Comparing ultrasound and nerve stimulation: time to ask the question?

Abstract

When reading (or writing) reviews supporting the use of ultrasound over nerve stimulation to facilitate needle guidance and nerve blockade, a considerable number of quoted studies still fail to show a significant difference in block success rates for surgical anaesthesia between the two techniques [1]. However, under closer scrutiny we believe that recent research casts new light on comparison trials and throws up an interesting question. Let us first accept that the difference between the two techniques is in the method by which a needle is introduced, directed and confirmed to be in an appropriate position before injection. When using ultrasound, the principal endpoint of needle guidance is generally accepted to be observation of the needle’s tip next to the nerve. Subsequent injection, with or without minor degrees of needle repositioning relative to the nerve, then results in circumferential spread of local anaesthetic i.e. a perineural injection. Our assumption has been that a similar effect was achieved using nerve stimulation. However, it would appear that minimum stimulation thresholds, previously thought to be a reliable indicator that the needle’s tip was close to but not within the nerve substance, might represent intraneural needle placement [2-4]. Although there appears to be variation between nerves in their minimum stimulation thresholds [2, 3], it is probable that a significant number of blocks performed in nerve stimulation limbs of comparison trials are intraneural injections. Is this important? If one wanted to make a closer comparison between the two techniques, it would be relatively easy to mimic an intraneural endpoint under ultrasound. However, anyone who uses ultrasound regularly will testify that breaching the epineurium is not necessary to achieve a successful block. Rather than debating which technique offers greater success, when both offer excellent results, should we instead ask the question ‘is it prudent to avoid intraneural injection?’ Enthusiasts of nerve stimulation may argue that although intraneural injection may be common, it seldom leads to nerve injury [2-4], and that overall, nerve injury related to regional anaesthesia is rare [5]. However, severe and permanent nerve injury in association with peripheral nerve blockade does occur, and our understanding of the pathophysiology is far from complete. In addition, we know that at clinically used concentrations, all commonly administered local anaesthetics induce neuronal apoptosis [6]. It would therefore seem counter-intuitive to continue to promote a technique known to be associated with intraneural injection, when an alternative exists that has the potential to exclude it and is at least as effective. Use of ultrasound will never completely prevent nerve injury, but it provides us with visual and potentially recordable evidence that injury was not a result of direct needle trauma or intraneural injection [7]. Use of ultrasound will not always prevent intraneural injection, but that challenge relates more to the provision of appropriate equipment, training and supervision, rather than to the technique itself. If you consider the answer to the question ‘is it prudent to avoid intraneural injection?’ to be ‘yes’, then current evidence would support using ultrasound guidance over other techniques. The authors have received a combination of equipment and honoraria for teaching from SonoSite Inc, Bothell, Washington. No other competing interests declared. Previously posted at the Anaesthesia Correspondence website: http://www.anaesthesiacorrespondence.com.