweigh the potential complications, establishing them as an indispensable tool within comprehensive pain management strategies. Continuous advancements in this area will further enhance the safety and effectiveness of nerve block administration within the realm of veterinary medicine.
 
References
 
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